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David Adamec

Abstract

The sensitivity of the separation of the western boundary current of an idealized single gyre circulation to the specification of various model parameters is investigated through a series of quasigeostrophic simulations. The model parameters considered are the value of β, bottom topography, lateral boundary conditions, deformation radius of the first baroclinic mode, horizontal friction, and model resolution. Changes in these model parameters affect western boundary current separation characteristics, but those parameter changes also produced changes in several measures of the global flow field such as level of mean and eddy kinetic energies. A number of flow field statistics are correlated with the average separation point, and it was found that total average baroclinic kinetic energy is the most highly correlated variable to the average separation point, with higher levels of kinetic energy being correlated with more poleward penetration. In the immediate vicinity of the separation point, the advection of relative vorticity is the largest contributor to tendencies conducive to separation. This contribution is especially dominant for the finest horizontal resolution run. An analysis of the convergence of zonal momentum revealed that it is the (uu′)x term that is the dominant contributor to zonal velocity component tendencies near the separation point. Contributions involving the mean flow tend to be smaller but of larger meridional scale than the terms involving eddy interactions. A wavelet analysis of the time-dependent separation point reveals that the greater the contribution of (uυ′)y in the convergence of zonal momentum, the greater the low-frequency energy at separation point location. The effect is most pronounced in the finest horizontal resolution experiment.

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David Adamec

Abstract

The effect of seamounts on Gulf Stream-like flow is examined using a quasi-geostrophic modal model that is spun-up from rest with idealized wind forcing. A flat-bottom simulation that resolves the flow with two vertical modes produces, on the average, a straight Gulf Stream-like jet across the domain along the latitude where the wind stress curl is zero. In a similar simulation that includes topographic effects, a chain of seamounts, approximately the width of the New England Seamounts, is responsible for a southward deflection of the jet as it passes over the seamounts. However, the seamounts do not appear to be a source mechanism for increased eddy activity. Simulations that include the effects of the second baroclinic mode produce a very different response from simulations that are resolved with the barotropic and first baroclinic mode only. Although 90% of the kinetic energy is contained in the two lowest modes, the higher modal interactions transfer energy to the lower modes and significantly alter the time evolution of those modes. In particular, the increased vertical shear due to the inclusion of the extra mode, enhances the energy transfer due to baroclinic instability by a factor of three relative to the 2-modal simulations and inhibits the zonal penetration of the Gulf Stream-like jet.

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David Adamec

Abstract

Results from a set of 2- and 3-mode quasi-geostrophic simulations are used to estimate the predictability time scale of a Gulf Stream-like flow and investigate the sensitivity of the predictability time scale to changes in model vertical resolution. For this study, two simulations, which differ initially by a specified “small” amount diverge from each other with a doubling time of approximately 16 days for simulations that are resolved by 2 modes and 13 days for simulations that are resolved by 3 modes. On average, higher vertical modes have longer predictability time scales. A spectral analysis of the differences in streamfunctions between a control simulation and a simulation with perturbed initial conditions shows that higher modes have shorter horizontal scales of maximum growth, and the horizontal scale of maximum growth expands at later times for the baroclinic flow. An error energetics budget is calculated to show that transports of error kinetic and error potential energies are initially responsible for exponential error growth of the difference fields. After sufficient buildup in the error potential energy, the conversion between error potential and error kinetic energies becomes as important as the transport terms in the error energy budget.

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David Adamec and Russell L. Elsberry

Abstract

Simulations of the oceanic mixed layer at Ocean Weather Ship Papa are used to study the sensitivity of the 30-day predictions of mixed-layer depth and temperature to the time resolution and averaging of the atmospheric forcing during spring, summer and autumn. The model simulations are sensitive to the length of the averaging window applied to the atmospheric forcing. Both the detail and trends in the mixed-layer depth and temperature deviate more from the control run when the length of the averaging window is increased. The effect of averaging the meteorological observations prior to calculating the surface fluxes is examined separately from the case in which the fluxes are calculated prior to the averaging. The cases which use forcing calculated from averages of the actual observations better simulate the detail and trend in mixed-layer depth of 30-day windows in the spring, summer and autumn than cases which use forcing based on the average of the calculated fluxes. By contrast, forcing based on an average of the calculated fluxes leads to better predictions of the detail and tend in the sea-surface temperature than the cases which use average observations to compute an average forcing.

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David Adamec and Russell L. Elsberry

Abstract

The effect of errors and biases in the atmospheric forcing for oceanic mixed layer model predictions is studied using data sensitivity techniques. First the bulk model of Garwood is used to predict 17 years of mixed layer evolution and temperature structure at Ocean Station Papa using forcing derived from the 3 h atmospheric observations. The model is then integrated again varying, one at a time, each atmospheric forcing variable by a Gaussian error whose spread is proportional to the standard deviations of that variable during late winter or midsummer. The results of those integrations are then compared with the control run to assess the effects of the added random errors or biases. A positive or negative bias in the atmospheric forcing is much more detrimental to the ocean prediction than is a random error with zero mean. The predicted mixed layer depths are more sensitive to errors introduced in the forcing in winter than in summer. Conversely, the mixed layer temperature is more sensitive to errors in summer than in winter. For both winter and summer, the wind speed is the most critical factor in predicting mixed layer depth and temperature. Dew point temperature is an important variable for mixed layer predictions during the winter. During summer, cloud cover becomes an important variab1e. The results of this study are compared with errors in mixed layer depth and temperature predictions that are due to errors in the initial profile. The errors in the predictions which are due to errors in the atmospheric forcing are comparable in magnitude to those errors which are due to imperfect initial conditions.

