Midlatitude Rossby wave modulation of the Indian summer monsoon

Abstract

The dominant mode of July–August seasonal variability of Indian summer monsoon rainfall obtained by performing an empirical orthogonal function analysis over India, excluding northeast India, for the period 1979–2014 is highly correlated to the quasi‐stationary midlatitude Eurasian Rossby wave train, known as the ‘Silk Road’ pattern. A new hypothesis has been proposed and tested using observed Indian rainfall and the ERA‐Interim dataset, by which the positive feedback between the midlatitude Rossby wave and the Indian summer monsoon circulation has been established. A Rossby wave train features successive upper‐tropospheric negative and positive geopotential height anomalies over the northeast of the Mediterranean and northwest of India, respectively. The negative height anomaly decreases the sum of the vertical integral of potential and internal energy, while this increases in the region of positive geopotential height anomalies. This is associated with middle to lower tropospheric midlatitude descent and ascent anomalies at 40°E and 50°E, which cause negative surface temperature anomalies over Syria, Iraq, Jordan and Saudi Arabia, and a positive surface temperature anomaly over Iran, respectively. The negative and positive surface temperature anomalies are associated with positive and negative surface pressure anomalies over Saudi Arabia and Iran, respectively, which generate anomalous northwesterly winds over eastern Saudi Arabia and the Persian Gulf. Further downstream this anomalous northwesterly wind combines with the climatological background cross‐equatorial southwesterly flow in the Arabian Sea. The flow further converges towards central and northwest India. The flow also acts as a source of moisture supply from the warm Arabian Sea and Persian Gulf for the deep convection over the Western Ghats, northwest and central India and hence reinforces active Indian summer monsoon conditions.