Bay of Bengal salinity stratification and Indian summer monsoon intraseasonal oscillation: 1. Intraseasonal variability and causes

Abstract

The huge freshwater flux of the Indian summer monsoon (ISM; May–October) gives rise to strong salinity stratification in the Bay of Bengal (BoB), causing a shallow mixed layer and a thick barrier layer, which potentially affects intraseasonal oscillations of the monsoon (MISOs). In this study, intraseasonal variability of the mixed‐layer depth (MLD) and barrier layer thickness (BLT) is investigated using in situ observations from Argo floats and moored buoys and an ocean general circulation model (OGCM). The average MLD in the BoB is typically 20–30 m during the ISM, while the BLT increases from ∼10 m in May–June to 20–40 m in September–October. MISOs induce in‐phase variations in MLD and isothermal layer depth (ILD), both of which are deepened by 8–15 m during MISO active phase, while the change of BLT is small and within the error range of Argo data sampling. In the northern (southern) bay, BLT increases by ∼5 m (2 m) during MISOs owing to a larger deepening of ILD than MLD. OGCM experiments are performed to understand the underlying mechanism. In the BoB intraseasonal variations of MLD, ILD and BLT arise largely from ocean internal instability, whereas those induced by MISOs are weaker. The in‐phase variations of MLD and ILD during MISOs are induced by different processes. The MLD deepening is primarily caused by wind stress forcing, while the ILD deepening is driven by surface heat fluxes via surface cooling. The limited variability of BLT is due to the offsetting of different forcing processes.

Origins of Eddy Kinetic Energy in the Bay of Bengal

“…However, the EKE patterns in the NoISO (Figure b) and NoSTRESS (Figure c) experiments are strikingly similar. This demonstrates that the effects of surface heat and freshwater fluxes on EKE are small, although their impacts on ocean temperature and salinity are large (Li et al, ). In Regions 1, EKE is 6.6% enhanced in the NoSTRESS experiment compared with the NoISO experiment, and in Region 2, EKE is 8.6% stronger in NoISO than in NoSTRESS (Figure ).…”

Seasonality and Buoyancy Suppression of Turbulence in the Bay of Bengal

“…Modeling studies have shown that the coupled dynamics of the ocean‐atmosphere system, at timescales as short as the diurnal, strongly affect the structure and variability of the large‐scale tropical atmosphere (Fu et al, ; Inness & Slingo, ; Woolnough et al, ). Observations have further demonstrated the coevolution of the atmosphere and ocean in response to monsoon forcing in the Indian Ocean, including within the Madden‐Julian Oscillation (McPhaden & Foltz, ; Moum et al, , ), and during boreal summer intraseasonal oscillations (Li et al, ; Sengupta & Ravichandran, ). While sea surface temperature (SST) controls coupling at the air‐sea interface, SST is itself influenced by net atmosphere‐ocean heat flux, turbulent entrainment of typically cooler deep water into the mixed layer (ML), horizontal and vertical advection, and the ocean's mixed layer depth (MLD).…”

The response of ocean parameters to tropical cyclones in the Bay of Bengal

“…The definition of the barrier layer is the region between the base of the mixed layer to the bottom of the isothermal layer (Balaguru et al ., ). Also, it exhibits seasonal variation showing strong stratification (and thick barrier layer) in the post‐monsoon season and less stratification (thin barrier layer) in the pre‐monsoon season (Li et al ., ). Therefore, considering this fact, post‐monsoon TCs are relatively stronger than those in the pre‐monsoon season (Mohapatra et al ., ).…”