Signatures of monsoon intra-seasonal oscillation and stratiform process in rain isotope variability in northern Bay of Bengal and their simulation by isotope enabled general circulation model

Abstract

Monsoon intra-seasonal oscillation (MISO) represents a quasi-periodic occurrence of rainfall spells over India during summer monsoon (June–September) associated with large-scale circulation and convection. The connection of MISO with the hydrological cycle has been explored in this study through stable oxygen (δ18O) and hydrogen isotope ratios (δD) in rainwater samples collected from two stations located near the north coast of the Bay of Bengal during 2004–2014. The δ18O and δD values of the samples (total 202) have wide ranges: − 18.2 to 2.8‰ and − 132 to 28‰ (rel. to VSMOW) respectively. Based on northward propagation of negative anomaly in Outgoing Longwave Radiation, 16 convective MISO phases were identified during this period. Out of 69 samples associated with these MISO phases, 46 samples have isotope ratios depleted in heavier isotopes. These ratios are well correlated (R = 0.81) with the MISO propagation speed and enhanced rainfall (R = − 0.71) over the Bay of Bengal. In addition, the isotope ratios also show a significant anti-correlation (R = − 0.94) with satellite-derived stratiform rain fraction. The observed isotopic ratios were compared with the results obtained from an Isotope Enabled Global Spectral Model (IsoGSM). The model simulation reproduces the amplitude of variation in the observed values reasonably well, but on average, the model values are higher in δ18O and δD by about 2‰ and 11‰ respectively. In contrast, the model d-excess values are much lower (by 4.5‰ on average). We speculate that the discrepancies in δ18O, δD and d-excess may arise due to an overestimation of raindrop evaporation effect in the model. Our study thus identifies two major physical processes controlling rain isotope variability in northern Bay of Bengal, namely, the effect of MISO propagation characteristics in inducing isotope depletion through rain out over the ocean and contribution from the stratiform rains.