Mapping the Mid-Atlantic Cold Pool Evolution and Variability with Ocean Gliders and Numerical Models

Transcription

1 Mapping the Mid-Atlantic Cold Pool Evolution and Variability with Ocean Gliders and Numerical Models W. Brown School for Marine Science and Technology University of Massachusetts Dartmouth 706 S Rodney French Blvd. New Bedford, MA W. Boicourt Center for Environmental Science University of Maryland PO Box 775 Cambridge, MD C. Flagg School of Marine and Atmospheric Sciences Stony Brook University Endeavour Hall, Rm 203A Stony Brook, NY A. Gangopadhyay School for Marine Science and Technology University of Massachusetts Dartmouth 706 S Rodney French Blvd. New Bedford, MA O. Schofield, S. Glenn, & J. Kohut Coastal Ocean Observation Lab Rutgers, State University of New Jersey 71 Dudley Rd. New Brunswick, NJ Abstract During the summer, distinctive, bottom-trapped, cold water mass called the Cold Pool resides over the mid to outer continental shelf in the Middle Atlantic Bight (MAB); strongly influencing parts of the ecosystem including important fisheries. Since 2003, repeated ocean glider temperature and salinity (TS) water property measurements along a seaward transect from the coast of New Jersey have helped to define the important variability in the cross-shelf structure of the Cold Pool there. To develop forecast capability, we need to better understand the relevant processes that control Cold Pool seasonal evolution and variability. To do this, we are now beginning to integrate ocean glider TS measurements with data assimilation models for the purpose of generating prototype Cold Pool forecast maps; with important support from the Mid-Atlantic Regional Association Coastal Ocean Observing System (MARACOOS). Here we report recent progress in having models assimilate zig-zag, along-shelf glider measurements of the Cold Pool. This year s 2012 measurements are revealing a shelf and Cold Pool water, which appear to be as much as 1 o C warmer than in previous years. Index Terms Mid-Atlantic Bight, Cold Pool mapping, water properties, modeling, data-assimilation I. INTRODUCTION The Cold Pool is a distinctive, highly-variable, bottom-trapped, cold water mass found during summer over the mid and outer continental shelf between Cape Cod to Cape Hatteras a region known as the Middle Atlantic Bight (MAB). The term Cold Pool continues to be used to describe this dynamic feature [13] [4] [2] [3], which is characterized by southwestward advection with an accompanying gradual increase in the heat and salt fluxes through the surface and lateral boundaries [12] [14] [15]. It has been shown that the Cold Pool affects phytoplankton productivity [16] [9] and the behavior and recruitment of pelagic and demersal fish on the shelf [21] [24]. During the mid to late spring - depending on latitude - the vertical water column from Georges Bank (GB) to Cape Hatteras stratifies thus isolating a significant portion of the previous winter s cold water from the surface. This process defines the Cold Pool (waters <10 o C) for a particular year in terms of water properties and geographical extent (Fig. 1). However, this highly variable Cold Pool

