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Scott B. Power

Abstract

A global version of the GFDL modular ocean model is forced using conventional restoring boundary conditions (BCs), mixed BCs (i.e., restoring the upper-level temperature but specifying a fixed salt flux), and stochastic fluxes of both heat and freshwater.

The climatology of the model is found to drift if stochastic freshwater fluxes are applied at high latitudes under mixed BCs. The drift is global in extent: the ocean is generally warmer in the North Pacific and Weddell Sea but cooler and fresher at depths elsewhere in the Southern Ocean and in the North Atlantic. There is a slight reduction (by about 5%) in the meridional overturning of the Southern Ocean and the North Atlantic. The drift of the barotropic flow is most pronounced in the Southern Ocean and is associated with a permanent meandering of the Antarctic Circumpolar Current.

The drift occurs within a few decades, suggesting that it may be important in enhanced greenhouse scenarios for early next century that have been obtained using coupled atmosphere-ocean GCMS. It is also possible that some of the intrinsic variability identified in the same models is actually a residual drift.

The drift depends upon convective adjustment to occur but can be amplified by the surface heat flux parameterization, both locally and by an additional feedback associated with large-scale flow changes. In an extreme case, the latter leads to a total collapse of the thermohaline circulation associated with North Atlantic Deep Water Formation. A similar mechanism underlies the drift that can occur when the switch from restoring to mixed BCs is made.

The heat flux feedback represents the atmosphere-ocean coupling in the model, so this aspect of the drift can be regarded as a coupled mode that actually contributes to the mean state of the coupled system. The existence of such modes makes some climatic drift in coupled models inevitable, if the individual components are equilibrated separately prior to coupling.

The applicability of these results to more sophisticated coupled models depends, in part, upon how well the restoring BC on temperature captures the heal flux feedback they exhibit.

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Richard Kleeman and Scott B. Power

Abstract

A simple model of the lower atmospheric layers and land/sea ice surface is described and analyzed. The model is able to depict with reasonable accuracy the global ocean heat fluxes. Due to the model's simplicity, insight into the mechanisms underlying particular heat flux responses is possible. Such an analysis is carried out for the regional Gulf Stream heat flux response (which is gualitatively correct in the model), and it is shown that atmospheric transient eddy heat transport is crucial to the modeled response. The perturbation response of the model to tropical SST anomalies is also analyzed, and it is demonstrated that the atmospheric transport processes incorporated in the model are responsible for a scale-dependent response. The magnitude of this response is shown to be significantly different to that obtained with formulations previously used by ocean modelers.

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Scott B. Power, Roger H. J. Grimshaw, and Jason H. Middleton

Abstract

Analytic solutions are obtained for forced, barotropic circulation at subinertial frequencies over a bilinear continental margin (shelf and slope) in situations where bottom friction is important. Three different alongshore forces are considered: wind-stress, offshore oceanic pressure gradients and offshore currents. Forcing functions are assumed to vary sinusoidally in time and in space alongshore. Steady models are found to perform adequately provided that the forcing functions do not move in the same direction as the free modes (continental shelf waves) propagate. Near resonance, when the alongshore velocity of the forcing approximates that of a free mode, the response is dominated by the mode. In the case of wind forcing, signals are trapped nearshore. If the shelf break occurs within this trapping length (as occurs near resonance) the shelf width becomes the elective trapping length. In this instance there can be significant horizontal shear in the alongshore velocity on the shelf near the shelf break.

When the velocity of an oceanic, alongshore pressure gradient signal approximates that of a free mode, the signal can be amplified towards the coast. For example, near a mode 2 resonance the signal is a maximum near the coast with a secondary maximum on the continental slope, near the shelf break. This amplification is in stark contrast to the solution forced by a signal which is either stationary or moving in a direction opposite to that in which the free modes propagate, which simply fall away from their maximum values offshore, resulting in weak coastal circulations.

Bottom friction affects the free continental shelf waves in three ways: their phase speeds are reduced, they decay with time and their altered structures exhibit phase differences across the continental margin whereby the flow nearshore leads that offshore in time. As a result, increased bottom friction reduces the response at resonance, broadens the range of frequencies over which responses are increased and detunes, or shifts, the frequency at which resonance occurs to a lower value. At practical parameter values, the reduction is minimal for the first mode but can he substantial for the second.

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Scott B. Power, Jason H. Middleton, and R. H. J. Grimshaw

Abstract

Analytic solutions am obtained for the barotropic shelf circulation caused by wind and deep-ocean forcing at subinertial frequencies. An Inclined beach model of the continental shelf is used and only situations in which bottom friction is important are considered. Three different alongshore forces are considered: pressure gradients and currents (maintained by the deep ocean) at the shelf break and wind stress, over the shelf. In each case the model is formulated as a boundary value problem in which the boundary conditions are determined by the forcing mechanism. In general, a damped resonant response occurs when the forcing function has the same longshore velocity as an unforced continental shelf wave and is most significant for the fim mode. In the case of forcing by an alongshore pressure gradient at the edge of the shelf, this leads to the amplification of the pressure signal toward the coast. The model frequencies and structures are determined for various frictional values. When friction is small the results are consistent with those of Brink and Allen in that phase speeds remain unchanged and cross-shelf phase differences are introduced. At larger frictional values, however, phase speeds are reduced, and the model structures and cross-shelf phase differences are further altered.

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