OC2m2

Fig. 19.11 Sensitivity of J to surface temperature at different times during the 3-day period. Top row. Southward down-welling winds. Bottom row. Northward upwelling winds. Panel at right shows sensitivity at day 0 (upwelling case) on a vertical section. (See the text for discussion)

o dynamical processes that were not incorporated in the model physics employed here that are worthy of incorporation in future model-based studies.

In the NYB, sea-land-breeze system (SLBS) activity can be pronounced during spring (Hunter et al. 2007, 2010) when ocean temperatures are still cool but the land is warming. Since this is precisely the time of year when river discharge peaks with the spring freshet, atmosphere-ocean interactions fundamental to SLBS dynamics are likely important to achieving realistic simulations of the plume circulation. Furthermore, mid-summer SLBS activity further south on the Jersey Shore is influenced by SST changes associated with wind-driven coastal upwelling (Bowers 2004). Full synchronous coupling of ROMS with an atmospheric forecast model has the potential to improve both ocean and atmosphere forecasts when SLBS conditions occur, and this capability has been added to ROMS by coupling to the COAMPS (Coupled Ocean Atmosphere Prediction System) (Warner et al. 2008b) and WRF (Weather Research and Forecasting) models.

Surface wind waves mediate air-sea interaction by modifying drag and hence net momentum exchange, plus surface wave radiation stress, Stokes drift and wave-current interaction processes in the bottom boundary layer drag are important in the ocean momentum balance itself. It was noted in Sect. 19.2.4 that these dynamical processes are now incorporated in ROMS, including the option to synchronously couple with the SWAN wave model. Studies of the Hudson plume that employed higher resolution than the 1 km grid used here and placed greater emphasis on processes in shallow waters near the coast (inside the 15-m isobath) or at the leading edge of the plume, may demonstrate that inclusion of these dynamics are important to faithful simulation of the plume evolution.

19.4.2 Ecosystem-Optics and Heating Interaction

Like most coastal ocean models, ROMS assumes constant absorption coefficients for shortwave radiation (Paulson and Simpson 1977) leading to a vertical exponential decay in internal solar heating. But optical properties of coastal waters can be far from spatially uniform, and observations during LaTTE exhibited distinct regions of turbid water associated with the river plume, motivating Cahill et al. (2008) to use the EcoSim model (Sect. 19.2.4) to examine coupling between shortwave radiation attenuation, buoyancy and photosynthesis. The solar heating parameterization was modified to make shortwave absorption dependent on the concentration of river source freshwater as a proxy for increased attenuation in the plume. The feedback between solar heating and vertical stratification was sufficient to modify the buoyancy driven circulation and mixed layer depth. This in turn raised concentrations of chlorophyll, detritus and coloured dissolved organic matter (CDOM) in the upper water column increasing attenuation of photosynthetically active radiation (PAR) and further impacting phytoplankton growth.

Simulations with full ecosystem-absorption-heating feedback (i.e. spectrally resolved 3-dimensional radiative absorption determined by optically active con stituents in the water column) have shown differences in simulated temperature can be as much as 2°C warmer at the surface, and correspondingly cooler some 10 m deeper, in the Hudson River plume. The associated changes in plume trajectory and ecosystem dynamics alter net export of particulate matter to mid shelf waters. Incorporating these optical properties into the 4-dimensional ocean state is a natural future step to enhance data assimilation in coastal ocean models.

19.5 Summary

We have described a series of model-based studies of circulation in the New York Bight region that utilize data from a sustained coastal ocean observing system complemented by extensive in situ observations from the LaTTE project.

Observations are used to evaluate the performance of traditional forward simulations where the model formulation is treated as an initial and boundary value problem. Circulation on the New Jersey inner shelf, and especially within the NYB, is strongly locally driven and direct forward simulations with ROMS are quite skil-ful—a result we attribute to the model being comprehensive and accurate in the suite of dynamical processes it represents and the numerical algorithms it employs, suitably configured in terms of bathymetric and coastline detail, and driven by meteorological, hydrological and tidal forcing with sufficient resolution and accuracy.

Using forward model simulations we have seen that the NYB circulation is particularly responsive to wind forcing, how buoyancy dynamics contribute to the retention of river source waters in the NYB apex through formation of a persistent anti-cyclonic recirculation, and that the model can be used to quantify this residence time by incorporating an age tracer. Long simulations reveal the pathways by which Hudson River borne material is ultimately dispersed across the New Jersey shelf.

Moving beyond traditional forward simulations, we have illustrated how coastal models are now being increasingly integrated with the growing network of regional coastal ocean observing systems. The creation of variational complements to the ROMS nonlinear forward model (i.e. the ROMS adjoint and tangent linear models) has enabled the implementation of 4-dimensional variational data assimilation in coastal ocean analysis with an attendant improvement in forecast skill. Variationalbased methods have further capabilities beyond data assimilation, through helping inform adaptive sampling strategies and observing system design targeted at improving predictive skill.

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