Abstract
Intraseasonal variations of upper and lower-level zonal winds, outgoing longwave radiation (OLR) and globally averaged angular momentum (GAM) for northern summers of 1977-1984 are studied using lag correlation statistics. The role of surface frictional drag on atmospheric angular momentum is also investigated by studying the temporal and spatial distribution of surface wind stress in the tropics and its relationship with zonal wind anomalies.
It is shown that the 30-60 day GAM fluctuation is accompanied by zonal propagation of convection and 850 mb zonal wind anomalies in the tropical belt. On the other hand, the associated 250 mb zonal wind and surface wind stress anomalies exhibit pronounced standing oscillations. The magnitude of wind stress anomaly is strongly influenced by the climatological zonal wind in the tropics. Large wind stress anomalies are only found in the tropical central Pacific where surface easterly from the climatological Walker circulation is strong and persistent. Therefore, although the tropical lower-level wind anomalies associated with the 30-60 day oscillation propagate continually from the Indian Ocean to the eastern Pacific, large momentum exchange between the earth and atmosphere only occurs at the time when the low level wind anomalies are in phase with climatological easterlies in tropical central Pacific. From the present results, it is suggested that momentum exchange between the lower and upper troposphere may occur in regions of active convection via vertical momentum transport. The tropical central Pacific plays a key role in the linking the atmosphere and earth through angular momentum exchange in intraseasonal time scales.
