John Wilkin

A semi-spectral primitive equation ocean circulation model using vertical sigma and orthogonal curvilinear horizontal coordinates

Abstract We describe a new diabatic primitive equation model for studying regional and basin-scale ocean circulation processes. The model features coordinate transformations that efficiently incorporate moderately irregular basin geometries and large variations in bottom topography, and permits the inclusion of both thermal and wind forcing. A novel semi-spectral solution procedure, in which the vertical structure of the model variables is represented as a finite sum of user-specifiable structure functions (e.g., Chebyshev polynomials), provides faster-than-algebraic convergence of the vertical approximation scheme. Model performance is assessed on a variety of test problems drawn from coastal and large-scale oceanography including unforced, linear wave propagation in both regular and irregular geometries; non-linear flow over rough bottom topography; and eddy/mean flow interaction in a wind-driven, midlatitude ocean basin. Computational efficiency of the model is found to be comparable to other existing primitive equation ocean models despite the utilization of the higher order spectral methods.

The Effect of Wind on the Dispersal of the Hudson River Plume

Abstract The dispersal of the Hudson River plume in response to idealized wind forcing is studied using a three-dimensional model. The model domain includes the Hudson River and its estuary, with a realistic coastline and bottom topography of the New York Bight. Steady low river discharge typical of mean conditions and a high-discharge event representative of the spring freshet are considered. Without wind forcing the plume forms a southward coastally trapped current at low river discharge and a large recirculating bulge of low-salinity water during a high-discharge event. Winds affect the freshwater export through the mouth of the estuary, which is the trajectory the plume takes upon entering the waters of the Mid-Atlantic Bight inner shelf, and the rate at which freshwater drains downstream. The dispersal trajectory is also influenced by the particular geography of the coastline in the apex of the New York Bight. Northward wind causes offshore displacement of a previously formed coastally trapped plume ...

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Denitrification effects on air-sea CO2 flux in the coastal ocean: Simulations for the northwest North Atlantic

[1] The contribution of coastal oceans to the global air-sea CO2 flux is poorly quantified due to insufficient availability of observations and inherent variability of physical, biological and chemical processes. We present simulated air-sea CO2 fluxes from a high-resolution biogeochemical model for the North American east coast continental shelves, a region characterized by significant sediment denitrification. Decreased availability of fixed nitrogen due to denitrification reduces primary production and incorporation of inorganic carbon into organic matter, which leads to an increase in seawater pCO2, but also increases alkalinity, which leads to an opposing decrease in seawater pCO2. Comparison of simulations with different numerical treatments of denitrification and alkalinity allow us to separate and quantify the contributions of sediment denitrification to air-sea CO2 flux. The effective alkalinity flux resulting from denitrification is large compared to estimates of anthropogenically driven coastal acidification. Citation: Fennel, K., J. Wilkin, M. Previdi, and R. Najjar (2008), Denitrification effects on air-sea CO2 flux in the coastal ocean: Simulations for the northwest North Atlantic, Geophys. Res. Lett., 35, L24608, doi:10.1029/2008GL036147.

Extracting Multiyear Surface Currents from Sequential Thermal Imagery Using the Maximum Cross-Correlation Technique

Ocean surface circulation can be estimated by automated tracking of thermal infrared features in pairs of sequential satellite imagery. A 7-yr time series of velocity, extracted from thermal imagery of the East Australian Current using the maximum cross-correlation (MCC) technique, provides enough measurements for a more statistical evaluation of the method than has previously been possible. Excluding 1 yr with extensive cloud cover, the method produces about 8000 velocity estimates per month with some seasonal variation. Method precision is estimated to be between 0.08 and 0.2 m s21 rms, the lower value with more restrictive compositing. Mean flow, time-dependent flow, and eddy kinetic energy from the time series are compared with values derived from a dynamic height climatology, altimeter analyses, and drifter datasets in the region. The observations reproduce similar features in the flow. The differences between the observations are discussed in relation to noise in the methods and differences in the types of velocities they measure.

Ocean currents and the larval phase of Australian western rock lobster, Panulirus cygnus

The return of Panulirus cygnus larvae to the coast of Western Australia after nearly a year at sea and its modulation by ocean currents were addressed with an individual-based larval-transport model. The simulations implied that offshore wind-driven transport of larvae is balanced by onshore geostrophic flow. Additional simulations revealed that vertical migration behaviour was essential to larval survival through its impact on advection. The six years simulated include two of high, two of low, and two of average puerulus settlement. The most robust interannual difference of the simulations was that, when coastal sea level was low and the Leeuwin Current was weak, more early-stage larvae were lost to the north and west under the influence of the wind. Conversely, many late-stage model larvae were carried south of the fishery in years when the Leeuwin Current was strong. The fraction of model larvae remaining or arriving offshore of the fishery and metamorphosing was essentially constant from year to year, so the variation in observed puerulus settlement was not explained by the model. The results imply that the nonadvective effects of fluctuations in the Leeuwin (e.g., on temperature and primary production) were primarily responsible for the high variation in natural settlement.

