PUBLICATIONS. Journal of Geophysical Research: Oceans

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1 PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE Key Points: Storm-induced currents are 7 10 times higher than background mean currents Storm impact depends on wind speed and several other factors Peak bottom currents reach 0.8 m s 21 and rotate clockwise as storms progress Correspondence to: M. Z. Li, mli@nrcan.gc.ca Citation: Li, M. Z., Y. Wu, R. H. Prescott, C. C. L. Tang, and G. Han (2015), A modeling study of the impact of major storms on waves, surface and near-bed currents on the Grand Banks of Newfoundland, J. Geophys. Res. Oceans, 120, , doi: / 2015JC Received 29 JAN 2015 Accepted 29 JUN 2015 Accepted article online 2 JUL 2015 Published online 3 AUG 2015 A modeling study of the impact of major storms on waves, surface and near-bed currents on the Grand Banks of Newfoundland Michael Z. Li 1, Yongsheng Wu 2, Robert H. Prescott 3, Charles C. L. Tang 2, and Guoqi Han 4 1 Geological Survey of Canada (Atlantic), Natural Resources Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada, 2 Ocean and Ecosystem Sciences Division, Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada, 3 Prescott and Zou Consulting, Halifax, Nova Scotia, Canada, 4 Northwest Atlantic Fisheries Centre, Fisheries and Oceans Canada, St. John s, Newfoundland, Canada Abstract Waves and current processes, both surface and near-bed were simulated for major storms on the Grand Banks of Newfoundland using integrated wave, 3-D tidal and ocean current models. Most storms track southwest to northeast and pass to the north or northwest of the Grand Banks. Significant wave heights can reach up to 14 m and are predominantly to the northeast at the peak of storms. Extreme surface currents reach approximately 1 m s 21 and are largely to the southeast. The strongest bottom currents, up to 0.8 m s 21, occur on St. Pierre Bank and are dominantly to the south and southeast. While wave height and wind-driven current generally increase with wind speed, factors such as storm paths, the relative location of the storm center at the storm peak, and storm translation speed also affect waves and currents. Surface and near-bed wind-driven currents both rotate clockwise and decrease in strength as the storm traverses the Grand Banks. While the spatial variability of the storm impact on surface currents is relatively small, bottom currents show significant spatial variation of magnitude and direction as well as timing of peak current conditions. These spatial variations are controlled by the changes of bathymetry and mixed layer depth over the model domain. The storm-generated currents can be 7 to 10 times stronger than the background mean currents. These strong currents interact with wave oscillatory flows to produce shear velocities up to 15 cm s 21 and cause wide occurrences of strong sediment transport over nearly the entire Grand Banks. VC American Geophysical Union. All Rights Reserved. 1. Introduction Storms can generate huge waves and strong currents in both the surface mixed layer and the bottom boundary layer [Cacchione et al., 1987; Wright et al., 1994; Shay et al., 1998; Williams et al., 2001]. Thus they may have a disproportionate impact on the erosion, transport and deposition of sediments on continental margins [e.g., Swift et al., 1986; Cacchione and Drake, 1990; Nittrouer and Wright, 1994]. Knowledge of storm processes thus is important for offshore engineering design, determination of sediment budget and strata formation, marine navigation and other physical and biological processes. The magnitude and pattern of the currents induced by a storm depends on atmospheric and oceanic variables, such as wind speed, storm size, path and transit speed, seabed topography and local stratification conditions [Price, 1981; de Young and Tang, 1990; Petrie, 1993; Tang et al., 1998]. Measurements with moored instruments can be used to describe storm processes over short time scales and for a fixed location but they generally lack good spatial coverage. Coastlines and topography are often complex, and sediment distribution is generally heterogeneous on continental shelves. Furthermore, the path along which each storm traverses the shelf and hence the flow field it produces are quite variable. Therefore numerical modelling, combined with field observations, is required to fully study the storm processes and their impact on sediment transport at regional and continental shelf scales [e.g., Ulses et al., 2008; Warner et al., 2008]. Various types of numerical models have been developed to simulate the ocean response to wind stress and pressure forcing of a moving storm [Chang and Anthes, 1978; Price, 1981; Greatbatch, 1983; Davies and Jones, 1992]. Three-dimensional hydrodynamic models have been applied to simulate the wind-induced circulations by real storms on various continental shelf regions [e.g., Davies et al., 1998; Sheng et al., 2006]. These LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5358

2 Figure 1. Map of the Grand Banks of Newfoundland with location names. studies demonstrate that the response of the upper ocean to a moving storm is characterized by a cold wake behind the storm, and inertial oscillations that are most energetic to the right of the storm track. Wave, hydrodynamic and sediment transport models have been integrated in several recent studies to model sediment resuspension and transport by storms on continental shelf settings [Blaas et al., 2007; Wang et al., 2007; Harris et al., 2008; Ulses et al., 2008; Warner et al., 2008; Guillou and Chapalain, 2011]. These modelling studies establish that while waves are predominately responsible for sediment resuspension, the length scale and direction of sediment transport are mainly controlled by the tidal currents and the winddriven currents generated by the storm winds. However, numerical studies investigating detailed temporal and spatial variations of waves, wind-driven currents, and sediment transport through the duration of a storm are sparse. The effect of wind speed, storm path, and translation speed on storm-induced currents is poorly understood. The Grand Banks of Newfoundland consist of a series of shallow banks (one of them is called Grand Bank) that are in water depths from 30 to 200 m, and are separated from each other by deeper channels or enclosed basins (Figure 1). While repeated glaciations have strongly affected topography and sediment distribution, the hydrodynamics and sediment transport and deposition on the Grand Banks occur under the combined influence of the Labrador Current and the large waves and strong wind-driven currents associated with intense winter storms [Barrie and Collins, 1989; Amos and Judge, 1991]. Influenced by this assemblage of processes, a suite of complex large-sized bedforms have developed on the seabed [e.g., Barrie et al., 1984; Dalrymple et al., 1992]. Therefore the interaction of the Labrador Current and intense storms over large-sized bedforms on the Grand Banks offers a unique setting to study the dynamics of waves, wind-driven currents and ocean circulation currents, and sediment transport processes on an energetic shelf environment. Numerous studies on storm waves and currents and sedimentary processes on the Grand Banks have indeed been conducted. Field current and sediment transport measurements have been used to investigate the current structure, pattern and generation of inertial and low-frequency currents, and near-bed wave and current dynamics and bedform mobility on the Grand Banks [de Young and Tang, 1990; LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5359

