Modeling seasonal ocean circulation of Prince William Sound, Alaska using freshwater of a line source

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1 Modeling seasonal ocean circulation of Prince William Sound, Alaska using freshwater of a line source /^ V.Patrick, J.Allen, S.Vaughan, C. Mooers,^ M.Jin^ International Arctic Research Center- Frontier, University of Alaska FmrWikc, Fairbanks, Af , C/&4 jwang@chukchi.iarc.uaf.edu Prince William Sound Science Center, P.O. Box 705, Cordova, AK 99574, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA * * Institute of Marine Science, University of Alaska Fairbanks, Fmr6aM&j,A# , [/&4 Abstract A three-dimensional, primitive equation ocean circulation model (Wang and Ikeda[7]), was applied to Prince William Sound, Alaska (3D-PWS model) under forcing of freshwater runoff of a line source, heat flux, Gulf of Alaska (GO A) water inflow/outflow (throughflow), and daily (synoptic), spatially varying winds. The 3-D structures and seasonal cycles of the circulation patterns, temperature, salinity, and density were examined in real bottom topography. The "river/lake" scenarios (i.e., the weak versus strong flushing throughflow) were captured on a monthly basis. The freshwater runoff of the line source significantly contributes to the basin-scale cyclonic circulation which can't be seen in previous simulations without freshwater runoff (Mooers and Wang [3]). Wind forcing due to the orographic effect substantially contributes to the circulation patterns in the Sound. Multiple circulation regimes (cyclonic, anticyclonic, and their combination) characterize the complexity of the system which depends on the intensity of the GOA water throughflow, freshwater discharge of the line source, and the synoptic wind. A winter circulation is characterized by a high flushing regime due to high throughflow and northeast winds, while the spring pattern is dominated by a basin-scale anticyclonic gyre. The summer (July to September) circulation is controlled by a basin-scale cyclonic gyre due to the maximum freshwater influence along the coastline. The autumn circulation is driven by a combination of the throughflow and the northeast wind-driven flow. The simulated cyclonic gyre in summer and late fall is supported by observations.

2 58 Coastal Engineering and Marina Developments 1 Introduction Prince William Sound (PWS or "the Sound") constitutes of multiple basins, fjords, channels, islands, inlets, and estuaries along the alpine coast of southern Alaska. The complexity of the seasonal circulation patterns is due to the complexities and uncertainties of seasonal inflow/outflow and various directions and magnitudes of winds due to the orographic effect in the mountainous region (Fig. 1). Mooers and Wang [3] implemented a 3-D circulation model in PWS using idealized wind forcing and fixed throughflow (0.3 Sv, 1 Sv =10^ m^/s), without heat flux and freshwater discharge. Therefore, the major circulation pattern is dominated by the throughflow from Hinchinbrook Entrance to Montegue Strait. For lack of freshwater runoff, the basin-scale circulation was not obtained. Nevertheless, the model performance was encouraging under idealized forcing which was confirmed by a study of the tracer transport experiments (Deleersnijder et al. [1]). The SEA (Sound Ecosystem Assessment) Program conducted major observational efforts. This numerical model is to 1) simulate the PWS seasonal circulation patterns under different atmospheric forcing and coastal inflow/outflow conditions using a sophisticated 3-D numerical model; 2) examine the seasonal variations of the river/lake scenarios (hypotheses) which are essential for understanding the ecosystem of the Sound; 3) investigate the forcing factors, such as orographic wind fields, freshwater runoff of a line source, and importance of ACC throughflow; and 4) understand the seasonal variations of temperature and salinity patterns. 2 The Mesoscale Ocean Circulation Model A modified version of the POM using a predictor- corrector scheme for the time integration and semi-implicit scheme without mode-splitting (Wang and Ikeda [7]) was implemented to the PWS. It has the following features: (1) Arakawa C grid; (2) sigma coordinates in the vertical with realistic bottom topography; (3) free surface; (4) level 2.5 turbulence closure model for the vertical viscosity and diffusivity (Mellor and Yamada [2]); (5)Smagorinsky's parameterization for horizontal viscosity and diffusivity; (6) semi-implicit scheme for the shallow water equations (Wang et al. [5]); and (7) a predictor-corrector scheme for the time integration to avoid inertial instability (Wang and Ikeda [6,7,8]). The model domain includes the entire PWS with two open boundaries (Hinchinbrook Entrance and Montague Strait, Fig 1). The model grid space is 1.2 km, which is eddy resolving because the internal Rossby radius of deformation is about 5 km in winter (50 km in summer) (Niebauer et al. [4]). There are 15 vertical sigma levels, with a relatively high resolution in the upper 50m to resolve the upper mixed layer. The integration time step is 100 seconds which is about ten times the CFL constraint because the semi-implicit scheme has been used for the shallow water equations (Wang [5]). The initial temperature and salinity fields are based on a typical spring profile (Fig.2, dashed, of Mooers and Wang[3]) and specified to be horizontally uniform. The model was spun-up for one year from these initial conditions under seasonal forcing to reach a dynamic and thermodynamic steady state.

