On the Strong Seasonal Currents in the Deep Ocean

Abstract

Using a set of models, including one with a resolution of ¼°, several aspects of the simulated seasonal currents in the deep ocean are considered. It is shown that over vast areas of the deep interior, particularly in the Indian Ocean, annual-mean circulation represents a small residual of much stronger seasonal flows. In many places the seasonal horizontal velocities are of the order of 10?2 m s?1, reaching locally to 10?1 m s?1; the corresponding vertical velocities are of the order of 10?5 m s?1. An idealized geometry model is employed to confirm the notion that much of this seasonal variability in the deep-ocean circulation can be attributed to the annual cycle of wind stress, combined with the significant increase in the vertical trapping depth for basin-scale seasonal forcing. It is suggested that, at least on seasonal time scales, the so-called bottom pressure torque can be an important term in the depth-integrated vorticity balance. An interaction of these relatively strong flows (of nontidal origin) with bottom topography may contribute to diapycnal mixing in the deep ocean in a manner similar to that proposed recently for the Southern Ocean. In addition, it is found that under a plausible climate change scenario, the amplitude of the mean annual cycle of wind stress may change. Among the regions where such changes are most pronounced is that in the extratropical North Pacific. It is shown that the data on surface wind stress can be effectively used to identify the seasons with the largest changes in the deep-reaching overturning cells. Finally, unlike what might be expected from the earlier theories, the annual-mean circulation simulated by the model with ¼° resolution has the deep interior flows that tend to group into jetlike structures, often having a predominant equatorward rather than poleward direction.