3 Tang and Belliveau, 1994; DeTracey et al., 1996; Li et al., 2011]. Several regional models were developed and used to study the seasonal or monthly mean circulation of the Newfoundland and Labrador shelves [e.g., Greenberg and Petrie, 1988; Han, 2005; Han et al., 2008]. These studies showed that the region is dominated by the strong equatorward-flowing Labrador Current along the shelf edge and relatively weak currents in the interior of the Grand Banks. With respect to numerical modelling of storm-driven currents, Amos and Judge [1991] described modeled spatial patterns of the depth-averaged wind-driven current and total sediment transport at the peak of selected storms. Petrie [1993] calculated the depth-averaged currents for selected storms on the Grand Banks using a two-dimensional barotropic model. Tang et al. [1998] studied the barotropic response of the Labrador and Newfoundland shelves to a moving storm using a linear barotropic ocean model. As an integrated part of the present study, Wu et al. [2011] are the first to apply a 3-D ocean circulation model in predicting extreme storm-induced currents over the Grand Banks. Their modelling results show that extreme surface currents reach about 0.8 m s 21 over a large portion of the Grand Banks, and the response of the water to storm forcing varies geographically since the mechanisms of current generation are spatially different. A recent study by Ma et al. [2015] examined the barotropic and baroclinic responses to Hurricane Igor in the Grand Banks region. The above review clearly suggests that a comprehensive understanding of the characteristics and patterns of waves and wind-driven currents during major storms on the Grand Banks needs to be achieved first before properly addressing how major storms impact seabed shear stresses and sediment transport processes. In the present modelling study, we analyzed wind and hindcast wave data to demonstrate the magnitude and characteristics of major storms on the Grand Banks. A three-dimensional ocean circulation model was applied to simulate the current fields for 22 selected storms. The ocean current data, wave data and tidal predictions from a 3-D tidal model were coupled in a shelf sediment transport model to predict seabed shear stress and sediment transport flux in major storms. These simulations are the first to explore the patterns of waves and wind-driven currents in major storms and their impact on sediment transport on the Grand Banks using integrated wave, 3-D tidal and ocean current models. Given the length limitation and difference in scopes, the focus of this paper is to present the results on the characteristics and patterns of waves and wind-driven currents. Only highlights of the storm impact on the intensity of seabed shear stress and sediment transport are presented in this paper. Details of storm impact on bed shear stress and sediment transport patterns, and how these change through space and time during major storms will be presented in a future publication. Through the applications of these integrated models, we intend to address the following questions: (1) What are the wave and extreme current characteristics of major storms on the Grand Banks, and what factors determine the storm impact in the study area? (2) How do wind-driven currents vary spatially and temporally through the process of a major storm? and (3) How do storm-induced currents alter the background surface and bottom currents and what are the implications to seabed stress and sediment transport intensities? 2. Methods Wave hindcast data, storm selection, tidal and ocean current models, and the scheme of bottom shear stress and sediment transport calculations are described in this section. Various models and their validation are described only briefly here but references to detailed model description and verification are provided Wind, Wave, and Storm Selection The wind and wave data and storm selection were based on the Meteorological Service of Canada 50 year wave hindcast data set (MSC50 data) [Swail et al., 2006]. The MSC50 data and climatology were developed by applying the third-generation wave model (OWI-3G), with the inclusion of shallow water effects, to predict the wave fields in the North Atlantic over a continuous 50 year period from 1954 to The wind data were the NCAR/NCEP (National Center for Atmospheric Research/U.S. National Centers for Environmental Prediction) global reanalysis wind fields [Kalnay et al., 1996, data.ncep.reanalysis.html]. The wave model predictions were hourly at 0.18 resolution for the Canadian Maritime region and 3 hourly at 0.58 resolution for the Atlantic basin domain. The model was extensively calibrated through comparison with buoy and platform measurement data. The root-mean-square (RMS) error of significant wave height prediction is about 0.3 m with respect to a measured mean of 1.7 m, and that of wave period prediction is about 0.9 s with respect to a measured mean of 7.1s. The wind data, wave model, model validation, and hindcast results are fully described in Swail et al. [2006]. LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5360

4 Table 1. List of Selected Major Storms on the Grand Banks Modeled in This Study and Associated Parameters a Storm Rank RI (year) Year-Date Time H s (m) T p (s) W dir WS (m s 21 ) WD U5 ws (m s 21 ) U5 dir : : : : : : : : : : : : : : : : : : : : : : a All parameter values are those at Hibernia site and the peak of the storm is defined as the time of maximum wave height. Storm Rank storm rank based on the peak significant wave height; RI storm return interval; Year-Date and Time are the year, date and time of storm peak; H s,t p and W dir are respectively significant wave height, spectral peak wave period and wave propagation-toward direction at the peak of the storm; WS and WD are respectively the wind speed and toward direction at the peak of the storm; U5 ws and U5 dir are respectively the speed and direction of wind-driven current at 5 m below surface at the peak of the storm. A peak-over-threshold (POT) analysis was performed in the MSC50 data set to define extreme storms with various return periods. Using the hourly hindcast significant wave height (H s ) data, a storm peak is defined as any event that is greater than the minimum significant wave height threshold and also must be separated from any other peaks by at least 48 h. The significant wave height threshold was 6.9 m for the grid point nearest to the Hibernia site (Figure 1). A total of 533 storms were selected using the above criteria and the maximum significant wave heights at the peak of these storms (peak significant wave height) were fitted to the Gumbel extremal distribution to derive the significant wave height and return intervals for extreme storms. The MSC50 extremal analysis plot shows severe overestimation of significant wave height for storms with greater than 25 year return intervals [Swail et al., 2006]. POT data theoretically should not be fitted to the Gumbel distribution. This likely is the cause for the overestimation by the MSC50 extreme value analysis. To obtain improved estimates of storm return intervals, the hourly hindcast significant wave heights at the MSC50 grid point nearest to the Hibernia site on Grand Bank (Figure 1) were extracted, and used to select the top 515 storms with peak significant wave height > 7 m for the period of The significant wave height POT data of these 515 storms were fitted to the Generalized Pareto Distribution (GPD) which is more suitable for POT data series. The GPD extremal analysis plot demonstrates better agreement between the data and best fit curve, particularly for large storms, and was used to assign the return interval to storms on the Grand Banks for the period of One of the objectives of the overall study is to evaluate how the storm pattern, as well as the magnitude, affects the currents and sediment transport. Thus, the selected storms should not only be the most intense but should also be representative of return intervals and storm track patterns on the Grand Banks. Hourly H s and wind speed and direction data for the strongest 100 storms on the Grand Banks were extracted and plotted as wind and wave parameter maps. These maps were used to assess the affected scope and storm track patterns with different return intervals. Twenty-two storms were eventually selected for model simulations and analyses (Table 1). The peak significant wave height of the selected storms varies from 10.6 to 13.8 m and their return intervals range from 1.0 to 34.1 years. Storms 1 to 12 are the 12 most extreme storms based on wave height, while the remaining 10 storms represent typical and more frequent storms with return intervals from 1 to 2.5 years Tidal Model The harmonic constants of tidal currents from a three-dimensional tidal model were used to predict hourly tidal currents for the duration of each of the 22 selected storms. The tidal model was based on a three- LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5361

5 dimensional finite-element model [Han et al., 2008] with a level-2.5 turbulence closure scheme for vertical mixing. The model domain covers the Newfoundland Shelf, the southern Labrador Shelf and adjacent deep waters, with a typical horizontal resolution of 5 10 km over the shelf [Han et al., 2008, Figures 1 and 2]. The vertical grid has 21 variably spaced nodes with a minimum spacing of 1 m near the seabed. Tidal heights of the five major semidiurnal (M 2,S 2, and N 2 ) and diurnal (K 1 and O 1 ) constituents were specified along the model open boundaries. Tidal currents are dominated by the M 2 constituent, which are largest over shallow areas such as Southeast Shoal (see more discussion in Han [2000]). The RMS amplitude differences between the model and observed M 2 currents are 0.04 m s 21 and 0.06 m s 21 for the eastward and northward components, respectively. At a location ( W, N) near the Southeast Shoal (Figure 1), the model and observed variances of tidal currents from all five constituents at 44 m depth for November 2003 (centered around Storm 43 modeled in this study) are and m 2 s 22 respectively, indicating approximate agreement Ocean Model Storm-driven currents and background circulation were predicted from the Canadian East Coast Ocean Model (CECOM), a 3-D coupled ice-ocean circulation model for the east coast of Canada [Tang et al., 2008]. The ocean component of the model is the latest version of Princeton Ocean Model (POM2K). The model has a free surface and contains an embedded second-order turbulence closure submodel [Blumberg and Mellor, 1987], which takes into account the effects of both wind mixing and wave dissipation [Mellor and Blumberg, 2004]. The vertical eddy viscosity is parameterized by a mixing length, the turbulence kinetic energy and a stability factor which depends on the vertical shear and buoyancy. The horizontal diffusivity is the Smagorinsky diffusivity. The ice component of the model is primarily based on a multicategory sea ice model developed by Hibler [1980]. Tang et al. [2007] found that under storm conditions the surface currents on the Grand Banks could be affected by waves due to the Stokes drift. The present study focuses on the storm effects on the wind-driven currents and does not include the effects of Stokes drift. While the influence of Stokes drift on near-bed currents is negligible for the study region, its influence on surface currents may be up to 0.16 m s 21. This is only a fraction of the maximum surface wind-driven currents during major storms ( 1ms 21 )[Wu et al., 2011 and this study]. Furthermore Coriolis-Stokes forcing may substantially alter the current profile in the mixed layer [e.g., Polton et al., 2005]. Our ocean model does not include wave-current interactions either. Therefore, future improvements could be made to account for the Stokes drift and wave-current interactions. The model domain covers the entire Canadian east coast [Wu et al., 2011] and the model grid has a 0.18 horizontal resolution on a rotated spherical coordinate system. There are 29 vertical levels in the generalized vertical coordinate system which permits both the z and the sigma levels so that the level spacing has higher resolution within the bottom and surface boundary layers. The detailed introduction of other implementation of the model can be found in Wu et al. [2012]. The atmospheric forcing to drive the model includes winds at 10 m above sea surface, air temperature at 2 m above sea surface, surface air pressure, precipitation, specific humidity and cloud coverage. As the storms used in this study go back to the 1950s (Table 1), the atmospheric forcing data were assembled from several data sources. The NARR data (North American Regional Reanalysis) [Mesinger et al., 2006, html] were used in the model calculation for storms in the period For storms before 1979, the wind fields were from the MSC50 data set and the other atmospheric forcing variables were from the NCEP reanalysis data set. Before the model was run for each storm, we generated the background mean circulation current field for each month (monthly mean circulation current) with 1 year model run forced by the NARR monthly climatology of the air forcing. The model was then run to produce 3 hourly ocean currents for each storm. In the model run for individual storms, the integration period was 20 days centered about the start time of each storm and the model time step was 150 s. The monthly mean circulation current was subtracted from the 3 hourly ocean currents to derive the wind-driven current. Extensive efforts have been made to validate CECOM [Wu et al., 2011, 2012]. CECOM simulated currents and observed currents at fixed locations and depths in several regions of the eastern Canadian seas were compared using both visual comparison and statistical analysis methods [Wu et al., 2012]. The comparison results indicate that the major features of the current fields present in the observations are successfully reproduced by the model. To evaluate the model performance under strong wind forcing, Wu et al. [2011] compared model results to mooring current meter data measured at three depths on northeastern Grand LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5362