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David Adamec and Russell L. Elsberry

Abstract

Shifts in location and strength of an intense oceanic flow such as the Gulf Stream to a cross-stream gradient in cooling are studied using two-dimensional numerical simulations. The gradient in cooling is imposed by removing more heat from the warm side of the associated baroclinic zone than is removed from the cold side. The initial flow is assumed to be in geostrophic balance. When only a vertical heat exchange associated with the convective overturning induced by surface cooling is allowed, the magnitude of the horizontal pressure gradient is reduced and the flow becomes supergeostrophic. The resulting cross-stream velocity will tend to shift the front toward the region of larger upward surface heat fluxes. When a vertical exchange of momentum is also allowed in the convective adjustment, the reduction of the initial surface velocities due to turbulent momentum exchange is not balanced geostrophically by a reduction in the horizontal pressure gradient. The flow becomes subgeostrophic and a cross-stream flow is produced that shifts the front toward the region of smaller upward surface heat fluxes. Although the along-stream current decreases near the surface, the current below the mixed layer is strengthened due to the exchange with relatively high momentum from above. The additional response due to an increase in the southward and eastward wind stress is compared to the response due to cooling only. Small changes in the temperature and flow fields occur when a southward wind stress is included. An eastward wind stress of 0.2 N m has a greater effect on the position of the simulated Gulf Stream than does a very strong gradient in cooling.

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David Adamec and Russell L. Elsberry

Abstract

The three-dimensional response of strong currents to cross-stream gradients of surface cooling is studied using numerical simulations. In particular, surface cooling is explored as a possible mechanism for explaining an observed 100 km southward shift in the mean position of the Gulf Stream during winter. The cooling increases in the downstream direction and in the direction of highest sea surface temperatures. In the immediate vicinity of the concentrated horizontal temperature gradient associated with the strong current system, most of the flow changes are induced by the cross-stream cooling gradient. The magnitude and direction of the cross stream circulation is highly dependent on whether or not a vertical mixing of momentum occurs when the water column convectively adjusts in response to the surface cooling. A weak cross-stream flow toward the higher sea surface temperatures occurs in the surface layer if momentum mixing does not occur, whereas a stronger flow toward lower sea surface temperatures results if momentum mixing does take place. In regions where the vertical shear is not large, the responses in the flow fields are due solely to the alongstream pressure gradient induced by the prescribed alongstream cooling gradient. The cross-stream response due to horizontal cooling gradient is not large enough to displace the Gulf Stream appreciably southward in any of the numerical simulations. By contrast, a moderate increase in the zonal wind stress is more effective in displacing the core of a strong current system than are very strong gradients in the surface cooling.

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David Adamec and Russell L. Elsberry

Abstract

No abstract available.

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David Adamec and James J. O'Brien

Abstract

A linear model on an equatorial β plane is integrated over a 120-day period in a basin that approximates the tropical Atlantic Ocean. An increase in the westward wind stress of 0.025 N m−2 in the western Atlantic excites an equatorially trapped Kelvin wave that propagates eastward along the equator, moves poleward at the eastern boundary, and produces upwelling throughout the Gulf of Guinea. Cases that study the effects of nonlinearities and the inclusion of a northward wind stress are included. Nonlinearities are shown to have the effect of amplifying the effects of the Kelvin wave and prolonging the upwelling event. The inclusion of a southerly wind stress in the eastern basin provides a secondary mechanism for upwelling south of the equator along the eastern basin. Local winds. cannot account for the seasonal upwelling in the Gulf of Guinea. The simple baroclinic ocean model is integrated from rest. The effects of mean currents and bottom topography are not considered in detail.

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Judith E. Ghirardelli, Michele M. Rienecker, and David Adamec

Abstract

Analyses of satellite-derived SSM/I winds and AVHRR sea surface temperatures are used to compute weekly estimates of global meridional ocean Ekman heat transport for the 4-year period 1987–1991. The heat transport is consistently poleward throughout the year over the Atlantic and much of the Pacific between 30°S and 30°N and equatorward at higher latitudes. The zonally integrated Ekman heat transport in the Pacific was weak and equatorward at 10°N in September 1989 and 1990, whereas in other years it is poleward throughout the year. In the Indian Ocean, equatorward heat transport was strongest in Northern Hemisphere summer 1990. The weekly time series provides better temporal resolution than previous studies that at best used monthly averages. The higher-frequency variations are explored through rotated empirical orthogonal functions (REOFs) of nonseasonal heat transport anomalies. The REOFs show large-scale coherence across the tropical and subtropical Pacific and Indian Oceans. The first REOF has a strong spectral peak at periods of 50–60 days and is dominated by the variability in the Southern Hemisphere Indian Ocean. The second REOF is dominated by the variance in the Northern Hemisphere Indian Ocean and western tropical North Pacific. The fist four REOFs, which explain 22.5% of the nonseasonal variance, have spectral peaks at periods consistent with the 30–60 day atmospheric Madden–Julian oscillation. These periods were not resolved in the monthly averaged data from COADS. Singular spectrum analysis has been used as a filter to show the long timescale variations. The early phase of the 1988–89 La Niña has a strong influence on the heat transport, indicating enhanced poleward heat transport in the eastern Indian Ocean, tropical North Pacific, and tropical and subtropical South Pacific and enhanced equatorward heat transport in the midlatitude North Pacific and western tropical Indian Ocean.

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