2 has a complex evolution during the summer. On one hand, the general southwestward along-shelf flow (~5 cm/s), transport relatively colder Cold Pool water from the Gulf of Maine (GoM)/GB region to the MAB during the late spring [18] [19] (Fig. 2). On the other hand, the Cold Pool is warmed (and salted) during summer by a poorly understood array of mixing processes around its periphery. Potentially important mixing processes include across-thermocline exchange, warm, salty intrusions in the bottom boundary layer, and a variety of across-shelf break front (SBF) exchange processes. The net result of these competing processes is that the minimum MAB Cold Pool temperatures are observed well into the summer in July; followed by a general warming of the still distinct Cold Pool until the autumn when storms mix the shelf water column until the Cold Pool disappears. The dynamic mysteries of the Cold Pool and its importance to the MAB ecosystem motivate our search for answers concerning the erosion processes. Such answers are within our reach because of the newly-available technologies, Figure 1 The Cold Pool is clearly revealed in a cross-new Jersey shelf temperature transect, which was constructed from data transmitted ashore between 3-13 Aug 2011 by the Rutgers glider RU-22. Figure 2 Total temperature transport of water with T< 10 o C - Cold Pool water - (in 10 5 m 3 - o C) normal to the 1979 Nantucket Shoals Flux Experiment (NSFE) mooring array. Positive is toward the MAB. (Adapted from [19]) including ocean gliders, remote high frequency radar-derived surface current mapping, modern data-assimilation coastal ocean numerical models, and the development of the Integrated Ocean Observing System (IOOS). This paper describes some of our efforts toward integrating those technologies for the real-time mapping and forecasting of the MAB Cold Pool II. METHODS Ocean Gliders: Beginning in 2007, a fleet of Teledyne/Webb Slocum ocean gliders a component of the Mid Atlantic Regional Association Coastal Ocean Observing System (MARACOOS) - has been measuring the evolution of MAB water properties in terms of temperature, salinity, oxygen, optical backscatter, and estimated velocity. A 9-year composite of repeated glider runs highlights our strategy in terms of (a) cross-shelf transects off of New Jersey and (b) lateral zig-zig transects from Massachusetts to New Jersey [20] (Figs. 3 and 4 ). During the summer, these glider missions slice through different parts of the Cold Pool; enabling us to map the seasonal evolution of the Cold Pool physical properties as well as associated biologically-important fields of chlorophyll and oxygen. In addition to the standard hydrographic analyses, the glider measurements are also assimilated into MARACOOS circulation models. These models output time series Cold Pool maps, volume estimations as well as forecasts. The standard zig-zag glider run for a typical 100m Slocum glider 26km/day takes about 4 weeks to transit from Massachusetts to New Jersey. Its flight will be aided by the estimated ~ 5 km/day (5 cm/s) westward alongshelf advection in the MAB/southern New England Bight (SNEB). These glider missions are guided by the MARACOOS Glider Technical Center - a network including a central node at Rutgers University and secondary nodes at University of Massachusetts Dartmouth (UMassD), University of Delaware, and University of Maryland Horn Point.

3 Complementary Measurements: This Cold Pool measurement project will benefit from other ongoing measurements, including MARACOOS (a) high frequency (HF) radar-derived maps of surface current; (b) satellite SST and color maps; (c) weekly Oleander cross-shelf transect TSV measurements; (d) the NOAA Northeast Fisheries Science Center (NEFSC) regional surveys of fish, plankton, and water properties; and (e) possibly repeat glider measurements of the Ocean Observation Initiative (OOI)-Pioneer Array region. Modeling: Two of the MARCOOS numerical models are capable of assimilating glider data. The Rutgers Regional Ocean Model System (ROMS), with a domain that extends from the GoM to Cape Hatteras, emphasizes shelf processes. The UMassD s Harvard Ocean Prediction System (HOPS), with a domain that extends from Cape Hatteras to the Grand Banks, including the Gulf Stream system to 50 o W, emphasizes the role of the Gulf Stream in cross-sbf mixing. ROMS has a horizontal grid resolution of approximately 5 km, and uses 36 vertical layers in a terrain-following s-coordinate system. The model will be driven by atmospheric forcing provided by the North American Regional Reanalysis. Bulk formulae [7] are used to compute the model surface fluxes of momentum and buoyancy from the 3- hourly re-analyses of surface air temperature, pressure, relative humidity, 10m vector winds, precipitation, downward long-wave radiation, and net short-wave radiation. Open boundary inputs will be specified by a climatology that is based on ROMS model simulations for the northeast North American shelf by [8]. Added boundary-forcing includes the inputs of the Hudson, Connecticut, Delaware, Susquehanna, Potomac, Choptank, and James rivers. Daily mean river discharge transports are operationally-available from the United States Geological Survey gauges. The Rutgers ROMS assimilates available data (with an emphasis upon Figure 3 Composite of MARACOOS glider trajectories from missions from 2003 through present; featuring repeat cross-shelf (yellow) and zig-zag pathways along outer-shelf (cyan) and innershelf (dark blue) [20]. Figure 4 (above right) The six-segment (red diamonds) MA (left) to NJ (right), glider-measured temperature field (above left) The glider pathway map on which the stars correspond to the diamonds for: a) 11 Sep-14 Oct Cold Pool loses distinctiveness b) 23 May-13 Jun Cold Pool established c) 14 Apr-09 May Cold Pool defined off New Jersey. d) 19 Mar-12 Apr winter waters.