6 Bank during a November 1997 storm (Storm 47 of Table 1). Good agreement in both current speed and direction was obtained in the storm period [Wu et al., 2011, Figure 3]. The mean error for the mean speed predicted at several depths was 7% relative to a measured mean speed of 0.16 m s 21 and the mean error for the predicted maximum speed was 12% relative to a measured maximum speed of 0.36 m s Bottom Shear Stress and Sediment Transport Computations The computations of shear stress and sediment transport are described in this section. However, as stated previously, the details of the storm impact on the shear stress and sediment transport will be presented in a sequel publication. The water depth was derived from the Northwest Atlantic (NWATL) data set [Varma et al., 2008] that were interpolated onto the 0.18 ocean model grid. Observed grain size data are very sparse on the Grand Banks. Thus a uniform grain size of 0.35 mm (medium sand) was used in this study. This mean grain size is representative of the average grain size in the study area [Barrie and Collins, 1989] and was also used in the sediment transport predictions by Amos and Judge [1991]. Model-predicted wave and ocean current data were interpolated temporally from 3 hourly to hourly intervals. The hourly ocean current was then added to the tidal current to derive the total current data at 2 m above seabed. The 2 m near-bed height was chosen to smooth out possible effects from suspicious values in the bathymetry data. The speed and direction of the near-bottom total current and the wave parameters of significant wave height H s, peak-spectral wave period, and wave propagation direction, together with water depth and the mean grain size of 0.35 mm, were input into a continental shelf sediment transport model (SEDTRANS) [Li and Amos, 2001] to calculate the combined wave-current shear velocity u*cws and sediment transport rate for each of the 22 selected storms. The shear velocity is related to seabed shear stress s through the quadratic law s 5 qu*cws 2 with q representing water density. The computation of the combined wave-current shear velocity u*cws is based on the combined wave-current bottom boundary layer theory of Grant and Madsen [1986], through iterative procedures. The full description of the bottom boundary layer theory and the iterative procedures of u*cws calculation can be found in Grant and Madsen [1986] and Li and Amos [2001]. In this application of SEDTRANS model on the Grand Banks, the Einstein-Brown bedload equation [Brown, 1950] was used to compute the bedload transport rate. The suspended load transport rate was obtained from the vertical integration of the predicted velocity and suspended sediment concentration profiles. The bedload and suspended load transport rates were then vectorially added to derive the total sediment transport rate. 3. Results 3.1. General Characteristics of Storms Of the top 100 storms identified using the hindcast significant wave height near the Hibernia site (for the 50 year period of ), the storm return interval (RI) ranges from 0.6 to 34.1 years. Peak significant wave height (H s ) is from 9.9 to 13.8 m, and wind speed ranges from 19.1 to 29.7 m s 21. Wave propagation direction at the peak of storms (based on maximum wave height near Hibernia) was dominantly to the northeast, accounting for 63% of the top 100 storms. Storms with waves to the southeast were also significant and accounted for 33% of the top 100 storms (Figure 2). If only the top 12 storms were considered, waves at storm peak were to the northeast in 83% of these major storms and only 17% of the major storms showed waves to the southeast at the storm peak (Table 1). However, the top five storms with peak significant wave height greater than 13 m all showed wave direction to the northeast at storm peak. The wind, wave, and wind-driven current parameters for the 22 storms modeled (Table 1) show that the maximum surface wind-driven current at Hibernia reaches 0.7 m s 21. The directions of the surface winddriven currents are predominantly to the east and southeast for major storms with winds to the northeast, and are generally to the southwest for major storms with winds to the southeast. However, peak winddriven current does not always occur at the time when wind is the maximum. Bottom wind-driven currents are more complex and show stronger spatial variations. These are discussed in later sections Factors Affecting Storm Impact on the Grand Banks The strongest wind does not necessarily produce the strongest waves and wind-driven currents. Of the 22 selected storms ranked using peak wave height (Table 1), the second strongest storm was ranked 20 th based on wind speed (with a peak wind speed of 25.2 m s 21 ). This moderately strong wind caused a very large significant wave height of 13.5 m and strong surface wind-driven current of 0.48 m s 21 at the peak of LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5363

7 the storm. In contrast, the wind of the 8 th ranked storm was stronger and reached 27.5 m s 21 (ranked fourth based on wind speed). Yet the peak significant wave height and the surface wind-driven current at the peak of the storm were both smaller in this storm (12.5 m and 0.36 m s 21 respectively). For a more representative and regional evaluation of the correlation between wind speed, wave height, and wind-driven current, these parameters were averaged over the entire Grand Banks (defined by the 200 m contour) to derive their domainaveraged peak values. The domainaveraged peak surface wind-driven current speed and peak significant wave height are compared against the domain-averaged peak wind speed for the selected 22 storms in Figure 3. These comparisons indicate a clear Figure 2. Polar plot of storm-peak significant wave height, H s (m), and wave propagation (toward) direction, W dir, of the top 100 storms for the Grand Banks. overall trend of increasing wave height and wind-driven current with increasing wind speed. However, significant variance exists. For example, the range of wind-driven current for a given wind speed can be 0.15 m s 21. Thus factors other than wind intensity (e.g., storm track, location of storm center, and translation speed) also impact waves and currents on the Grand Banks. Wind and surface wind-driven current (at 5 m below surface, U5 ws ) for selected hours of Storm 7 and Storm 11 are compared in Figure 4 to demonstrate how storm track and the distance of storm centre to the Grand Banks at the peak of storm affect wind-driven currents on the Grand Banks. Both storms had a southwestnortheast track, and nearly identical wind speed and direction (southwesterly wind with 27 m s 21 speed, Table 1). However, Storm 7 passed over northern Newfoundland (Figures 4a and 4b), while Storm 11 passed directly over the Grand Banks (Figures 4d and 4e). Also the distance of the storm center to the Grand Banks at the peak of Storm 11 was much closer than that of Storm 7 (Figures 4b and 4e). As a result, Storm 11 generated a strong domain-averaged peak current of 0.48 m s 21 (Figure 4f) while the domain-averaged peak current of Storm 7 only reached 0.27 m s 21 (Figure 4c). Therefore under similar wind speeds, storms that follow tracks closer to the Grand Banks and have shorter distance between the storm center and the Grand Banks at the storm peak, will likely generate stronger surface wind-driven currents on the Grand Banks. The effect of storm translation speed (how fast the storm crosses the modelling domain) is demonstrated by comparing the wind and surface wind-driven current of Storm 7 (Figures 4a 4c) against those of Storm 8 of February 1995 (Figure 5). Both storms followed a SW-NE track and passed to the northwest of Grand Bank. Peak wind speed (27 m s 21, Table 1) and the distance from the storm center to the Grand Banks at storm peak (Figures 4b and 5b) were also similar. However, Storm 7 took 10 h from the time its center left Newfoundland to the time the peak condition was reached (Figures 4a and 4b). The same process took 15 h for Storm 8 (Figures 5a and 5b). The faster translation speed of Storm 7 resulted in relatively low domainaveraged peak current (0.27 m s 21, Figure 4c) while the slower translation speed of Storm 8 allowed the wind longer time to impact the water surface and hence generated stronger domain-averaged peak current (0.39 m s 21, Figure 5c). Thus if storm track, wind speed and storm-center distance to the Grand Banks at the storm peak are similar, slower moving storms will produce stronger wind-driven currents in the study region Spatial and Temporal Patterns of Major Storms As previously stated, waves at storm peaks propagate to the northeast for approximately 80% of the major storms and generally to the southeast for the remaining storms (Table 1). Storm 3 was one of the top storms LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5364

