Revisiting Moisture Transport During the Indian Summer Monsoon Using the Moisture River Concept

Abstract

The moisture river (MR) concept is used here to revisit the characteristics of moisture transport during the Indian Summer Monsoon (ISM) season from June to September. The study is carried out using Modern-Era Retrospective analysis for Research and Applications (MERRA) data products for the 30-year period from 1982 to 2011. MRs are detected from vertically integrated vapor transport (IVT) computed using validated MERRA mixing ratios and horizontal wind speeds at vertical levels from the surface to 300 hPa. The characteristics of MR and moisture transport associated with major rainfall episodes of the 2011 ISM are studied in detail. The large moisture content, together with high wind speeds associated with the monsoon low-level jet, contributes to the observed structure of the MR. The outflux of moisture from the Arabian Sea is on average 40% larger than the cross-equatorial flux from the southern Indian Ocean, indicating increased contribution from the Arabian Sea. The upward movement of moisture to the layers above the jet is a decisive factor in the strengthening of the river and major rainfall episodes. Back-trajectory analyses reveal that the path of moisture transport from the southern Indian Ocean is confined to the lower levels, whereas higher-level trajectories originate from the Red Sea, Arabian Sea and southern Indian Ocean. The MR statistics show significant intra-seasonal and inter-annual variability in occurrence and strength, with a near-linear relationship between the IVT strength and the corresponding daily ISM rainfall, especially over the west coast. Wavelet cross-spectral analyses carried out using the daily Arabian Sea IVT maximum and rainfall over the west coast and central India during a deficit and surplus monsoon year depict significant differences in the dominant periodicities, associated with intra-seasonal and inter-annual monsoon variations.