The Indian Monsoon Circulation Response to El Nino Diabatic Heating

The response of the boreal summer mean tropical circulation to anomalies in diabatic heating during the strong El Nino events of 1972, 1987, and 1997 is studied, with particular focus on the Indian region. In experiments with the atmospheric general circulation model of the National Center for Atmospheric Research, anomalous diabatic heating fields are added to the full temperature tendency of the Community Atmosphere Model, version 3 (CAM3). The boundary conditions are specified climatological sea surface temperatures everywhere but over the Indian and western Pacific Oceans, where a slab-ocean model is used. The vertical structure of the added heating is idealized with a single maximum at 600 hPa. The added heating in the experiments was chosen on the basis of the 1972, 1987, and 1997 diabatic heating anomalies in the Pacific and Indian Oceans diagnosed from reanalyses. Integrations extended from May to August with 20 different initial conditions. The 1972 and 1987 experiments produced an anomalous anticyclonic circulation extending westward toward the Indian region, accompanied by negative total (added plus CAM3 produced) diabatic heating anomalies over India. A similar result was obtained for 1997 when only the Pacific Ocean diabatic heating was added. The heating over the central Pacific is shown to be more important than the western Pacific cooling. When the added heating also took into account anomalies over the Indian Ocean, the anomalous anticyclonic circulation weakens, while the total Indian heating anomaly is quite small. These results suggest the importance of the Indian Ocean heating for the 1997 monsoon circulation, but do not constitute a complete explanation since the Indian Ocean heating was given a priori.