8 Figure 3. Scatter plots of (a) peak domain-averaged significant wave height H s (m) and (b) peak domain-averaged surface wind-driven current speed U5 ws (m s 21 ) versus peak domain-averaged wind speed WS (m s 21 ). (3 rd ranked) that had the dominant track pattern of northeasterly wave direction at the storm peak. Storm 19 was one of the stronger storms having the pattern with southeasterly waves at the storm peak. These two representative storms were analyzed in detail to demonstrate the spatial and temporal patterns of wind, waves, and wind-driven currents of the major storms and the differences in storm impact between these two contrasting storm track patterns Wind, Waves, and Wind-Driven Currents of Storm 3 This storm affected the Grand Banks for 3 days from 14 to 16 February 1982 with storm force wind (>10 m s 21 )lasting more than 56 h from 0300Z on 14 February to 1000Z on 16 February (all time used in this paper is UTC time). Time series of wind, wave, surface (U5 ws ) and bottom (5 m above seabed U5 wb ) wind-driven currents of the storm are shown in Figures 6 9, respectively. Statistics are values averaged over the Grand Banks domain defined by the 200 m isobath unless stated otherwise. The center of the storm passed to the northwest of Grand Bank (Figures 6a 6c). Winds reached peak intensity at 1800 on 14 February as the storm centre crossed southeastern Newfoundland (Figure 6b). Maximum wind speed reached 28.7 m s 21 and maximum significant wave height reached 13.4 m (Figure 7b) at Hibernia which defines the storm as the third ranked storm with a 20 year return interval (Table 1). Winds and waves were both to the NE at the peak of the storm Wind and Waves At the early stage of the storm, 0800 on 14 February, the storm center first entered the modelling domain at the southwest corner (Figure 6a). Domain-averaged wind speed was 18 m s 21 and to the NW. Peak wind was reached at 1800 on 14 February with storm center located over southeastern Newfoundland (Figure 6b). Domain-averaged wind speed reached the peak value of 25.4 m s 21 and was to the NE. The storm started to weaken at 0000 on 15 February (Figure 6c) when wind speed decreased to 24.6 m s 21 and wind direction was dominantly to the E and ESE. Toward the later stage of the storm at 0600 on 15 February (Figure 6d), domain-averaged wind speed decreased to 21 m s 21 and was to the SE. Waves generally followed the wind pattern. At the early stage of the storm, waves were to the NW with a domain-averaged significant wave height of 4 m (Figure 7a). The peak wave condition lagged the wind by 2 h and occurred at 2000 on 14 February with a domain-averaged H s of 10 m and a dominant direction to the NE (Figure 7b). During the waning stage of the storm, waves were largely to the SE and the domain-averaged H s decreased to 8 m (Figures 7c and 7d) Surface Currents At the early stage of the storm (0800 on 14 February), moderate surface wind-driven currents were detected, up to 0.5 m s 21 to the NW on St. Pierre Bank (SPB) and Green Bank, and up to 0.35 m s 21 to the N on Grand Bank (Figure 8a). At the time when wind and waves reached peak conditions (approximately 1800 on 14 February), surface wind-driven currents significantly increased (Figure 8b) although peak condition LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5365

9 Journal of Geophysical Research: Oceans Figure 4. (top) Wind speed (m s21) and direction at early stage and approximately peak (middle) of (left) storm 7 and (right) storm 11 respectively. (bottom) The surface wind-driven current (m s21) approximately at the peak of each storm. All times are in UTC. LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5366

10 Figure 5. (top) Wind speed (m s 21 ) and direction at early stage and approximately peak (middle) of storm 8. (bottom) The surface winddriven current (m s 21 ) approximately at the peak of the storm. was not reached yet. Surface currents were up to 0.8 m s 21 and current direction was rotated clockwise to the east (Figure 8b). Surface current reached peak conditions at 0000 on 15 February, with maximum values of 1 m s 21 and predominantly to the SE (Figure 8c). At the early waning stage (0600 on 15 February, Figure 8d), surface currents were largely less than m s 21 and dominantly to the SW. At the later waning stage (1300 on 15 February), surface currents further rotated clockwise to become westerly on SPB, Green Bank and over the southern Avalon Channel, and the speed was reduced to m s 21 (Figure 8e). Wind-driven currents were minimal on the Grand Bank proper Bottom Currents The effect of winds on bottom currents first became detectable at 1400 on 14 February with current speed reaching 0.3 m s 21 to the NW on SPB, and m s 21 to the E and NE over patches on central and southeastern Grand Bank (Figure 9a). When wind peaked at 1800 on 14 February, bottom currents were also further developed (Figure 9b). U5 wb increased to m s 21 and rotated clockwise to the NE on SPB. Bottom currents up to 0.3 m s 21 occurred widely on western, central and southern Grand Bank. Currents were to the NE on western Grand Bank and to the E and SE respectively on central and southern Grand Bank. As the surface current peaked at 0000 on 15 February, peak bottom currents first occurred on SPB and Green Bank, up to 0.7 m s 21 and dominantly to the SE (Figure 9c). Bottom currents on Grand Bank also increased significantly: up to 0.45 m s 21 and to the E on western Grand Bank, up to 0.3 m s 21 and to the N on southeastern Grand Bank, and up to 0.25 m s 21 and dominantly to the W on northeastern Grand Bank. This pattern defined a counterclockwise gyre centered over NE Grand Bank. The peak of the domainaveraged bottom current, however, lagged the surface current by 3 h and occurred at 0300 on 15 February (Figure 9d). Bottom currents on SPB decreased slightly and further rotated clockwise to become southerly. Maximum values were reached on the Grand Bank proper and the counterclockwise gyre was strengthened: currents were up to 0.5 m s 21 on western Grand Bank, and up to 0.35 m s 21 on SE Grand Bank. At the early waning stage, bottom currents on SPB decreased to <0.5 m LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5367

11 Figure 6. Time series of wind speed (m s 21 ) and direction for storm 3 with northeastward peak waves. Peak wind condition occurred at 1800 on 14 February in B. s 21 and further rotated clockwise to the SW (Figure 9e). The southwesterly currents over Avalon Channel reached the peak of 0.45 m s 21 and expanded to cover part of northern and central Grand Bank. This expansion weakened the counterclockwise gyre although it was still recognizable (Figure 9e). At 1800 on 15 February (Figure 9f), 24 h after the storm peak, moderate near-bed wind-driven currents were still observed on St. Pierre Bank (0.4 m s 21 to the NW), over the Avalon Channel (0.3 m s 21 to the W and SW), and on SE and NE Grand Bank (0.3 m s 21 to the SE and NW respectively) Wind, Waves, and Wind-Driven Currents of Storm19 This storm affected the Grand Banks for 3 days from 22 to 25 January 2002 with storm force wind (>10 m s 21 ) lasting 48 h from 1000Z on 22 January to 0900Z on 24 January. Time series of wind, wave, surface and bottom wind-driven currents of this storm are shown in Figures respectively. The storm center also passed to the northwest of the Grand Banks (Figures 10a 10c), similar to Storm 3. Winds reached peak intensity at 1600Z on 23 January (Figure 10c) when the storm centre was located to the northeast of the Grand Banks. Maximum wind speed reached 25.9 m s 21 and maximum significant wave height reached 11.5 m (Figure 11c) at Hibernia which defines the storm as the 19 th ranked storm with an approximately 2 year return interval (Table 1). In contrast to Storm 3, winds and waves were both to the southeast at the peak of the storm. LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5368

