Dynamical Influence of Subtropical Jet Stream (STJ) and Tropical Easterly Jet (TEJ) on Indian Summer Monsoon Rainfall

The subtropical jet stream (STJ) and tropical easterly jet (TEJ) are critical upper-tropospheric features shaping the Indian summer monsoon (ISM). BY analysing data for the period 2000–2023, this study investigates the positional dynamics of these jets and their relationship with rainfall variability over the Indian region. Using ERA5 reanalysis data and daily rainfall records from the India Meteorological Department (IMD), we analysed the zonal and meridional wind fields at 200 hPa along with rainfall observations.

Four distinct jet stream cases were examined: both southern and northern hemispheric STJs shifting: (i) equator-ward, (ii) pole-ward, (iii) northward, and (iv) southward. Results reveal that equatorward shifts of the STJ weaken the TEJ and displace it southward, reducing rainfall over central India. Conversely, poleward migration of the STJ strengthens the TEJ, driving its northward extension and intensifying monsoonal rainfall, including extreme rainfall events. Northward shifts of both hemispheric STJs enhance TEJ strength, while southward shifts suppress it, altering the spatial distribution of rainfall. Strengthening of TEJ is expected to enhance the vertical velocity and LLJ through easterly vertical shear mechanism, and result in enhanced rainfall over the central Indian region. A TEJ Index (TEJI) was developed by area-averaging the TEJ core at 200 hPa, demonstrating strong correlations with rainfall intensity.

These findings underscore the complex interplay between STJ and TEJ and their dynamical role in modulating ISM rainfall. Understanding these mechanisms provides essential insights into atmospheric circulation patterns and their influence on monsoonal extremes, aiding improved prediction and climate resilience strategies in the region.