Influences of the North Pacific Anomalies on the Indian Summer Monsoon Onset

Abstract:

The linear trend (1979-2018) of April-July mean Sea Level Pressure (SLP) over the Asian Monsoon region and composite differences of meteorological variables during early and late Indian Summer Monsoon (ISM) onset have been investigated. We can see a significantly decreasing and increasing trend of SLP over coastal East Asia and far North Pacific during May and June, respectively. It resembles the negative (positive) phase of Pacific Decadal Oscillation (PDO) (North Pacific Oscillation, NPO). Sea Surface Temperature (SST) composite difference shows that early (late) ISM onset is accompanied by the negative PDO (positive PDO). Whereas in SLP, an intense low pressure over north-western India, the Arabian Sea to southeast Asia (far North Pacific, East Asia) results in an early (late) onset. During the early onset, warm SAT anomalies over southeast Russia propagate towards Central Asia to the Middle East and the northwest Indian subcontinent strengthening land-sea thermal contrast, which subsequently helps to establish a stronger monsoon circulation over the ISM region. Thereafter, we show how NPO, PDO, and Bering Sea Ice (SIC) are related to the SAT anomalies. We find that negative, positive phase of PDO, NPO, and higher Bering SIC cover are associated with the early onset. In wave activity flux (WAF) analysis, wave-packets generated in response to these North Pacific anomalies propagate towards the Atlantic and European region reaching and converging over the north-west Indian sub-continent. The warm anomalies perhaps draw westward propagating WAF towards India, thus intensifying the monsoon low over there. While in the CAM5 simulation of PDO, we also observe a similar pattern of atmospheric responses, where warm SAT anomalies in association with stationary Rossby wave trains propagate from the North Pacific towards the North Atlantic to Central Asia and the north-west Indian subcontinent strengthens low pressure over there. Hence, the study highlights the key role of the North Pacific Ocean in modulating the ISM onset via an atmospheric bridge. Based on these results, we conclude that the North Pacific anomalies, as a result of internal variability and unabated increases in greenhouse gas emissions will continue to influence the ISM onset by changing the land-sea thermal gradient over the Asian summer monsoon region.