Extra Tropical Influence on Early Season Deficit of Indian Summer Monsoon Rainfall

El Niño is often considered to be a major tropical forcing that has a tendency to suppress Indian summer monsoon rainfall (June through September) through various teleconnection pathways. However, historically, nearly half of all Indian monsoon droughts observed in the past century have occurred in the absence of an El Niño. We recently reported that these non-El Niño droughts are subseasonal in nature, characterised by a moderate deficit in June compounded by an abrupt late season deficit. The late season deficit was shown to be a consequence of a Rossby wavetrain from the mid latitudes. The work presented here systematically characterises the mid latitude influence on early season Indian monsoon rainfall deficit. Firstly, we show that the June rainfall deficit in non-El Niño drought years also has a mid latitude connection following a similar pathway as the late-season deficit. Following this, we extend the scope of our diagnostic analysis beyond drought years. Specifically, we identified 24 years (1901-2015) consisting of both drought and normal years in which June rainfall was in deficit. Here too, based on ERA 20th century reanalysis, we find that when the Pacific Ocean sea surface temperatures are near neutral, the primary pathway by which early season Indian rainfall is suppressed is via an extra tropical Rossby wavetrain originating from persistent (2 to 3 weeks) deep tropospheric vorticity forcing located in the North Atlantic. This finding suggests that the extra tropical influence on Indian rainfall deficit is more prominent than has been reported, especially early and late in the season.