12 Figure 7. Time series of significant wave height (m) and direction for storm 3. Peak wave condition occurred at 2000 on 14 February in B Wind and Waves At the early stage of the storm (1400 on 22 January), storm center first entered the modelling domain from the western side (Figure 10a). Wind was to the NW with a domain-averaged speed 14 m s 21. Wind reached peak conditions at 1600 on 23 January with the storm center located northeast of the Grand Banks (Figure 10c). Domain-averaged wind speed reached 24 m s 21 and was to the SE. During the waning stage of the storm (at 0100 on 24 January; Figure 10d), wind speed was reduced to 17 m s 21 but the southeastward direction was maintained. The patterns of the waves again were similar to that of the wind. At the early stage of the storm (1400 on 22 January), waves were to the NW and the domain-mean significant wave height was 3.6 m (Figure 11a). The peak wave condition occurred at 1700 on 23 January with a domainmean H s of 8.4 m and a dominant direction to the SE (Figure 11c). During the waning stage of the storm, wave direction remained to the SE but the domain-mean H s decreased to 7.2 m (Figure 11d) Surface Currents At the early stage of the storm (1400 on 22 January), surface current was mainly affected on southern Grand Bank (Figure 12a). Current speed was up to 0.5 m s 21 and to the north. During the storm build-up stage at 2100 on 22 January, much broader effects on surface current occurred on Grand Bank (Figure 12b). U 5ws increased to 0.6 m s 21 and rotated clockwise to eastward on western and southern Grand Bank. As the storm peak was approached, strong southeasterly winds affected the western Grand Banks (not shown but the pattern was similar to that of Figure 10c). This caused the peak surface currents to first occur on SPB at LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5369

13 Journal of Geophysical Research: Oceans Figure 8. Time series of surface wind-driven currents (m s21) for storm 3. Peak domain-averaged currents occurred at 0000 on 15 February in C on 23 January: U5ws was southward and up to 1 m s21 (Figure 12c). The domain peak of U5ws occurred at 1800 on 23 January (Figure 12d) and lagged the wind peak by 2 h. Currents were predominantly to the southwest on Grand Bank with strongest current of 1 m s21 occurring on central and southeastern Grand LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5370

14 Figure 9. Time series of bottom wind-driven currents (m s 21 ) for storm 3. Peak domain-averaged currents occurred at 0300 on 15 February in D. LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5371

15 Figure 10. Time series of wind speed (m s 21 ) and direction for storm 19 with southeastward peak waves. Peak wind condition occurred at 1600 on 23 January in C. Bank. Strong currents of 0.7 to 0.8 m s 21 also occurred on the northeastern bank and over Avalon Channel. Currents on SPB, however, decreased to 0.7 m s 21 and were to the WSW now. During the waning stage of the storm (at 0100 on 24 January), surface currents continued the clockwise rotation and were significantly reduced in speed (Figure 12e): currents were up to 0.6 m s 21 and to the NW on SPB, up to 0.55 m s 21 and to the WSW in Avalon Channel and on northern Grand Bank, and <0.35 m s 21 to the SW on central and southern Grand Bank. Toward the end of the storm at 0900 on 24 January, storm effects on surface currents diminished significantly (Figure 12f) Bottom Currents Effects on bottom currents became apparent at 2100 on 22 January (Figure 13a). Currents were up to 0.35 m s 21 and were to the northeast on western Grand Bank, and to the east and southeast on central and southeastern Grand Bank. Northeasterly currents up to 0.2 m s 21 were also predicted on SPB. By 0200 on 23 January, stronger winds (20 25 m s 21 ) to the southeast affected the western Grand Banks (not shown). This generated stronger bottom currents in these areas (Figure 13b). Currents increased to m s 21 on SPB and Green Bank, and rotated clockwise to be dominantly to the east. Currents on SE Grand Bank rotated clockwise to become southward and were slightly reduced. By 0900 on 23 January, bottom currents reached peak conditions first on SPB (Figure 13c). Currents were to the SE and S and up to 0.5 m s 21. Bottom currents also increased up to 0.35 m s 21 LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5372

16 Journal of Geophysical Research: Oceans Figure 11. Time series of significant wave height (m) and direction for storm 19. Peak wave condition occurred at 1700 on 23 January in C. on Grand Bank and developed into a counterclockwise gyre with the center on the central bank. Domain peak bottom currents occurred at 1100 on 23 January (Figure 13d) which was several hours ahead of peak wind and peak surface current, and lasted 8 h until Currents on SPB maintained the speed of 0.45 to 0.5 m s21 but were to the SW now. The counterclockwise gyre on Grand Bank was further developed with bottom currents ranging from 0.25 to 0.4 m s21. In the waning stage of the storm (at 0100 on 24 January), bottom currents decreased significantly (Figure 13e). Currents were to the NW but <0.4 m s21 on SPB. Currents over Avalon Channel and northern Grand Bank maintained the southwestward direction but were reduced to 0.3 m s21. Bottom currents were <0.2 m s21 on other parts of Grand Bank and the counterclockwise gyre had disappeared. At the end of the storm (0900 on 24 January), bottom currents on Grand Bank were mostly <0.25 m s21 and occurred only in small patches (Figure 13f) General Patterns of Top Storms and Differences Between Storms With Peak Wind to NE and SE With the exception of Storm 6, the top 7 storms (with RI > 8 years, see Table 1) show a similar storm pattern, i.e., storms start with wind build up south of the Scotian Shelf, the storm center is first recognized in the southwest quadrant of the model domain (Figure 6a) and then moves to the NE over Newfoundland or the northern Grand Banks (Figure 6b), then onto the Northeast Newfoundland Shelf. At the storm peak, the storm center is to the north or northwest of the Grand Banks (Figure 6b), and waves are to the NE over the model domain (Figure 7b). These top storms are all large-scope storms that affect the Scotian Shelf, the Grand Banks, and the Northeast Newfoundland Shelf. Analyses of spatial and temporal patterns of individual storms also demonstrate that while wind and waves tend to peak at approximately the same time, LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5373

17 Journal of Geophysical Research: Oceans Figure 12. Time series of surface wind-driven currents (m s21) for storm 19. Peak domain-averaged currents occurred at 1800 on 23 January in D. storm-induced surface currents often lag by several hours. Storm-induced bottom currents then lag the surface currents by another several hours. It is also found that while the magnitude and direction of surface wind-driven currents at the peak of a storm do not change significantly over the modelling domain (Figures LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5374

18 Journal of Geophysical Research: Oceans Figure 13. Time series of bottom wind-driven currents (m s21) for storm 19. Peak domain-averaged currents first occurred at 1100 on 23 January in D. LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5375

