Horizontal Structure and Seasonality of Large-Scale Circulations Associated with Submonthly Tropical Convection

Abstract

The relationship between deep tropical convection and large-scale atmospheric circulation in the 6?30-day period range is examined. Regression relationships between filtered outgoing longwave radiation at various locations in the Tropics and 200- and 850-mb circulation are mapped for the standard seasons, and the spatial structure and seasonal dependence of the results are interpreted in view of the basic-state circulation. In regions where the convection is embedded in upper-level easterlies, anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal, along with patterns of meridional outflow into subtropical anticyclonic perturbations. Lagged relationships suggest that the convection is forcing the circulation in many of these cases. The outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons, with more symmetric signals about the equator seen in the equinoctial seasons. The longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection. There tends to be a first baroclinic mode vertical structure to these circulations, such that equatorial westerlies are present at 850 mb within the convection, with closed circulations on either side of the equator resembling equatorial Rossby modes especially common over the Atlantic and Pacific sectors. As a contrast, in regions located within upper-level westerlies or along the margin of influence of upper westerly disturbances, convection appears to be forced by upper-level wave energy propagating into the deep Tropics, with the heating located in the upward motion region ahead of upper-level troughs. This occurs over the Atlantic and eastern Pacific sectors during northern winter and spring, and over Australia, the South Pacific, and South America during southern summer, when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection. The results confirm theoretical and modeling ideas that suggest that Rossby wave energy is able to propagate into the deep Tropics in regions where upper-level westerlies exist in the Tropics.