Dynamics of rainfall extremes over India: A new perspective

• Published in: International journal of climatology 2020-10, Vol.40 (12), p.5223-5245
• Main Authors: Sooraj, KP, Terray, Pascal, Shilin, Anokha, Mujumdar, Milind
• Format: Article
• Language: English
• Subjects: Anomalies; Arid regions; Arid zones; Atmospheric waves; Circulation anomalies; Clustering; Daily; Domains; Dynamics; Eurasian wave train; Extreme values; Extreme weather; Geophysics; Geostrophic wind; Geostrophic winds; Heavy rainfall; Hydrodynamics; hydrodynamic‐radiative teleconnection; Intrusion; Latitude; Lower atmosphere; Moist static energy; Monsoon circulation; monsoon core zone; Monsoon rainfall; Monsoon trough; Monsoon winds; Monsoons; Physics; Pressure anomalies; Rain; Rainfall; Specific humidity; subtropical‐midlatitude interaction; summer monsoon rainfall extremes; Surface temperature; Wave packets; Wave trains; west Asian arid regions; Westerlies; Wind
• ISSN: 0899-8418; 1097-0088
• DOI: 10.1002/joc.6516

A comprehensive analysis of the role of Indo‐Pakistan arid region (IPAR) in modulating the boreal summer heavy extreme rainfall events over monsoon core zone (MCZ over Indian subcontinent) at subseasonal to daily time scales is presented. The objective is to bring new perspectives on the dynamics of such rainfall extremes. Time‐lagged composite analysis shows the clustering of Monsoon Intra‐Seasonal Oscillation with monsoon rainfall daily extremes. However, our results additionally bring out the distinct role of surface thermal forcing and moist processes over IPAR as another potential and significant precursor, 16–10 days in advance, causing monsoon rainfall daily extremes over MCZ. Further analysis reveals that the warming over IPAR is initially found in the northern part of the domain, 20–15 days before the rainfall events over MCZ and is triggered by subtropical‐midlatitude interactions characterized by the intrusion of a midlatitude (i.e., Eurasian) atmospheric wave train into the Asian domain. Our analysis further reveals strong moist static energy (MSE) build‐up over the IPAR 12 days in advance, which is initially dominated by anomalous warming in the lower atmosphere, but with an incremental contribution of low‐level specific humidity as we moved forward in time towards the rainfall event. Hydrodynamic‐longwave radiative teleconnections are the primary mechanism for the persistence of the surface warming and MSE buildup over IPAR after the initial intrusion of the Eurasian wave train. The persistent warming over IPAR reinforces the land‐sea contrast between the Middle East and western equatorial Indian Ocean to its south, and this subsequently leads to pressure anomalies and monsoon circulation changes through geostrophic wind adjustment, thus enhancing westerlies over Arabian Sea. All these factors lead to the development of a gigantic monsoon trough over central India thus causing the rainfall extremes over MCZ through the genesis of synoptic vortices. The study demonstrates the distinct role of surface thermal forcing and moist processes over Indo‐Pakistan arid region (IPAR) as another potential and significant precursor (16–10 days in advance), in causing monsoon rainfall daily extremes over the monsoon core zone (MCZ). The IPAR warming is initially triggered by subtropical‐midlatitude interactions characterized by the intrusion of Eurasian wave train into the Asian domain. Hydrodynamic‐longwave radiative teleconnections are the primary mechani