19 8c and 12d), bottom wind-driven currents at the storm peak often show strong spatial variability in current speed and direction (Figures 9d and 13d). As suggested by Table 1, wind and wave directions of the top storms are predominantly to the northeast although about 20% of the major storms show peak wave direction to the southeast. Wind, wave and winddriven current processes have been analyzed for storms with peak wind to NE and those with peak wind to SE respectively to demonstrate the different characteristics of these two dominant types of storms with distinctive storm-peak wind and wave patterns. For major storms with peak waves to NE, the storm center is generally located to the north or northwest of Grand Bank at the peak of the storm (Figure 6b). Wind and waves at the peak of the storm are to NE (Figures 6b and 7b). Maximum surface wind-driven currents reach 1 ms 21 and are predominantly to the southeast (Figure 8c). Maximum bottom wind-driven currents are up to 0.6 to 0.7 m s 21 and are dominantly to SE and E but to N and NW on central and NE Grand Bank (Figures 9c and 9d). Counterclockwise gyres are often developed on northeastern Grand Bank. For storms with peak waves to the SE, the storm center at the peak of the storm tends to be located to the NE of Grand Bank (Figure 10c). Wind and waves at the peak of the storm are to the SE (Figures 10c and 11c). Maximum surface wind-driven currents also reach 1 ms 21 but are predominantly to the southwest (Figure 12d). Peak near-bed wind driven currents (Figures 13c and 13d) reach m s 21, which are roughly 30% less than those for storms with peak waves to the NE. Current directions are dominantly to the south and southwest on SPB while a counterclockwise gyre forms over nearly the entire Grand Bank Vertical Variation of Wind-Driven Currents Time series of wind and wind-driven currents at various depths are shown in Figure 14 for a selected point on southeastern Grand Bank, and in Figure 15 for a site on northeastern Grand Bank to evaluate the vertical variations of wind-driven current during Storm 3. At the peak of the storm (approximately 0000 on 15 February), wind was essentially eastward (Figures 6c and 14a). Surface wind-driven current was generally to the southeast to form 458 clockwise angles from the wind direction (Figures 14b and 15b). At the southeastern Grand Bank site in about 40 m depth (Figure 14), the direction of the surface (Figure 14b), middepth (Figure 14c) and near-bed (Figure 14d) wind-driven currents was similar and to the southeast. But the speed decreased from 1.2 m s 21 at the surface and middepth, to 0.3 m s 21 at 5 m above the bottom. A similar vertical pattern was also found for SPB. This suggests that for relatively shallow depths (70 m) on the bank tops, strong winds during major storms generate a surface mixed layer that extends all the way to the seabed (see further discussion in Section 4.3) so that the direction of the wind-driven current is nearly the same through the entire water column, although the current strength decreases with depth due to friction of the sea floor. At the NE Grand Bank site where the water is relatively deep (90 m; Figure 15), the surface wind-driven current was significantly less than that on southeastern Grand Bank and the maximum speed only reached 0.4 m s 21 at the peak of the storm (at 0000 on 15 February; Figure 15b). Vertically, the current speed and direction did not change much from the surface (Figure 15b) to the middepth (Figure 15c). The nearbottom currents, however, not only decreased significantly to 0.2 m s 21 but also were nearly opposite in direction to the surface wind-driven currents. For instance, the near-bottom current was to NNW at 0300 on 15 February (Figure 15d) while currents at the surface and the midvdepth were to the southeast. Analysis of moored current meter data on northeastern Grand Bank by de Young and Tang [1990] also demonstrated a similar pattern in which the upper and lower-layer currents were roughly equal in amplitude but were 1808 out of phase. The current below the mixed layer was believed to be introduced by mass continuity. Similar vertical variation patterns of wind-driven currents at these selected points were also found for Storm 1, although these are not presented in this paper Significant Alterations of Ocean Currents by Major Storms The time series data of wind-driven currents at various stages of a storm (Figures 8, 9, 12, and 13) already show that major storms significantly impact both the surface and near-bed currents on the Grand Banks. The modeled surface and near-bed total ocean currents before and at the peak of Storm 3 (based on maximum surface current) are compared in Figure 16 to further illustrate the dramatic alterations to the magnitude and direction of ocean currents on the Grand Banks. Before the storm at 1900 on 13 February (Figure 16a), the dominant feature of the surface current is the Labrador Current following the contours of the shelf break. Current speed is up to 0.5 m s 21 off the northeastern and southeastern edges of Grand Bank but LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5376

20 Figure 14. Vector plots of (a) wind and (b d) wind-driven current at different depths for a site on southeastern Grand Bank (see location in Figure 18a) for storm 3. decreases to 0.3 m s 21 off the southwestern edge and on SPB. Over the interior of the Grand Banks, the surface currents are minimal (< 0.1 m s 21 ) and show no distinct patterns. At the peak of the storm (0000 on 15 February, Figure 16b), the strong wind causes a tenfold increase of the currents over the interior of the Grand Banks: surface currents are up to 1 m s 21 and are dominantly to the southeast. Storms also significantly affect the currents along the shelf edge and upper slope. The direction of the equator-ward currents off the northeastern and southeastern edges does not change but their magnitude is increased from 0.5 to 0.8 m s 21. Northwesterly currents off the southwestern edge now flow to the southwest and the speed is increased from 0.3 to 0.6 m s 21. Similar changes were also described by Wu et al. [2011]. The near-bed currents before the storm (Figure 16c) show similar patterns to the surface currents. The Labrador Current hugs the contours of the shelf break, and the current speed is generally <0.3 m s 21. Bottom currents for the interior Grand Banks are again less than 0.1 m s 21 with no clear patterns. The near-bed currents at the peak of the storm are strongly enhanced so that maximum bottom currents reach 0.7 m s 21 and are dominantly to the southeast and east-southeast (Figure 16d). A counterclockwise gyre is developed on northeastern Grand Bank with currents up to 0.35 m s 21.Moderately strong currents of 0.5 m s 21 with a southwestward direction are also found over Avalon Channel. Due to the greater depths over the bank edge and upper slope, storms do not significantly impact the near-bed currents in these areas. Thus the magnitude and direction of the near-bottom Labrador Current on the edges of Grand Bank are not altered by the storms. Figure 15. Vector plots of (a) wind and (b d) wind-driven current at different depths for a site on northeastern Grand Bank (see location in Figure 18a) for storm Discussions 4.1. Differences in Storm Impact on Surface and Bottom Currents Descriptions of individual storms in section 3.3 demonstrate that the spatial and temporal variations of the surface and bottom wind-driven currents share some common features but are also distinctively different in other aspects. One of the important features demonstrated by the time series maps of wind-driven currents presented in Figures 8 and 9 (for Storm 3) and Figures 12 and 13 (for Storm 19), is that both surface and near-bed currents LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5377

21 Journal of Geophysical Research: Oceans Figure 16. Comparison of (a, b) surface and (c, d) near-bed modeled ocean currents (m s21) (left) before and (right) at the peak of storm 3. rotate clockwise as the storm moves across the Grand Banks. The surface current rotates clockwise almost uniformly over the entire modelling domain with little spatial variations. For instance, surface wind-driven current was dominantly northward at the start of Storm 3 (0800 on 14 February, Figure 8a), became easterly 10 h later (1800 on 14 February, Figure 8b), and further rotated clockwise to the southeast at the peak of the storm (0000 on 15 February, Figure 8c). A similar rotation pattern is also found for Storm 19 (Figures 12a 12d). This is expected because wind direction rotates clockwise (see Figure 6) as the storm center migrates across the domain and the surface Ekman layer determines that surface wind-driven currents should be 45 degrees to the right of the wind in the northern hemisphere. The clockwise rotation of the bottom wind-driven currents, however, does not occur uniformly over the entire Grand Banks. The clockwise rotation is clearly recognizable on SPB and the western Grand Banks, less so on southeastern Grand Bank, and nonexistent over Avalon Channel and northeastern Grand Bank. The direction of bottom currents on SPB and the western Grand Banks changed from northeasterly, through southeasterly, to southwesterly from 1800 on 14 February to 0900 on 15 February (Figures 9b 9e). On central and southeastern Grand Bank, bottom wind-driven currents were largely to the east at 1400 on 14 February (Figure 9a) and became southeasterly at 1800 on 14 February (Figure 9b). This pattern, however, had largely deteriorated at 0000 on 15 February (Figure 9c) as currents became minimal on central Grand Bank and those on southeastern LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5378

