Indian Ocean warming during peak El Niño cools surrounding land masses

Understanding the interactions between the Pacific and other ocean basins during extreme phases of the El Nino-Southern Oscillation (ENSO) is necessary for explaining its global climate impacts. Here climate model experiments are used to highlight a mechanism by which the characteristic warming of the Indian Ocean during peak El Nino months can cool North Africa and South Asia, an area encompassing over three billion people. It is found that warming of the Indian Ocean during extreme El Nino events leads to broader upper tropospheric geopotential height anomalies than would otherwise occur. This weakens the extratropical Rossby wave response initiated in the tropics and leads to higher pressure and reduced cloud forcing over North Africa and South Asia. Reanalysis data provides empirical support for this mechanism, although it is likely only to be prominent during strong El Nino events when Indian Ocean warming tends to be larger. This dampening effect needs to be taken into account in understanding the climatic impact of extreme El Nino events, which are projected to increase under global warming.