On the Importance of Low-Frequency Gravity Modes for the Evolution of Large-Scale Flow in a General Circulation Model

Abstract

The impact of low-frequency baroclinic gravity modes on the tropical and extratropical circulation has been studied in a global spectral model. The procedure used was to filter out these gravity modes at every time step in a 120 day integration. Comparing the results of this integration with a control integration in which these modes were retained shows that they have a major influence on the tropical divergent circulation, which is severely damped in their absence. The precipitation rate in the filtered integration is considerably reduced and this leads to lower temperatures in the troposphere. The changes in the tropical circulation are accompanied by a considerable reduction in the amplitude of 40–50 day oscillation as indicated by the time variation of the mean sea level pressure and the upper and lower tropospheric winds at five tropical stations indicating the important role of the gravity modes in these oscillations. Finally the control and filtered integrations were repeated with a positive sea surface temperature anomaly of 4°C in the Indonesian-North Australian region. Then integrations indicate that the low frequency gravity modes resulting from convective activity in the tropics also play a strong role in tropical-midlatitude interactions.