Extreme rainfall propagation within Boreal Summer Intraseasonal Oscillation modulated by Pacific sea surface temperature

TL;DR

This study uncovers how Pacific sea surface temperatures influence the propagation modes of the BSISO, affecting extreme rainfall patterns in South Asia, and proposes early warning signals based on these insights.

Contribution

It identifies three distinct BSISO propagation modes and links them to Pacific SST variations, enhancing understanding of monsoon variability and improving early warning capabilities.

Findings

Three BSISO propagation modes identified: north-eastward, eastward-blocked, stationary.

Pacific SSTs modulate propagation modes via changes in circulations and Kelvin wave.

Potential for four-week early warning signals of extreme rainfall.

Abstract

Intraseasonal variations of the South Asian Summer Monsoon (SASM) contain alternating extreme rainfall (active) and low rainfall (break) phases impacting agriculture and economies. Their timing and spatial location are dominated by the Boreal Summer Intraseasonal Oscillation (BSISO), a quasi-periodic movement of convective precipitation from the equatorial Indian Ocean to the Western Pacific. However, observed deviations from the BSISO's canonical north-eastward propagation are poorly understood. Utilizing climate networks to characterize how active phases propagate within the SASM domain and using clustering analysis, we reveal three distinct modes of BSISO propagation: north-eastward, eastward-blocked, and stationary. We further show that Pacific sea surface temperatures modulate the modes - with El Ni\~no- (La Ni\~na-) like conditions favoring the stationary (eastward-blocked) - by changing local zonal and meridional overturning circulations and the BSISO Kelvin wave component. Using these insights, we demonstrate the potential for early warning signals of extreme rainfall until four weeks in advance.