22 Grand Bank now became northwesterly. For eastern and NE Grand Bank, bottom wind-driven currents were respectively to the northwest and west at 0000 on 15 February (Figure 9c). As the storm progressed, currents did rotate clockwise initially on eastern Grand Bank to be northeasterly at 0300 on 15 February (Figure 9d) and became insignificant after 0900 on 15 February (Figure 9e). However, currents on NE Grand Bank essentially remained to the west in this period. Bottom currents over Avalon Channel were dominantly to the SW over the entire storm duration (Figures 9c 9f). The spatial distribution of current magnitude and the time of peak current conditions are quite different for surface and bottom wind-driven currents. Wind impact on the surface currents is largely uniform without significant spatial variations and temporal delays. For instance, at the early stage of Storm 3 (1800 on 14 February, Figure 8b), moderate wind-driven currents of m s 21 occur widely over SPB, Green Bank and most parts of Grand Bank and are predominantly to the east. At the peak of the storm (0000 on 15 February, Figure 8c), maximum currents of 1 ms 21 are uniformly distributed over the modelling domain and the direction is predominantly to the southeast. In contrast, the impact on the bottom currents shows significant temporal delays and spatial variability. Wind impact on the bottom wind-driven currents starts on SPB and then migrates eastward over the rest of the Grand Banks as the storm progresses (Figures 9a and 9b). Peak bottom currents first occur on SPB at 0000 on 15 February (Figure 9c). Maximum current speed reaches 0.7 m s 21 and is to the SE. Peak bottom currents on the western, central and SE Grand Banks occur 3 h later at 0300 on 15 February (Figure 9d). Maximum current speed reaches 0.35 m s 21 and is to the NE on southeastern Grand Bank. Peak bottom currents are further delayed for another 6 h over Avalon Channel where the maximum current occurs at 0900 on 15 February with a speed of 0.45 m s 21 and a southwestward direction (Figure 9e). The timing differences of peak bottom currents imply that during the process of a major storm, the peak sediment transport could occur at different times and with different magnitudes and directions over different areas on the Grand Banks Inertial Rotation of Bottom Wind-Driven Currents Inertial currents are an important part of the current climatology on the Grand Banks [e.g., de Young and Tang, 1990] and are significant even when vertical density variations are small [Petrie, 1993]. Inertial currents are strongly oscillatory, rotating in a clockwise direction in the northern hemisphere with decreasing periods with the increase of latitude. Analyses of mooring current data from the eastern Grand Bank show that the inertial oscillation represents on average over 50% of the current variance, and that the inertial currents rotate clockwise with a period of 17 h and reach up to 0.35 m s 21 in maximum speed [de Young and Tang, 1990]. The time series of near-bed wind-driven current on southeastern Grand Bank during Storm 3 (Figure 14d) shows that there is the inertial rotation of bottom wind-driven currents with 18 h period at this site. For instance, bottom currents were to the SE at 0300 on 15 February, and went through one circular cycle to flow to the SE again at 2100 on 15 February. The time series of model-predicted near-bed wind-driven currents for Storm 1 are presented in Figure 17 to further demonstrate the spatial variation of inertial rotation of the bottom current during major storms on the Grand Banks. At the peak of the storm (0000 on 12 February, Figure 17a), domain wind was to the east at 22 m s 21 and near the maximum. Moderately strong near-bed wind-driven current of 0.45 m s 21 occurred on SPB, Green Bank, Whale Bank and the central and southeastern Grand Bank, and the direction was dominantly to the southeast. Wind remained to the east but the speed decreased to 11 m s 21 in the next few hours (Figure 17d). As the storm abated, the bottom current over these locations rotated clockwise as inertial oscillation flows. The inertial currents were still strong (0.4 m s 21 ) on SPB, Green Bank and Whale Bank, while current speed decreased to <0.3 m s 21 on central and southeastern Grand Bank (Figure 17d). For the next 12 h, the wind over the modelling domain further decreased to <5 ms 21 and wind direction remained to the east and northeast. However, the nearbed wind-driven currents continued to rotate clockwise so that current on southeastern Grand Bank was to the southeast again by 1800 on 12 February and the speed remained 0.25 m s 21 (Figures 17e 17i; also Figure 14d). Our model predictions therefore show that the inertial near-bed currents range from 0.25 to 0.45 m s 21 and rotate clockwise with 18 h period on southeastern Grand Bank. It is important to note that the near-bed currents are much stronger and rotate with longer periods on SPB and Green Bank. For instance currents on these banks were to the SE and up to 0.45 m s 21 at 0000 on 12 February (Figure 17a). By 1500 on 12 February (Figure 17h), currents on these banks were to the northwest with speed still up to 0.4 m s 21. This indicates that the near-bed wind-driven currents have a rotational period of 30 h in these LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5379

23 areas. The increased periods are probably due to the fact that these banks are closer to land and that nearbed currents experience stronger bottom friction because of the reduced depths over these banks. The relatively higher currents with longer periods imply that higher sediment transport rates in rotational motions could occur on SPB, Green Bank and possibly Whale Bank many hours after the peak of a storm. Interestingly, only moderate northerly near-bed wind-driven currents of 0.25 m s 21 were predicted at the peak of the storm on eastern and northeastern Grand Bank (Figure 17a) due to deeper depths (90 m) in these areas. The currents did rotate clockwise in the next 2 4 h (Figures 17b and 17c) but the current speed quickly diminished as the storm moved away (Figures 17e 17i; also Figure 15d). This suggests that the inertial oscillation of near-bed wind-driven current may not exist on eastern and northeastern Grand Bank despite the fact that such oscillation was found in the upper and middle parts of the water column [de Young and Tang, 1990]. Similar spatial variations of inertial current oscillation were also found for near-bed current predictions of Storm 3 although these are not as well established as shown by the Storm 1 data Spatial Pattern of Extreme Bottom Currents and the Effect of Mixed Layer Depths The extreme bottom currents were computed as the maximum near-bed current speed at each model grid for all 22 storms (Figure 18a). The strongest bottom current occurs on SPB where the maximum current speed reaches 0.8 m s 21. Strong currents up to 0.5 m s 21 also occur on Green Bank, in patches over Whale Bank, central and southeastern Grand Bank, and over Avalon Channel. Maximum near-bed wind-driven currents are relatively weaker over central-northeastern Grand Bank and southwestern Grand Bank where maximum current speeds reach 0.25 m s 21 which is still 2.5 times greater than the mean circulation current speed of 0.1 m s 21 (Figure 16c). The extreme storm-induced currents presented in Wu et al. [2011] show that the response of the water to storm forcing on the Grand Banks varies from place to place. Detailed analyses of surface and near-bed wind-driven currents for selected major storms in this study further demonstrate the complex spatial and temporal variations of storm-induced currents in the study area. Following the method of Wu et al. [2011], the current speeds (Figure 18b) and the density difference from the surface values (Figure 18c) along a cross section at the peak of Storm 1 are used to explore how bathymetry and mixed layer depth affect the response of the water to storm forcing on the Grand Banks. The cross section starts from SPB and crosses Green Bank and central-southeastern Grand Bank (see cross-section location in Figure 18a). The threshold density difference value of 0.05 kg m 23 was used to define the mixed layer depth (shown as white line in Figure 18c). Because of the shallow depths over SPB, Whale Bank, and central and southeastern Grand Bank, mixing is stronger and the mixed layer extends nearly all the way to the seabed. As a result, storminduced currents over these areas are strongest ( m s 21 ; Figure 18b) in the upper water column and moderately strong currents ( m s 21 ) also occur near the bottom (Figures 18a and 18b; also Figures 9, 13, and 17a). The mixed layer extends over only approximately half of the total depth in Halibut Channel, Haddock Channel and the Whale Deep depression, generally shallower than that over the adjacent banks (Figure 18c). Evaluation of density profiles showed that the vertical density difference before the storm was greater over these deep channels and therefore more wind energy was dissipated in overcoming the greater density difference in these areas. As a result, the wind-forced surface velocity layers at the peak of the storm were deeper in these channels than those on the adjacent banks (Figure 18b). This would in turn mean that the remaining wind energy would be distributed over thicker surface velocity layers. The combination of the two aspects thus led to lower overall wind-driven currents in these channels (Figure 18b). Profiles of current speed over these channels show that current speeds in the upper water column are 0.4 to 0.5 m s 21 and decrease to 0.2 m s 21 below this layer. Current speeds, however, increase again to 0.3 m s 21 near the seabed despite the presence of less well mixed profiles in the lower water column. Processes other than storm mixing are likely responsible for this occurrence of moderate bottom currents in these submarine channel and depression areas. Cascading dense shelf water [e.g., Puig et al., 2013] and propagating trapped shelf waves along the coastline [e.g., Tang, et al., 1998] are possible other processes. Another area showing strong near-bed storm-induced currents is Avalon Channel with current speeds of 0.5 m s 21 (Figure 18a). The presence of moderately strong bottom currents in this area is explained by the peak southerly and southwesterly wind-driven currents that often occur in the late stage of storms (Figures 9e and 17d). Trapped shelf waves propagating along the coastlines [Tang et al., 1998; Thiebaut and Vennell, 2010; Han et al., 2012; Ma et al., 2015] probably also contribute to this. LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5380

