A Limited-Area Model of the Gulf Stream: Design, Initial Experiments, and Model-Data Intercomparison

J. Dana Thompson Ocean Sensing and Prediction Division, Naval Ocean Research and Development Activity, Stennis Space Center, Mississippi

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W. J. Schmitz Jr. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Abstract

A primitive-equation, n-layer, eddy-resolving creation model has been applied to the Gulf Stream System from Cape Hatteras to cast of the Grand Banks (78°–45°W, 30°–48°N). Within the limitations of the model, realistic coastlines, bottom topography, and forcing functions have been used. A two-layer version of the model was driven by observed mean climatological wind forcing and mass transport prescribed at inflow. Outflow was determined by a radiation boundary condition and an integral constraint on the mass field in each layer. Specification of a Deep Western Boundary Current (DWBC) was included in some model runs.

Six numerical experiments, from a series of over fifty integrated to statistical equilibrium, were selected for detailed description and intercomparison with observations. The basic case consisted of a flat bottom regime driven by wind forcing only. Realistic inflow transport in the upper layer was then prescribed and two different outflow specifications at the eastern boundary were studied in experiments 2 and 3. Three additional experiments involved (4) adding bottom topography (including the New England Seamount Chain), (5) adding a DWBC to experiment 4 with 20 Sv (Sv ≡ 106 m3 s−1) total transports and (6) increasing the DWBC, to 40 Sv. A brief discussion of the influence of parameter variations includes modifications of dissipation (lateral eddy diffusion and bottom friction) and stratification.

Results from the sequence of experiments suggest an important role for the DWBC in determining the mean path of the Gulf Stream and consequently the distribution of eddy kinetic energy, and the character of the deep mean flow. The most realistic experiment compares to within a factor of two or better with observations of the amplitude of eddy kinetic energy and rms fluctuations of the thermocline and sea surface height. Abyssal eddy kinetic energy was smaller than observed. The mean flow is characterized by recirculations to the north and south of the Gulf Stream and a deep cyclonic gyre just east of the northern portion of the New England Seamount Chain, as found in the data.

Abstract

A primitive-equation, n-layer, eddy-resolving creation model has been applied to the Gulf Stream System from Cape Hatteras to cast of the Grand Banks (78°–45°W, 30°–48°N). Within the limitations of the model, realistic coastlines, bottom topography, and forcing functions have been used. A two-layer version of the model was driven by observed mean climatological wind forcing and mass transport prescribed at inflow. Outflow was determined by a radiation boundary condition and an integral constraint on the mass field in each layer. Specification of a Deep Western Boundary Current (DWBC) was included in some model runs.

Six numerical experiments, from a series of over fifty integrated to statistical equilibrium, were selected for detailed description and intercomparison with observations. The basic case consisted of a flat bottom regime driven by wind forcing only. Realistic inflow transport in the upper layer was then prescribed and two different outflow specifications at the eastern boundary were studied in experiments 2 and 3. Three additional experiments involved (4) adding bottom topography (including the New England Seamount Chain), (5) adding a DWBC to experiment 4 with 20 Sv (Sv ≡ 106 m3 s−1) total transports and (6) increasing the DWBC, to 40 Sv. A brief discussion of the influence of parameter variations includes modifications of dissipation (lateral eddy diffusion and bottom friction) and stratification.

Results from the sequence of experiments suggest an important role for the DWBC in determining the mean path of the Gulf Stream and consequently the distribution of eddy kinetic energy, and the character of the deep mean flow. The most realistic experiment compares to within a factor of two or better with observations of the amplitude of eddy kinetic energy and rms fluctuations of the thermocline and sea surface height. Abyssal eddy kinetic energy was smaller than observed. The mean flow is characterized by recirculations to the north and south of the Gulf Stream and a deep cyclonic gyre just east of the northern portion of the New England Seamount Chain, as found in the data.

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