Michael M. Whitney

Simulating the Delaware Bay Buoyant Outflow: Comparison with Observations

Coastal buoyant outflows from rivers and estuaries previously have been studied with field research, laboratory experiments, and numerical models. There is a dire need to evaluate model performance in light of coastal current observations. This research simulates the Delaware Bay outflow and compares results with observations of estuarine and shelf conditions. Observations include an estuarine salinity climatology, a record of freshwater delivery to the shelf, coastal current salinity mappings, and surface drifter data. Simulation efforts focus on spring 1993 and spring 1994, the primary field study period. The simulation is forced with river discharge, winds, and tides; only tidal-averaged results are discussed. Estuarine salinity results are consistent with the observed lateral salinity pattern, vertical structure, and response to river discharge. Salinities within the lower bay agree with observations, but the simulation overestimates the along-estuary salinity gradient. Observed and simulated freshwater delivery exhibit the same amplitude of response to river discharge and winds. The simulation produces a buoyant outflow that is generally consistent with the observed buoyancy signature, width, length, and vertical structure over a variety of river discharge and wind conditions. The simulated coastal current, however, tends to be somewhat shorter and fresher than observed. Simulated surface drifter paths exhibit the observed onshore advection during downwelling winds as well as offshore transport and current reversals during upwelling winds. A statistical evaluation based on shelf salinity mappings indicates that the model reproduces the observed variance and has only a small bias (less than 10% of plume buoyancy signature). The rms error of 1.2 psu is linked to the shorter and fresher nature of the simulated coastal current. Observational comparisons discussed in this paper indicate that the model can simulate many coastal current features and its response to river discharge and wind forcing.

Response of a Large Stratified Estuary to Wind Events: Observations, Simulations, and Theory for Long Island Sound

AbstractThe response to wind events in the Long Island Sound (LIS), a large macrotidal estuary influenced by rotation and stratification, is studied using long-term ferry-based current observations near the mouth, unstratified and stratified numerical simulations forced with along-estuary winds, and analytic solutions based on linear barotropic theory. The observed wind-event velocity anomalies for down-estuary winds have surface-intensified downwind flows flanking a deeper central upwind flow. Response to up-estuary wind events has a weaker magnitude and a broader and thicker downwind flow. The downwind and upwind flows are more laterally aligned than vertically layered, as determined by a newly defined dimensionless lateral alignment index. Simulation results and analytic solutions share the gross spatial patterns of the observed response, though statistical measures indicate weak agreement. Along-estuary variations in the simulation results and analytic solutions follow similar trends and are strongly ...

The Physical Oceanography of Long Island Sound

Coastal ocean ecosystems are strongly influenced by circulation, tides, waves, and the rates of mixing of the water. Many shoreline communities are increasingly threatened by the same phenomena, most notably through flooding and coastal erosion. In this review we summarize the observations that have been acquired in LIS to describe and explain the magnitude and variability of these physical processes. We also comment on the status of our theoretical understanding of the links between them and some of their consequences. Analysis of available buoy and coastal wind observations suggests that the shear stress due to wind over LIS is under-predicted by a factor of between 2 and 3 if shore station winds are used to make the estimates. This difference is significant to both water quality and wave forecasting. We describe the magnitude of seasonal variations in wind and waves and use long-term records from coastal stations to show that there are decadal-scale variations in both wind speed and directions. Available wave data from two buoys suggest that the wave field is consistent with that predicted by fetch- limited wind forcing. Semi-diurnal tidal sea level variations and vertically averaged currents are well described by theoretical models, however, recent observations show high amplitude over tides in the western LIS that remain to be explained. The vertical structure of the tidal currents is much more complex and a closer examination of model predictions, particularly of the across Sound velocity components, should be conducted. The interaction of the vertical structure of tidal currents and the salinity and temperature distributions may lead to significant heat and salt transport vertically and horizontally. Observations of the mean density, temperature, and salt distributions and the mean circulation in LIS are qualitatively consistent with several models and we summarize the recent work. A more critical evaluation is now appropriate. We also discuss evidence of long-term trends. The role of shorter time-scale meteorological forcing and the bathymetry of the Sound on the structure and variability of the circulation is summarized using observations and simulation. Long-term observations of both hypoxia duration and hypoxia areal extent in western and west central Long Island Sound are analyzed to determine the directional response to wind forcing. We show that a substantial fraction of the inter-annual variability in area and duration can be explained by the directional statistics of wind. Using simulation, we demonstrate that the geometry of the basin and across isobath flow may be significant. The response of the Sound to severe storms is outlined and the technical developments in simulation and observation that are necessary to the improvement of model predictions are suggested.