4 the proposed glider measurements) via a four dimensional variational (4DVAR) approach [17] [23] [25] [26]. ROMS can also assimilate the MARACOOS high frequency (HF) radar-derived surface currents, satellite sea surface temperature (SST), and all available in situ data from other gliders, moorings and vessels. ROMS also assimilates Jason-2 along-track altimeter data that has been reprocessed with new coastal corrections to extend coverage across the shelf. The series of monthly-average ROMS simulations (without glider data assimilation; Fig 5) consistent with dataderived seasonal Cold Pool maps [12] - demonstrate its basic skill in producing a realistic Cold Pool. Future plans for the ROMs models include producing near real-time 48-hour forecasts in support of glider field operations. One of the other operational numerical circulation models in the MARACOOS ensemble of models is HOPS. It has been built in particular to capture the mesoscale dynamics of the Gulf Stream (GS) system and its interaction with adjacent shelf circulation. Thus it is particularly well-suited for investigations of GS warm core ring SBF/Cold Pool interactions. The data assimilation-hops runs are initialized using a suite of feature models for the GS and its rings, SBF, buoyancy-driven shelf currents, and GoM gyres [10]. When running HOPS, first the GS region feature models are scaled using a ring and front analysis of recent satellite sea surface temperature (SST) measurements. The MAB/GoM region feature models are scaled with available ocean temperature (T) and salinity (S) measurements and/or their climatologies. (Historical glider measurements are useful in this regard). To produce the initial field for a HOPS simulation, the multi-scale, velocity-based, GS region and the Gulf of Maine (GoM)/MAB region water mass-based feature models are melded across the SSF. Before the start of a typical weekly HOPS run, the initial field is dynamically-adjusted with wind-forcing that is ramped up to the start-time wind field. From there the model run proceeds forward for a few daysassimilating available SST on the fly- until pausing to compute the mid-week nowcast/forecasts according the methodology described by [5] [6]. This data assimilation-hops version was validated for the July-September 2006 Shallow Water Experiment (SW06). Following [22], the HOPS forecast skills were found to be higher than persistence for six out of the eight weeks of the test. In addition, HOPS was able to forecast the first 3 days of the observed drifter trajectories especially well. This validated version of HOPS has been run operationally every week since from March More recently, HOPS has also been used to assess impact of its real-time ocean SST forecasts on atmospheric forecasts [1]; and has been adapted to assimilate ocean glider data as described by [11]. A preliminary application of the approach in detecting the Cold Pool using May-June 2007 glider data is demonstrated in Fig. 6. III. RESULTS TO DATE Figure 5 The sequence of monthly-average ROMS 10 o C isotherm maps (depicting the model Cold Pool - T<10 o C) highlight the late summer erosion of its extent. This spring/summer (2012), the UMassD glider called BLUE completed two short two-week missions part way across the shelf south of Martha s Vineyard (MA). Between 1 and 13 April, the UMassD glider was able to measure the 2012 winter water precursor to the 2012 Cold Pool (Fig. 7). A preliminary comparison of the early April 2010 (Fig. 4) and 2012 measurements reveals an unusually warm 2012 MAB spring/summer coastal ocean (perhaps 1 o C warmer in 2012).This motivated