The Summertime Heat Budget and Circulation of Southeast New England Shelf Waters

A modeling study of summer ocean circulation on the inner shelf south of Cape Cod, Massachusetts, has been conducted. The influences of winds, air–sea heat fluxes, tides, and shelfwide circulation are all incorporated. The model reproduces recognized features of the regional summer circulation: warm temperatures and weak eastward flow in Nantucket Sound, cool tidally mixed waters and an associated anticyclonic flow encircling the Nantucket Shoals, and strong stratification south of Martha’s Vineyard. Comparisons with satellite and in situ observations show the model simulates the major features of the temperature patterns that develop during summer 2002. The evolution of the summer heat budget is characterized by three regimes: Nantucket Sound heats rapidly in June and then maintains warm temperatures with little net air–sea heat flux; tidal mixing on the Nantucket Shoals maintains perpetually cool ocean temperatures despite significant air–sea heating; and midshelf south of Martha’s Vineyard the surface waters warm steadily through July and August because of sustained air–sea heating with only modest cooling resulting from the mean circulation. In the environs of the Martha’s Vineyard Coastal Observatory tidal eddy heat flux emanating from Nantucket Sound produces a bowl of warm water trapped against the coast and significant local variability in the net role of advection in the heat budget. A suite of idealized simulations with forcing dynamics restricted, in turn, to only one of winds, tides, or shelfwide inflows shows that tidal dynamics dominate the regional circulation.

Seasonal evolution of hydrographic fields in the central Middle Atlantic Bight from glider observations

[1] The first sustained glider observations in the Middle Atlantic Bight are used to describe the seasonal evolution of hydrographic fields off New Jersey. Near-surface temperatures respond to the seasonal cycle of surface heating, while waters at depth are primarily influenced by advection of cold waters from the north in the cold-pool during spring/summer, and warming due to mixing during fall. The thermocline thickness increases in the offshore direction. Salinity presents seasonal variability due to river discharge and wind variations, with low-salinity waters spanning ∼100 km across the shelf from May to September in a ∼10 m thick surface layer. Stratification intensifies from April/May to late summer, especially within 80 km from the coast. The pycnocline deepens in the water column during late summer, while the passage of storms during fall rapidly reduces the stratification. The glider high-resolution observations allowed for unprecedented detailed characterization of the spatial scales of variability.

Scattering of Coastal-Trapped Waves by Irregularities in Coastline and Topography

Abstract The scattering of freely-propapting coastal-trapped waves (CTWs) by large variations in coastline and topography is studied using a numerical model which accomodates arbitrary density stratification, bathymetry and coastline. Particular attention is paid to the role of stratification which in moderate amounts can eliminate backscattered free-waves which occur. theoretically, in a barotropic ocean. Numerical simulations using widening and narrowing shelf topographies show that the strength of the forward scattering into transmitted CTW modes is proportional to a topographic warp factor which estimates the severity of the topographic irregularities. The forward-scattering is further amplified by density stratification. Within the scattering region itself, the strengths of the scattered-wave-induced currents exhibit substantial variation over short spatial scales. There is generally a marked intensification of the flow within the scattering region, and rapid variations in phase. On narrowing shelves...

Eddy kinetic energy and momentum flux in the Southern Ocean: Comparison of a global eddy‐resolving model with altimeter, drifter, and current‐meter data

The ability of a seasonally forced high-resolution global ocean general circulation model to simulate eddy variability and associated energy and momentum transfer processes in the Southern Ocean is assessed by comparing model statistics with observations. The observations include Geosat altimeter data analyzed for surface velocity variance at satellite ground track crossover points, current-meter data from the Agulhas and Campbell plateaus, and surface drifter data in the Tasman Sea. In western boundary currents and energetic regions of the Antarctic Circumpolar Current model eddy kinetic energy is lower than observed by typically a factor of 4, and in less energetic regions by a factor of 10. Differences in the location and extent of energetic regions are related to smoothness of the model bathymetry and other features of the model configuration. Eddy momentum flux divergence and eddy to mean kinetic energy conversion at the surface are diagnosed from the model. These show regions where eddy activity accelerates the mean flow through instability processes. Observational estimates of these terms are computed using mean flow gradients from hydrography climatology and altimeter eddy statistics. Several features of the spatial distribution of the observational estimates are consistent with the model and suggest that future calculations of mean currents from altimeter data will allow direct computation of eddy to mean current momentum and energy conversion terms.