Effect of the variation in the lower tropospheric temperature on the wind onset of the Indian summer monsoon

There is a large thermal contrast between the Arabian Peninsula and India (ΔθAI) at the mature stage of the Indian summer monsoon (ISM). The forming process of ΔθAI is investigated analyzing various datasets. It forms earlier in the lower troposphere than in the middle and upper layers. The potential temperature in the lower troposphere over the west coast of India (θIW) abruptly decreases in advance of the rapid enhancement of the westerly wind over the Arabian Sea corresponding to the ISM onset. Such a process was observed for all the target years and the rapid decrease in θIW could trigger the ISM onset. The decrease in θIW had two patterns. In one case, cooler air is brought by the strong winds around a cyclone over the Arabian Sea. In another case, θIW decreases gradually by a synergy of a southwesterly wind over the Arabian Sea and the enlargement of ΔθAI. © 2009 Springer-Verlag