The role of surface air temperature over the\xa0east Asia on the early and late Indian Summer Monsoon Onset over Kerala

Devanil Choudhury,Debashis Nath,Chen Wen

doi:10.1038/s41598-019-47945-4

Abstract

This study investigates the physical mechanism for late Indian Summer Monsoon onset over Kerala (MOK). 14 early and 9 late onset years are selected based on the criteria when the onset is 5 days or more prior and after normal onset date (i.e 1st June according to India Meteorological Department) respectively. Then, we perform composite analyses of mean May monthly and daily evolution during early and late onset years to examine the differences in monsoon circulation features prior to the MOK. We find that advection of Surface Air Temperature (SAT) from the northern to the southern China and the eastern Tibetan Plateau (TP) plays an important role to modulate the MOK processes. In the late onset years, more low-level jet (LLJ) from the Bay of Bengal (BOB) divert towards the east Asia before the onset, which is due to an extension of the low sea level pressure and high SAT over the east Asia (eastern TP, east-central China). This strengthens the low-level convergence and upper level divergence over the eastern TP and southern China. As a result, a significant amount of moisture from the BOB is transported towards the eastern TP and southern China. Thereby, a comparatively weaker LLJ and deficit low-level moisture supply over the eastern BOB maintain the key roles in modulating the MOK processes.

Highlights

In order to investigate the anomalous changes of SLP, we address SAT
Zhang (2001) already mentioned that strong water vapor transport from the Indian monsoon region is accompanied by less water vapor transport over the east Asia, leading to less rainfall over the middle and lower reaches of the Yangtze River valley[24]
Several monthly and daily data sets are employed in this study

Summary

Results

The early onset features include deep convection over the southern BOB and southern AS, stronger cross-equatorial LLJ over the tropical Indian Ocean, more updrafts over the south, eastern India, BOB, TP and SEA, more moisture convergence over the India, AS and BOB and higher precipitation over the BOB, which are usually regarded as the normal meteorological features during normal MOK These changes in mean circulation during the late onset warrant further investigation and we Figure 4. During the late onset years, the extension of low SLP over the east Asia (eastern TP, south-east-central China, SCS, and Sea of Japan) due to anomalous high SAT over the eastern TP, southern China and the adjacent region strengthen the south-westerly LLJ over the east Asia from the BOB. The background atmospheric conditions for the early and late MOK and the role of east Asia are shown in the schematic diagram (Supplementary Fig. S19)

Summary and Conclusion

Data and Methods

Additional Information