24 Figure 17. Time series of model-predicted near-bed wind-driven currents (m s 21 ) of storm 1 demonstrating the inertial rotation of bottom currents during major storms on the Grand Banks. Time and domain-averaged wind speed are shown at the top of each map. Clearly, besides wind intensity, paths and translation speed of storms, areas with shallow depths favor better mixing of the water column and thus occurrence of stronger wind-driven currents during major storms on the Grand Banks Differences Between Depth-Averaged and Near-Bed Wind-Driven Currents Limited previous modelling studies of sediment transport in storms on the Grand Banks [e.g., Amos and Judge, 1991] described depth-averaged wind-driven currents at the peak of storms and used these to predict sediment transport patterns. These studies showed that the depth-averaged currents at the peak of the February 1982 storm (Storm 3) were dominantly to the southeast similar to the surface wind-driven current pattern predicted in our study (Figure 8c). The sediment transport at the peak of the storm predicted by Amos and Judge [1991] displayed that sediment transport was to the southeast on northeastern Grand Bank and to the south-southeast on southeastern Grand Bank. However, the direction of sediment transport during storms is largely determined by the bottom wind-driven currents, not by the depth-averaged or surface wind-driven currents. The 3-D modelling results from the present study show that while the near-bed wind-driven currents at the peak of the storm are to the southeast on SPB and western Grand Banks (Figure 9c), they are to the northwest and west-northwest respectively over southeastern and northeastern Grand Bank which are nearly 1808 opposite to the depth- LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5381

25 Journal of Geophysical Research: Oceans averaged current. As a result, the sediment transport at the peak of the storm predicted using the near-bed wind-driven currents is to the west and the northwest respectively (see Figure 20d and discussion below). This difference emphasizes the importance of using near-bed wind-driven currents for correct prediction of the magnitude and direction of sediment transport during storms on a continental shelf setting Relative Magnitudes of Near-bed Circulation, Tidal and Storm-Driven Currents The background circulation current at 5 m above the bottom for the month of February and the peak ebb tidal current at 5 m above bottom on the Grand Banks are shown in Figure 19 which are compared to the peak near-bed wind-driven currents during Storms 3 and 1 (Figures 9d and 17a respectively) to assess the relative scales of near-bed circulation, tidal and storm-driven currents. Circulation currents are essentially less than 0.1 m s21 on the Grand Banks except for stronger currents, up to 0.3 m s21, on the bank edge and upper slope due to the presence of the Labrador Current (Figure 19a). Peak near-bed tidal currents can reach 0.3 m s21, particularly on eastern and southeastern Grand Bank (Figure 19b). In contrast, peak near-bed storm-induced currents can reach m Figure 18. (a) Extreme near-bed currents of all storms, and (b) cross sections of currents and (c) density difference at the peak of storm 1 along a s21 (Figures 9d and 17a). Therefore maxitransect (black line in Figure 18a) across the modeling domain over the mum near-bed storm-driven currents on the Grand Banks. Circles in Figure 18a mark the sites for data presented in FigGrand Banks are 7 times higher than the ures 14 and 15 respectively. The white line in Figure 18c with a value of 0.05 kg m23 represents the mixed layer depth. background mean currents, and about 2 times higher than the peak tidal currents. Near-bed tidal currents are relatively strong on eastern and southeastern Grand Bank where maximum winddriven currents during major storms only reach 0.4 m s21 (Figures 9d and 17a). While the impact of the background circulation currents is small compared to that of the storm-induced currents, the inclusion of tidal currents can enhance the magnitude of near-bed currents by more than 50% over some areas, and thus the effect of tidal current on bed shear stress and sediment transport should not be neglected on the Grand Banks Storm Impact on Seabed Shear Stress and Sediment Transport Although detailed analysis of temporal and spatial patterns of bed shear stresses and sediment transport during major storms on the Grand Banks will be addressed in a future paper, highlights of the impact on seabed shear and sediment transport flux by major storms are briefly discussed here. Combined wave-current shear velocity (u*cws) and sediment transport rate for medium sand before and at the peak of Storm 3 are compared in Figure 20 to quantify how major storms enhance the bed shear stress and sediment transport level on the Grand Banks. Before the storm (Figure 20a), maximum shear velocity reaches 3 cm s21 and mainly occurs on central and southeastern Grand Bank largely due to moderate tidal currents in these areas (Figure 19b). Shear velocity is zero or less than 1 cm s21 over a significant part of the modelling domain. Transport of medium sand is predicted in a few isolated spots and the transport rates are less than kg m21 s21 (Figure 20b). LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5382

26 Figure 19. Vector maps of (a) background circulation current at 5 m above the bottom for the month of February and (b) peak ebb tidal current at 5 m above bottom on the Grand Banks. At the peak of the storm (Figure 20c), the combined action of storm waves and storm-induced currents dramatically increases the shear velocity so that u*cws is greater than 3 cm s 21 over most parts of the Grand Banks. The strongest shear velocity reaches 15 cm s 21 and occurs on central and southeastern Grand Bank. Strong values of >5 cms 21 are also predicted on SPB and northeastern Grand Bank. As a result of this enhanced seabed forcing, sediment transport occurs widely over the entire Grand Banks during storms (Figure 20d). Maximum sediment transport rates occur on central and southeastern Grand Bank and reach about 5 kg m 21 s 21 which is more than 3 orders of magnitude higher than those before the storm. 5. Summary and Conclusions Wave, current and sediment transport processes during major storms on the Grand Banks of Newfoundland have been simulated by integrating the results of a wave model, 3-D tidal and ocean current models, and a combined-flow sediment transport model. The investigation of the detailed temporal and spatial variations of waves and storm-induced currents through the duration of major storms is unprecedented. This modelling study also utilized information of bottom tidal and storm-driven currents not available to previous studies of sediment transport on the Grand Banks. The following are the major findings on the characteristics and spatial and temporal patterns of storm-generated waves and current on the Grand Banks under major storms considered. Most of the major storms track southwest to northeast and pass to the north or northwest of the Grand Banks. Significant wave heights can reach up to 14 m at the peak of these storms. The majority of storms and the five strongest storms in terms of wave height all show peak waves to the northeast. But storms with peak waves to the southeast are also significant (33% of the top 100 storms). The maximum surface wind-driven currents can reach 1 ms 21 for either type of these major storms. However, the direction of the peak surface currents is to the southeast for storms with peak waves to the NE and to the southwest for storms with peak waves to the SE. The bottom wind-driven currents reach a maximum speed of 0.8 m s 21 during storms with peak waves to the NE. The directions of these peak bottom currents are dominantly to the southeast although the bottom currents tend to show a counterclockwise gyre on NE Grand Bank. The maximum bottom wind-driven currents for storms with peak waves to the SE are slightly smaller and reach LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5383

27 Figure 20. Model predicted (a, c) combined wave-current shear velocity u*cws (base 10 log m s 21 ) and (b, d) sediment transport rate (base 10 log kg m 21 s 21 ) over uniform medium sand (0.35 mm diameter) for storm 3. (top) Before storm at 2200 on 13 February and (bottom) the peak of the storm at 0000 on 15 February. Arrows only show direction. Solid lines represent 200 m contours. 0.5 m s 21. The direction of the near-bed current is dominantly to the south and southwest on St. Pierre Bank while a giant counterclockwise gyre occupies nearly the entire Grand Bank. While there exists an overall trend of increasing wave height and wind-driven current with increasing storm wind speed, other factors also significantly affect waves and wind-driven currents on the Grand Banks. Storms with paths closer to the Grand Banks, shorter storm-center distances to the Grand Banks at the storm peak, and slower translation speed will likely generate stronger wind-driven currents. Both surface and near-bed currents rotate clockwise and decrease in strength as the storm moves across the Grand Banks. Wind impact on the surface currents does not show significant spatial variations and temporal delays, and the surface wind-driven currents rotate clockwise almost uniformly over the entire modelling domain. The storm impact on the bottom currents, however, shows significant temporal delays and spatial variability. Storm impact on the bottom wind-driven currents tends to first occur on St. Pierre Bank and then migrate eastward over the rest of the Grand Banks. The magnitude of bottom wind-driven currents is also strongest on St. Pierre Bank and decreases substantially over other parts of the Grand Banks. Clockwise inertial rotation of the bottom wind-driven currents are evident on St. Pierre Bank, Green Bank, western and southeastern Grand Bank but is weak or nonexistent over other parts of the Grand Banks. On bank tops with shallower depths, the direction of the wind-driven current is similar from the surface to the LI ET AL. STORM PROCESSES ON THE GRAND BANKS 5384