Abstract
The existing theories for the tropical teleconnections to Indian summer monsoon (ISM) are diverse in approaches. As a result, it is impossible to quantify the relative impacts of different tropical climate patterns on ISM, complying with a single physical mechanism. Here, we show that tropical teleconnections to ISM can be explained through net moisture convergence driven by surface pressure (Ps) gradients surrounding the Indian region. The positive and negative phases of major tropical climate patterns modulate these pressure gradients asymmetrically in the zonal and/or meridional directions leading to asymmetric changes in moisture convergence and ISM rainfall (ISMR). Stronger El Nino droughts than La Nina floods are due to greater decreased eastward moisture flux over the Arabian Sea during El Nino than the corresponding increase during La Nina driven by proportionate meridional Ps gradients. While the equatorial Atlantic Ocean’s sea surface temperature in boreal summer and El Nino Southern Oscillation in the preceding winter changes ISMR significantly, moisture convergence anomalies driven by the Indian Ocean Dipole were insignificant. Moreover, while ISMR extremes during ENSO are due to asymmetric changes in zonal and meridional gradients in Ps, non-ENSO ISMR extremes arise due to the zonal gradient in zonally symmetric Ps anomalies.
Highlights
Ow different tropical climate patterns modulate these parameters
We aimed to develop a physically-based theory for the interannual variations of the Indian summer monsoon rainfall (ISMR) that, at the same time, can be used to explain the relative impacts of different tropical climate phenomena
We showed that relative change in the moisture fluxes along the boundaries of the Indian monsoon region determines by year-to-year variations in the moisture convergence
Summary
Ow different tropical climate patterns modulate these parameters. Often more than once forcing appear simultaneously. While one could linearly separate their influences, for example, using SST-based indices, the actual superposition of individual effects could be non-linear. Recent researches indicate coupled interaction and evolution of tropical ocean basins[60]. It is difficult to completely separate out the impact of one forcing from the other on ISM. Our study illustrates the signature of four major tropical climate patterns on monsoon teleconnection using a common mechanism. This theory would be helpful to test the teleconnection mechanisms simulated by general circulation model
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