How Do Atmosphere and Land Surface Influence Seasonal Changes of Convection in the Tropical Amazon?

Abstract

Although the wet season in the tropical Amazon (10°N?20°S) at any one place and in any one year is initiated rapidly by synoptic systems, large-scale thermodynamic conditions modulate the frequency and intensity of these synoptic systems and hence control the climatology of the wet season. In this study, the satellite radiances, radiosondes, and assimilation data of the atmosphere are analyzed to show that the conditioning of the large-scale thermodynamics for the onset of the wet season is controlled by a moistening of the planetary boundary layer (PBL) and a lowering of temperature at its top, hence reducing convective inhibition energy (CINE). These changes occur either in phase with or lagging by one month the enhancement of low-level moisture convergence. Integration of a slab mixed-layer model shows how a higher humidity can reduce the drying effect of the entrainment and increase the humidity of the daytime PBL. Hence, the increase of low-level moisture convergence may provide enough moisture to initiate the wet seasons. In the southern part of the basin (5°S?20°S), the land surface warming from austral winter to spring reduces the strong stability of the dry season and increases the frequency of unstable profiles for deep convection (fCUS), but convection remains infrequent until, in addition, the PBL is moistened and the inversion decays to lower CINE in October. The latter occur one month after the moisture becomes convergent. The seasonal changes in land surface temperatures are stronger than those over the adjacent oceans and hence have more influence on the gradient between land and ocean, and so on the changes in the large-scale circulation. In the equatorial western Amazon, a warmer land surface provides high fCUS all year round, but the seasonal changes of convection are more controlled by CINE. In the eastern basin, a lower fCUS in spring suppresses the expected wet season. Hence, convection is most frequent during austral fall, but also occurs in austral spring in the western Amazon. Higher fCUS and lower CINE are largely contributed to by the increases in humidity and weakening of the inversion. In contrast to the southern part of the basin, changes in local land surface temperature are small so that changes of the surface temperature in the adjacent oceans and southern Amazon largely control the changes in temperature gradient between land and ocean, and hence the seasonal onset of equatorial convection.