Unique relationship between tropical rainfall and SST to the north of the Mozambique Channel in boreal winter

Abstract

In this study, we investigate a possible mechanism for the dichotomy in climatology of marine rainfall and sea surface temperature (SST) over a part of the Southwest Indian Ocean (SWIO) during boreal winter (January and February) with state-of-the-art satellite and reanalysis data sets. Rainfall to the north of the Mozambique Channel, bounded 10°–5°S and 40°–50°E, is found to be quite feeble despite being in the warm SST regime of up to 29–29.5 °C. The rainfall intensity is rather found to be highly associated with the atmospheric surface divergence. The vigorous rainfall is associated with the more convergence over the Intertropical Convergence Zone (ITCZ), while the weak rainfall is linked with the divergence to the north of the Mozambique Channel. The surface divergent flow to the north of the Mozambique Channel is associated with a deep southward penetration of the northerly Indian Winter Monsoon (IWM). Corresponding to the surface divergent field, a relatively high sea level pressure (SLP) compared to the SLP in the ITCZ, weak subsidence, and low-level stratiform clouds are formed to the north of the Mozambique Channel, despite the warm, tropical SST. These atmospheric conditions are most likely conductive to the inhibition of cumulus convection over the region and the unique relationship between rainfall and SST seems peculiar. Our analysis also shows that the rare occurrence of tropical cyclones over the area is attributed to a high-pressure surge and the associated positive surface vorticity (anti-cyclonic). This study suggests that the area to the north of the Mozambique Channel is dynamically interesting for climatological studies.