Coastal Wind-Driven Circulation in the Vicinity of a Bank. Part I: Modeling Flow over Idealized Symmetric Banks

This study examines how coastal banks influence wind-driven circulation along stratified continental shelves. Numerical experiments are conducted for idealized symmetric banks; the standard bank (200 km long and 50 km wide) has dimensions similar to the Heceta Bank complex along the Oregon shelf. Model runs are forced with 10 days of steady winds (0.1 Pa); upwelling and downwelling cases are compared. The bank introduces significant alongshelf variability in the currents and density fields. Upwelling-favorable winds create an upwelling front and a baroclinic jet (flowing opposite coastal-trapped wave propagation) that bend around the standard bank, approximately centered on the 90-m isobath. The upwelling jet is strongest over the upstream bank half, where it advects a tongue of dense water over the bank. There is a current reversal shoreward of the main jet at the bank center. Upwelling is most intense over the upstream part of the bank, while there is reduced upwelling and even downwelling over other bank sections. Downwellingfavorable winds create a near-bottom density front and a baroclinic jet (flowing in the direction of coastaltrapped wave propagation) that bend around the standard bank; the jet core moves from the 150-m isobath to the 100-m isobath and back over the bank. The downwelling jet is slowest and widest over the bank; there are no current reversals. Results over the bank are more similar to 2D results (that preclude alongshelf variability) than in the upwelling case. Downwelling is weakened over the bank. The density field evolution over the bank is fundamentally different from the upwelling case. Most model results for banks with different dimensions are qualitatively similar to the standard run. The exceptions are banks having a radius of curvature smaller than the inertial radius; the main jet remains detached from the coast far downstream from these banks. The lowest-order across-stream momentum balance indicates that the depth-averaged flow is geostrophic. Advection, ageostrophic pressure gradients, wind stress, and bottom stress are all important in the depth-averaged alongstream momentum balance over the bank. There is considerable variability in alongstream momentum balances over different bank sections. Across-shelf and alongshelf advection both change the density field over the bank. Barotropic potential vorticity is not conserved, but the tendency for relative vorticity changes and depth changes to partially counter each other results in differences between the upwelling and downwelling jet paths over the bank. Only certain areas of the bank have significant vertical velocities. In these areas of active upwelling and downwelling, vertical velocities at the top of the bottom boundary layer are due to either the jet crossing isobaths or bottom Ekman pumping.

Sensitivity of Simulated Sea Breezes to Initial Conditions in Complex Coastal Regions

AbstractMesoscale simulations of sea breezes are sensitive to the analysis product used to initialize the simulations, primarily due to the representation of the coastline and the coastal sea surface temperatures (SSTs) in the analyses. The use of spatially coarse initial conditions, relative to the horizontal resolution of the mesoscale model grid, can introduce errors in the representation of coastal SSTs, in part due to the incorrect designation of the land surface. As a result, portions of the coastal ocean are initialized with land surface temperature values and vice versa. The diurnal variation of the sea surface is typically smaller than over land on meso- and synoptic-scale time scales. Therefore, it is common practice to retain a temporally static SST in numerical simulations, causing initial SST errors to persist through the duration of the simulation. These SST errors influence horizontal coastal temperature and humidity gradients and thereby the development of the sea-breeze circulations.The a...

Tidal Cycles in Stratification and Shear and Their Relationship to Gradient Richardson Number and Eddy Viscosity Variations in Estuaries

AbstractTidal cycles in stratification and shear lead to tidal variations in mixing in many estuaries. This study 1) defines readily observable dimensionless parameters for establishing the sense and magnitude of gradient Richardson number Ri and eddy viscosity K changes from maximum to minimum stratification during a tidal cycle and 2) calculates where representative estuaries fit in this parameter space. The dimensionless parameters are Ri calculated with tidal-averaged stratification and shear, scaled stratification amplitude, and a scaled shear parameter. The scaled stratification amplitude is approximately the tidal amplitude of stratification divided by the tidal-averaged stratification. The scaled shear parameter depends on the scaled tidal amplitude of shear and the phase difference between the tidal cycles of stratification and shear. Over most of the parameter space, Ri is larger at maximum stratification. If the scaled stratification amplitude falls below a threshold value defined in terms of t...

