Principal modes of intraseasonal variations in atmospheric angular momentum and tropical convection

Abstract

The seasonal and intraseasonal variation of angular momentum and its relationship with tropical convection is studied using 5‐day mean wind and outgoing longwave radiation (OLR). It was found that the zonally averaged u component and OLR both show pronounced periodicities around 30–60 days in the tropics. The globally averaged zonal wind at 250 mbar exhibits a very pronounced peak at 50 days, with strong seasonal and intraseasonal variations that are linked to the expansion and contraction of the lower level easterlies and the equatorward and poleward migration of the subtropical jet streams at upper levels. In the 30‐ to 60‐day time scale, three basic meridional modes of angular momentum are found. First, the tropical modes have Gaussianlike meridional structures, which decay rapidly away from the equator. These modes are related to both symmetric and antisymmetric tropical heating about the equator. Second, the tropical‐subtropical modes feature a strong amplitude in angular momentum in the tropics and subtropics. They are likely to be related to the seasonal and intraseasonal fluctuations of tropical heating, subtropical jet streams, and transfer of angular momentum through localized meridional circulations. Finally, the extratropical modes have a large amplitude poleward of 30° latitude. They appear to have some relationship with tropical convection, but it is not very strong. Most importantly, they do not seem to contribute significantly to the global angular momentum fluctuations in the 30‐ to 60‐day time scale. Our results indicate that angular momentum generated within the tropics and subtropics contributes to a large part of the observed global angular momentum changes in the intraseasonal and interannual time scales. In these time scales, angular momentum inside the tropics is exported outside the tropics (poleward of 20°N) via jet stream variations associated with regional atmospheric teleconnections.