Variability in the Mozambique Channel Trough and impacts on southeast African rainfall

The Mozambique Channel Trough (MCT) is a cyclonic region prominent in austral summer in the central and southern Mozambique Channel. It first becomes evident in December with a peak in strength in February when the Mozambique Channel is warmest and the Mascarene High (MH) is located furthest southeast in the Indian Ocean basin. The strength and the timing of the mean MCT are linked to that of the cross equatorial northeasterly monsoon in the tropical western Indian Ocean which curves as northwesterlies towards northern Madagascar. The interannual variability in the MCT is associated with moist convection over the Mozambique Channel and is modulated by the location of the warm sea surface temperatures in the South Indian Ocean. Variability of the MCT shows a strong relationship with the equatorial westerlies north of Madagascar and the latitudinal extension of the MH. Summers with strong MCT activity are characterized by a prominent cyclonic circulation over the Mozambique Channel, extending to the midlatitudes. These are favorable for the development of tropical-extratropical cloudbands over the southwestern Indian Ocean and trigger an increase in rainfall over the ocean but a decrease over the southern African mainland. Most years with a weak MCT are associated with strong positive South Indian Ocean subtropical dipole events, during which the subcontinent tends to receive more rainfall whereas Madagascar and northern Mozambique are anomalously dry.