3 Coastal Engineering and Marina Developments 59 3 Seasonal Forcing Factors According to observations at Hinchinbrook Entrance (Niebauer et al. [4]), the coastal inflow varies seasonally (Fig. 2a). The outflow through Montague Strait is of the same order of magnitude, although the water volume in the Sound may increase or decrease in response to transient forcing. Hence, an inflow was specified over the seasonal cycle through Hinchinbrook Entrance while a radiation boundary condition for the normal velocity was applied to Montague Strait. The open boundary condition at Montague Strait for the temperature and salinity is free advective. The monthly heat flux (Fig. 2b) originating from the GOADS was specified spatially homogenous with the restoring surface temperature boundary condition to the seasonal climatology from The monthly freshwater runoff of the line source (Fig. 2c) was calculated from the hydrological Digital Elevation Model (DEM) along the coast. The restoring boundary condition is the observed seasonal salinity climatologies from The wind forcing has high spatial and seasonal variations over PWS due to the orographic effect and synoptic time scales from days to weeks. Three-hourly records of the wind speed and direction, humidity, air temperature, and shortwave solar radiation were taken at nine meteorological stations over PWS (shown in Fig. 1). The wind speed and direction are spatially variable among stations (Fig. 3), at Mid Sound (Fig. 3a), the wind direction varies from time to time due to a relatively flat region in the central Sound. However, at Potato Point (Fig. 3b) channeled by high mountains on both sides, the wind direction is either southwestward or northeastward, and the wind speed is stronger than at the other stations. Thus, annually averaged wind is southwest. At Valdez (Fig. 3c), the wind direction is also channeled by the mountains, as at Whittier (Fig. 3d) where the wind is normally northeasterly or southwesterly, with the northeast winds dominating. The annual mean northeast wind is about 3m s. As the orographic effect over PWS is overwhelming, theoretically, a mesoscale meteorological boundary model is needed to calculate the surface wind fields. For the first cut, however, an empirical, wind-fetch model (nine wind fetches responding to 16 possible prevailing wind directions) was used to optimally interpolate the wind records from the nine stations. Figure 4 demonstrates the daily wind fields on days 15 (January), 105 (April), 195 (July) and 285 (October). These show spatially variable features that cannot be captured by any of the wind records at a single station. 4 Seasonal Circulation, SST, and SSS Patterns The surface circulation patterns in January, April, July, and October are shown in Fig. 5, along with the sea surface temperature (SST, Fig. 6) and sea surface salinity (SSS, Fig. 7) fields. Due to the strong northeast wind in winter, the surface currents are driven out of the Sound (Fig.5a) at very high flushing rate. There are two regimes in SST and particularly SSS with a division between the western and eastern Sound. The SST (Fig. 6a) and SSS (Fig. 7a) signatures suggest a significant oceanic influence on the central and eastern Sound rather