Coastal Wind-Driven Circulation in the Vicinity of a Bank. Part II: Modeling Flow over the Heceta Bank Complex on the Oregon Coast

Abstract This study investigates wind-driven circulation in the vicinity of the Heceta Bank complex along the Oregon shelf. Numerical experiments forced with steady winds (0.1 Pa) are conducted; upwelling and downwelling cases are compared. The asymmetric bank bathymetry is the only configurational difference from the symmetric bank runs analyzed in Part I (Whitney and Allen). Upwelling-favorable winds generate an upwelling front and southward baroclinic jet. Model results indicate the upwelling jet is centered on the 100-m isobath along the straight shelf. The jet follows this isobath offshore around the northern part of the bank but separates from sharply turning isobaths in the southern half and flows over deeper waters. The jet turns back toward the coast farther downstream. Inshore of the main jet, currents reverse and flow back onto the bank. These reversed currents turn southward again (at the bank center) and join a secondary southward coastal upwelling jet. This secondary coastal jet converges wi...

Sill effects on physical dynamics in eastern Long Island Sound

This study investigates how Mattituck Sill influences circulation patterns and physical dynamics in eastern Long Island Sound, a major estuary on the U.S. east coast. Observations show there is pronounced across-estuary transport in the area and suggest there may be subtidal anticyclonic flow around the sill. Model runs, with and without sill bathymetry, exhibit this across-estuary transport and anticyclonic circulation. Comparison between these runs indicates that the sill intensifies the anticyclonic circulation. This study finds the sill does not exert internal hydraulic control during neap, mean, or spring tidal conditions. Nevertheless, along-estuary exchange is reduced over the sill and across-estuary fluxes are increased. The Connecticut River plume enters close to the estuary mouth. The sill deflects more of the plume waters towards the mouth, causing less freshwater to take the long looping route through the estuary. The subtidal circulation balance around the sill indicates a barotropic balance between the tidal advection of tidal vorticity and friction. The subtidal vorticity balance indicates the net effect of tidal advection of relative vorticity is balanced with frictional curl associated with lateral speed gradients and vorticity dissipation. Previously developed scalings based on the circulation balance (Nature 290:549–555, 1981), frictional vorticity generation mechanisms (Deep-Sea Res 28:195–212, 1981), and tidal diffusion of potential vorticity (J Phys Oceanogr 29:821–827, 1999) are applicable to Mattituck Sill and predict circulation with a similar magnitudes to model results.

Subtidal Exchange in Eastern Long Island Sound

AbstractLong Island Sound (LIS) is a large and wide macrotidal estuary with distributed river inputs, including the Connecticut River (the largest freshwater source) that flows into the eastern LIS near the mouth. In 2010, shipboard surveys of salinity, temperature, and currents were collected along an across-estuary transect in eastern LIS. Numerical model results are compared to these observations and used to study the spatial and temporal variability of salinity, velocity, and freshwater and salt fluxes over a 4-yr period. For all low wind conditions, observations and model results indicate an outward-flowing, low-salinity wedge on the south side with an inward-flowing, higher-salinity area underneath and to the north. Observations and model results during the low wind surveys indicate that stratification substantially decreases with increased tidal amplitude and decreased river discharge; the velocity field is less variable among surveys. Model analysis indicates strong sensitivities to both tides and...

Collaborative Project: Improving the Representation of Coastal and Estuarine Processes in Earth System Models

This project aimed to improve long term global climate simulations by resolving and enhancing the representation of the processes involved in the cycling of freshwater through estuaries and coastal regions. This was a collaborative multi-institution project consisting of physical oceanographers, climate model developers, and computational scientists. It specifically targeted the DOE objectives of advancing simulation and predictive capability of climate models through improvements in resolution and physical process representation.