5 us to track the evolution of the 2012 Cold Pool with glider/model methodologies. Between 31 May and 13 June, the UMassD glider detected the inshore edge of a warmer-than-usual 2012 Cold Pool just south of Massachusetts/Rhode Island. As we go to press, BLUE is being prepared for a MA-NJ zig-zag September run that will help us map the 2012 Cold Pool before it disappears shortly thereafter. Stay tuned. IV. ACKNOWLEDGMENTS This research, which has benefitted from the assistance of Richard Arena, has been supported in part through a NOAA grant NA07NOS for the implementation of the Mid Atlantic Regional Association Coastal Ocean Observing System (MARACOOS). NJ Mass Figure 6 (right) The 12 June 2007 HOPS model nowcast temperatures (color-coded between 0-12 o C) on a 0-60 m depth versus distance along a Massachusetts (Mass) to New Jersey (NJ) section. Glider-measured temperatures from the 23 May 12 June 2007 glider trajectory (on the left) were assimilated by HOPS via Optimal Interpolation at initialization. Note the glider temperature slice through the Cold Pool on the right. MA 10 km/tick -> Figure 7 Two 2012 north to south glider temperature transects that start south of Massachusetts: in which (above) the 1-5 April section shows well-mixed 7 o C to 8 o C winter water; (below) the 1-5 June section shows the inshore edge of the 2012 Cold Pool. The temperature bars are the same as in Fig. 4. V. REFERENCES [1] Agel, L., A. Gangopadhyay A., and F. Colby, An analysis of fall 2009 North Atlantic coastal storm forecasting using real-time SST. (Submitted to Cont. Shelf Res.) [2] Beardsley, R.C., W.C. Boicourt, and D.V. Hansen (1976). Physical oceanography of the Middle Atlantic Bight. The Middle Atlantic Continental Shelf and New York Bight, November 3-5, 1975, New York, N.Y. American Society of Limnology and Oceanography. Special Symposium Series 2: [3] Beardsley, R.C., and W.C. Boicourt, On estuarine and continental-shelf circulation in the Middle Atlantic Bight. Evolution of Physical Oceanography. B.A. Warren and C. Wunsch, Cambridge, MA, The MIT Press: [4] Boicourt, W.C. and P.W. Hacker, Circulation on the Atlantic continental shelf of the United States, Cape May to Cape Hatteras, in Memoires de la Societe Royale des Sciences de Liege, edited by J.C.J. Nihoul, pp , Univ. of Liege, Liege, Belgium. [5] Brown, W. S, A. Gangopadhyay, F. L. Bub, Z. Yu and G. Strout, and A. R. Robinson, 2007a. An Operational Circulation Modeling System for the Gulf of Maine/Georges Bank Region, Part 1: The Basic Elements, IEEE J. Oceanic Engineering, 32 (4), [6] Brown, W. S, A. Gangopadhyay, and Z. Yu, 2007b. An Operational Circulation Modeling System for the Gulf of Maine/Georges Bank Region, Part 2: Applications, IEEE J. Oceanic Engineering, 32 (4), [7] Fairall, C.W., E.F. Bradley, J.E. Hare, A.A. Grachev, and J. Edson, Bulk parameterization of air sea fluxes: Updates and verification for the COARE algorithm. J. of Climate, 16: [8] Fennel, K. J. Wilkin, J. Levin, J. Moisan, J. O'Reilly, and D. Haidvogel, 2006, Nitrogen cycling in the Middle Atlantic Bight: Results from a three-dimensional model and implications for the North Atlantic nitrogen budget, Global Biogeochemical Cycles, 20, GB3007. [9] Flagg, C.N., C.D. Wirick, S.L. Smith, The interaction of phytoplankton, zooplankton and currents from 15 months of continuous data in the Mid-Atlantic Bight, Deep Sea Res., 41, [10] Gangopadhyay, A., A.R. Robinson, P.J. Haley, W.J. Leslie, C. J. Lozano, James J. Bisagni, and Z. Yu, 2003: Feature Oriented Regional Modeling and Simulation (FORMS) in the Gulf of Maine and Georges Bank, Cont. Shelf Res., 23(3-4), [11] Gangopadhyay, A., A. Schmidt, L. Agel, L., O. Schofield, and J. Clark, Multiscale forecasting in the western North Atlantic: Sensitivity of model forecast skill to glider data assimilation, (Submitted to Cont. Shelf Res.) [12] Houghton, R., R. Schlitz, R.C. Beardsley, B. Butman, and J. L. Chamberlin The Middle Atlantic Bight cold pool: Evolution of the temperature structure during summer J. Phys. Oceanogr, 12, [13] Ketchum, B. H., and N. Corwin, The persistence of winter water on the continental shelf south of Long Island, New York, Limnol. Oceangr., 9, [14] Lentz, S., K. Shearman, S. Anderson, A. Plueddemann, and J. Edson, Evolution of stratification over the New England shelf during the Coastal Mixing and Optics study, August June1997. J. Geophys. Res., 108 (C1), doi: /2001JC001121, 2003.

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