4 60 Coastal Engineering and Marina Developments than on the west. The SST in the west (about 3 C) is about 2 C lower than in the central and eastern Sound (about 5 C), and the SSS difference is about 2 psu (28 vs. 30 psu) which dominates the density field. When the minimum throughflow occurs in April, anticyclonic circulation occurs in the central Sound (Fig. 5b). This feature cannot be seen in the previous simulations (Mooers and Wang [3]). The SST (Fig. 6b) rises to 7-8 C due to solar warming. Freshwater of the line source is set up along the northwestern coast and estuaries (Fig. 7b) and generates the along-shore current. The SSS decreases due to the setup of the freshwater discharge. As the freshwater runoff continues in July and southwest wind dominates, the cyclonic gyre in the central Sound starts to form, while an anticyclonic eddy is developing in the northern Sound (Fig. 5c). Despite of the freshening, the northeastward current on the west coast is generated by the southwest wind, whose direction is opposite to that of the density-driven along-shore current. There is freshwater tongue from Valdez Arm and strong freshening along the northern and western coasts (Fig. 7c). A strong freshening occurs during August through September, so the central Sound cyclonic circulation continues to develop from August to September (Fig. 8b). The circulation pattern in October (Fig. 5d) is dominated by a basin-scale cyclonic gyre. The northeast wind-driven current is consistent with the alongshore density-driven current, leading to stronger current along the western coast. Oceanic influence through Hinchinbrook Entrance on SST (Fig. 6d) and SSS (Fig. 7d) is a striking feature, because the ACC transports the warmer and more saline water into PWS and leads to strong SST and SSS gradients in the southeast-to-northwest direction. The seasonal "river/lake" scenario can be captured by the streamfunction (volume transport). During the lower throughflow months, the circulation pattern dominated by the anticyclonic gyre (Fig. 8a) tends to be a lake scenario because the PWS water recirculates anticyclonically. However, in the summer season, the cyclonic circulation dominates (Fig. 8b), enhancing the flushing of the water out of the Sound through Montegue Strait: the river scenario. In the winter season, the strong northeast winds along with the strong throughflow, maximizes the river scenario (Fig. 8c). A mixed scenario between the lake and river states can be seen in February (Fig. 8d) when the cyclonic and anticyclonic eddies are comparable. There are 2-3 pairs of cyclonic-anticyclonic eddies along the deep basin of the Sound; the deep water allows easier development of eddies than the shallow region of sloping topography (Wang and Ikeda [9]). These seasonal circulation patterns depict the complexity caused by external forcing. Time series of the surface velocity vectors (Fig. 9) reflect the combination of wind forcing, density, and throughflow forcing. Obviously, the winds play significant role in the surface current. Because no moorings were deployed inside PWS during the SEA program, no direct comparison in time series between model outputs and observations were conducted. With respect to the mesoscale eddy development and decay, the time series of total kinetic energy (TKE), zonal kinetic energy (ZKE), and eddy kinetic energy (EKE) of the entire PWS domain indicate the seasonal variations (Fig. 10, upper panel), where TKE = ZKE + EKE. ZKE (that is smaller than EKE) represents

5 Coastal Engineering and Marina Developments 61 the strengths of the throughflow [which well matches the magnitude of the throughflow (Fig. 2a)] and the density-driven flow (which peaks in August- September). For instance, ZKE is low from April through August when the throughflow is weak and freshwater runoff starts to build up. In September, the throughflow increases and the freshwater runoff reaches its maximum, leading to a basin-scale cyclonic circulation in the Sound. As a result, ZKE rises sharply in September. In summary, ZKE represents the strength of the basin-scale gyre or throughflow circulation, either cyclonic or anticyclonic. Another significant ZKE peak can be seen during February and March when there are more active basinscale gyres and eddies (Fig. 8a and 8d). Mesoscale eddies are another phenomenon in the Sound which can be seen in the time series of EKE and the growth rate (Fig. 10, lower panel). The higher EKE is accompanied by the higher ZKE, indicating that the higher basin-scale gyres are the sources (possibly due to baroclinic instability together with the topographic effect) of the mesoscale eddies (Wang and Ikeda [9]). EKE is much higher than ZKE because the active mesoscale eddies or transient current dominates. The eddy duration time scales are from a few days to months, based on the growth rate time series (Fig. 10, low panel). The negative values of the growth rate indicate the decaying phase of eddies. 5 Conclusions and Discussions The 3D-PWS model has produced reasonable seasonal circulation patterns under forcing of the throughflow, freshwater runoff of a line source, and daily (synoptic) winds. The above investigations can be summarized as follows: 1) Seasonal circulation patterns are generated by the equally important external forcing factors: throughflow, freshwater runoff of the line source, and synoptic wind fields. Without one of these forcing factors, the simulated seasonal circulation would not be reliable. The circulation patterns vary from month to month due to the highly variable forcing fields. Generally speaking, the anticyclonic circulation patterns more likely occur during the time of low freshwater runoff, weak throughflow, and weak northeast wind, such as in March, while cyclonic circulation patterns dominate during the period of high throughflow and high freshwater runoff, such as from July to October. 2) There are more than lake and river scenarios over a seasonal cycle. The lake scenario is detected in March to June when the anticyclonic circulation gyres prevail. The river scenario most likely occurs in July to October when the cyclonic circulation patterns are strong. The most energetic river scenario may occur in the winter months (November to January) due to both strong throughflow and persistent northeast wind controlled by the persistent Aleutian Low (Fig. 6, Wang et al. [10]). However, in February, comparable cyclonic and anticyclonic gyre and eddy pairs mark the transit from river to lake scenario. 3) Without freshwater runoff of a line source, the seasonal circulation patterns in the upper layer would be impossible to be reproduced and the seasonal SSS and SST patterns would not be realistic. The freshening due to runoff generates current along the northwest coast that has not been shown in the previous simulations.

