Quasi-biweekly oscillations in the Bay of Bengal in observations and model simulations

Abstract

Intraseasonal oscillations (ISOs) significantly contribute to the variability and strength of rainfall associated with the Indian Summer Monsoon. The westward-propagating, 10–20-day, quasi-biweekly ISO (QBWISO), in association with the zonal double-cell structure, contributes to an increase in momentum and moisture from the western Pacific Ocean and South China Sea to the Bay of Bengal (BoB) and Indian subcontinent that intensifies monsoonal rainfall rates. The QBWISO also has a meridional double-cell structure positioned over 15–20°N and 0–5°N, with the northernmost cell significantly contributing to monsoonal precipitation. The atmospheric systems associated with this QBWISO, particularly the northernmost cell, induce shifts in circulation that directly impact the strength and timing of active and break monsoon periods, which are respectively characterized by wet and dry conditions. Here we conduct a multivariate analysis of the atmospheric QBWISO to both assess its overall characteristics in multiple oceanic variables and analyze how this atmospheric signal interacts with the underlying ocean. We utilize a combination of satellite observations and the Nucleus for European Modeling of the Ocean version 3.4 (NEMOv3.4) ocean model simulated temperature, salinity and mixed layer depth to examine the characteristics of QBWISO for the 2016–2018 Indian Summer Monsoon seasons. This study reveals that both the zonal and meridional double-cell structures in QBWISO precipitation are phase-locked during the southwest monsoon and positively enhance/suppress the precipitation over northern BoB leading to higher amplitudes in QBWISO sea surface salinity (SSS), compared to those in the central and southern BoB. The NEMO SSS also supports the occurrence of higher SSS anomalies at QBWISO period in the northern BoB. We find that NEMO temperature has a strong biweekly signal in the central and southern BoB down to 250 m depth, with the mixed layer temperature showing a marked decrease after the QBWISO precipitation maximum. Comparatively, at subsurface depths the QBWISO signal in NEMO salinity shows a slight increase in thermocline in the central and southern BoB, suggesting that the subsurface Arabian Sea high salinity water mass is affected by the QBWISO.