Structural Characteristics of Moisture Transport Systems over the Indian Subcontinent during the Summer Monsoon

Extreme precipitation and flooding during the monsoon season in India are closely linked to anomalies in atmospheric moisture transport. Two of the primary mechanisms that govern moisture flux into the Indian region are Monsoon Low-Level Jets (LLJ) and Atmospheric Rivers (ARs), the latter of which are under-recognized over the tropics, especially in the monsoon dominated regions. The low level jets are characterized by nocturnal intensification and boundary-layer thermal forcing and ARs are synoptic scale, transient corridors of intense horizontal water vapor transport. While these two phenomena have been extensively characterized individually, their degree of structural correspondence, coexistence, and synergistic impact on monsoon rainfall and extremes over India remains poorly quantified. This study presents a comparative structural analysis of LLJ and ARs that landfall over the Indian subcontinent during the Indian summer monsoon season (June to September). We apply standard detection methodologies on the horizontal winds and Integrated Water Vapor Transport (IVT) fields, derived from high resolution reanalysis data for AR and LLJ detection. We then stratify events into combinations of LLJ and AR occurrences, and construct composites to characterize the horizontal structural properties and vertical structures quantitatively. The results reveal quantitative distinctions in the wind and moisture characteristics associated with the LLJs and ARs that traverse. This study addresses a critical gap in distinguishing the ARs from the large-scale monsoon moisture transported by LLJs, by quantifying the structural distinctness between these transport mechanisms. Providing process level insights on these mechanisms may have implications in improving their representation in weather and climate models and potential predictability.

Keywords: Atmospheric Rivers, Low Level Jets, Indian Summer Monsoon, Monsoon Moisture Transport