The Strengthening Association Between Siberian Snow and Indian Summer Monsoon Rainfall

ERA5 reanalysis data shows significant correlation between Siberian snow depth in March and following Indian summer monsoon rainfall (ISMR). Whilst the Siberian snow depth is negatively correlated with seasonal monsoon rainfall over most of India, it is positively correlated with the monsoon rain over north-east Indian regions. This relationship has significantly strengthened during the past 2 decades. We show that the strengthening relationship is more likely related to recent atmospheric circulation changes owing to persistent global warming, particularly the changes in the atmospheric circulation over the North Atlantic. There are indications that during 1999-2018, the extratropical climatic variability (e.g., Arctic Oscillation [AO] and North Atlantic Oscillation [NAO]) significantly influenced Siberian snow in the month of March. In contrast, during the period 1979-1998, the influence of NAO and AO on Siberian snow was negligible. Analysis indicates that Siberian High, a dominant atmospheric circulation system that exerts strong influence on Eurasian weather and climate, has also significantly weakened during the past 2 decades. We hypothesize that the March Siberian snow influences the ISMR through a delayed hydrological response, in which the increased (reduced) accumulated snow during springtime provides wetter (drier) soil during the following summer. This leads to the alternation of the meridional tropospheric temperature gradient. The heavy (less) spring snowfall in north (south) Siberia led to changes in summertime meridional tropospheric temperature gradient, which resulted in the weakening of subtropical westerly jet and Tibetan anticyclone. The reduced intensity of tropical easterly jet connected to Tibetan anticyclone resulted in the weakening of summer monsoon Hadley cell, leading to subsequent suppression of rainfall. This association between Siberian snow and ISMR provides seasonal prediction potential.