6 62 Coastal Engineering and Marina Developments 4) Although heat flux is not as important as freshwater runoff in terms of density-driven current, it is important to determine the seasonal cycle of the upper layer temperature structure, as well as 3D structures. Therefore, accurate heat flux is necessary to the thermodynamics that affects biological components. 5) Highly spatially variable directions and magnitudes of the wind fields provide a topic for Mure research. Acknowledgements: Financial support from the SEA Program through Prince William Sound Science Center, Alaska is appreciated. JW acknowledges support from IARC-FRSGC for providing the computer power and from Prof. M. Ikeda of Hokkaido University, Japan, for his helpful discussions. References 1. Deleernijder D., J. Wang and C.N.K. Mooers, A two compartment model for understanding the simulated three-dimensional circulation in Prince William Sound, Alaska. Continen. Shelf Res,lS, pp , Mellor, G.L. and T. Yamada, Development of a turbulence closure model for geophysicalfluidproblem, Rev. Geophys. Space Phys., 20, pp , Mooers, C.N.K. and J. Wang, On the implementation of a three-dimensional circulation model for Prince William Sound, Alaska. Continen. Shelf Res., 18, pp , Niebauer, H.J., T.C. Royer and TJ. Weingartner, Circulation of Prince William Sound, Alaska. /. Geophys. Res., 99, pp. 14,113-14, Wang, J., L.A. Mysak and R.G. Ingram, A 3-D numerical simulation of Hudson Bay summer circulation: topographic gyres, separations and coastal jets. /. Phys. Oceanogr., 24, pp , Wang, J. and M. Ikeda, Stability analysis of finite difference schemes for inertial oscillations in ocean general models. Computer Modeling of Seas and Coastal Regions., Vol. 2, C.A. Brebbia. Et al, Computational Mechanics Publications, Southampton, pp Wang, J. and M. Ikeda, A 3-D ocean general circulation model for mesoscale eddies-i: Meander simulation and linear growth rate. Acta Oceanologica Sinica, 15, pp , Wang, J. and M. Ikeda, Inertial stability and phase error of time integration schemes in ocean general circulation models. Mon. Wea. Rev., 125, pp , 1997a. 9. Wang, J. and M. Ikeda, Diagnosing ocean unstable baroclinic waves and meanders using quasi-geostrophic equations and Q-Vector method. /. Phys. Oceanogr., 27, pp , 1997b. 10. Wang J., R.T. Cheng and P.C. Smith, Seasonal sea-level variations in San Francisco Bay in response to atmospheric forcing, Estuarine, Coastal and Shelf Science, 45, pp , 1997c.

7 Coastal Engineering and Marina Developments 63 POTATO POUft 78 DISTANCE (KM) Fig.l. Bottom topography of PWS and stations. Six CM in 'o', ten TIDE in '*', and nine meteorological stations 'A'. C B b0 Fig.3. The 1996 time series of daily wind velocity vectors at Mid Sound (a), Potato Point (b), Valdez (c), and Whittier (d). The thick solid line with the dot (direction) denotes the annual mean wind velocity. Fig.2. The time series of inflow from the Hinchinbrook Entrance (a), heat flux (b), and freshwater runoff (c). Fig.4. The snapshots of the wind fields derived from the empirical wind-fetch model on days 15 (a), 105 (b), 195 (c), and 285 (d).

8 Fig.5. The surface circulation patterns on days 15(a), 105(b), 195(c) and 285 (d). H9W 148W p - 51.ON

9 Coastal Engineering and Marina Developments 65 DISTANCE (KM) Fig.6. Sea surface temperature patterns on day 15(a), 105(b), 195(c) and 285 (d). DISTANCE (KM) LONGITUDE N STANCE (KM Fig.7 Sea surface salinity patterns on day 15(a), 105(b), 195(c) and 285 (d).

10 66 Coastal Engineering and Marina Developments r M p 61.0N DISTANCE (KM) ' a- 61.0N DISTANCE (KM) Fig. 8. The simulated streamf unction (transport) in March (a), August (b), December (c) and February (d) SOLD) TKE SHORT DASHED ZKE LONG D JULIAN DAYS Fig.9. The time series of the simulated surface velocity vectors at the five stations. JULIAN DATTS Fig. 10. The time series of simulated TKE, ZKE, and EKE (upper panel), and the mesoscale eddy growth rate (lower panel)