Circulation Anomalies Associated with Tropical Convection during Northern Winter

Abstract

Lagged cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses are utilized to isolate the preferred upper-level and surface circulation anomalies associated with tropical convection during northern winter. Three intraseasonal time scales are studied: 30?70, 14?30, and 6?14 days. In the 30?70-day band, the upper-level circulation signals are zonally elongated, with zonal wavenumbers 0?2 dominant. Higher-frequency signals are dominated by zonal wavenumbers 5 and 6. In the 14?30-day band, convection over the eastern hemisphere is associated with upper-level anticyclones in the subtropics and appears to be linked in some cases to midlatitude wave trains. The strongest signals are for convection over Africa, Australia, and the eastern Indian Ocean. Only weak signals are seen for convection over Indonesia. In these regions of upper-level easterlies, OLR anomalies peak prior to the maximum anomalies in wind, suggesting forcing of the circulation by tropical heating. In contrast, 14?30-day and 6?14-day convection over the eastern tropical Pacific, eastern South America, and central South Pacific is primarily associated with the intrusion of troughs in the westerlies originating in the extratropics. These are regions of mean upper level westerly flow, or where upper-westerlies lie adjacent to tropical convergence zones overlain by only weak easterly flow aloft. The large amplitude of these troughs prior to the OLR anomaly is indicative of the forcing of the convection by these disturbances.