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Abstract

Historically, nearly half of all Indian monsoon droughts observed since 1901 have occurred in the absence of an El Niño. We recently demonstrated that such non-El Niño droughts are subseasonal in nature, characterised by an abrupt late season deficit caused by a Rossby wavetrain from the mid latitudes. This summer-time Rossby wavetrain is a result of the interaction of the upper-level winds with a persistent deep tropospheric vorticity forcing collocated above cold North Atlantic waters. Following this, we also find a similar teleconnection for early season deficit of Indian monsoon rainfall, especially in the absence of any major tropical oceanic forcing. Based on these findings, we investigate the role of a strong deep tropospheric vorticity forcing over an anomalously cold ocean in the North Atlantic, using two sets of idealised experiments: (i) a simple 1-layer shallow water model; and (ii) an Aquaplanet mode of the Community Earth System Model (CESM2). The shallow-water model demonstrates similar Rossby wave response to the vorticity forcing over the North Atlantic that could reach over India. The aquaplanet model shows modest success in simulating coherent episodic vorticity forcing over the North Atlantic, in response to an underlying north-south temperature gradient. The SST gradient results in a baroclinic instability leading to a strong vertical wind shear which in turn produces a deep tropospheric vorticity over the North Atlantic. This anomalous vorticity forcing in turn triggers waves which travel towards the Equator (and India).