Variability of Intraseasonal Oscillations and Synoptic Signals in Sea Surface Salinity in the Bay of Bengal

Abstract

Intraseasonal oscillations (ISOs) significantly impact southwest monsoon precipitation and Bay of Bengal (BoB) variability. The response of ISOs in sea surface salinity (SSS) to those in the atmosphere is investigated in the BoB from 2005 to 2017. The three intraseasonal processes examined in this study are the 30–90-day and 10–20-day ISOs and 3–7-day synoptic weather signals. A variety of salinity data from NASA’s Soil Moisture Active Passive (SMAP) and the European Space Agency’s (ESA’s) Soil Moisture and Ocean Salinity (SMOS) satellite missions and from reanalysis using the Hybrid Coordinate Ocean Model (HYCOM) and operational analysis of Climate Forecast System version 2 (CFSv2) were utilized for the study. It is found that the 30–90-day ISO salinity signal propagates northward following the northward propagation of convection and precipitation ISOs. The 10–20-day ISO in SSS and precipitation deviate largely in the northern BoB wherein the river runoff largely impacts the SSS. The weather systems strongly impact the 3–7-day signal in SSS prior to and after the southwest monsoon. Overall, we find that satellite salinity products captured better the SSS signal of ISO due to inherent inclusion of river runoff and mixed layer processes. CFSv2, in particular, underestimates the SSS signal due to the misrepresentation of river runoff in the model. This study highlights the need to include realistic riverine freshwater influx for better model simulations, as accurate salinity simulation is mandatory for the representation of air–sea coupling in models.

Monitoring Intraseasonal Oscillations in the Indian Ocean Using Satellite Observations

“…The 10‐20‐day mode, or quasi‐biweekly oscillation, is a westward propagating system over the Bay of Bengal (BoB) and equatorial Indian Ocean (Chatterjee & Goswami, ; Kikuchi & Wang, ; Murakami, ; Roman‐Stork et al, ). The 3‐7‐day mode is due to oscillations in the monsoon trough and is chiefly responsible for the active and break cycles of the southwest monsoon that have spectral peaks between 4 and 5 days (Trott et al, ). These ISOs are responsible for a significant portion of monsoonal variability, and while there has been extensive ISO research on the atmospheric side, there remains a gap in knowledge and understanding how oceanic parameters respond to and influence both the atmosphere and other oceanic parameters.…”

“…While the structure and dynamics of the 10‐20‐day mode in the atmosphere are reasonably well understood, the corresponding signature and dynamics in the ocean are less so, with few studies focused on the ocean (Chatterjee et al, ; Miyama et al, ; Roman‐Stork et al, ; Sengupta et al, ; Sengupta et al, ; Subrahmanyam et al, ; Trott et al, ; Trott & Subrahmanyam, ). Ocean currents and meridional surface winds in the eastern equatorial Indian Ocean were found to exhibit strong biweekly variability, with fluctuations in this wind stress exciting a westward wave response that is associated with a pattern of upwelling and downwelling along the coastal boundary (Sengupta et al, ; Sengupta et al, ).…”

“…Ocean currents and meridional surface winds in the eastern equatorial Indian Ocean were found to exhibit strong biweekly variability, with fluctuations in this wind stress exciting a westward wave response that is associated with a pattern of upwelling and downwelling along the coastal boundary (Sengupta et al, ; Sengupta et al, ). Within the BoB, SSS responses have been found to be predominantly forced by river runoff in the northern Bay and by wind‐induced Ekman processes in the central Bay for all ISOs, with the signal in the northern Bay being the strongest (Roman‐Stork et al, ; Subrahmanyam et al, ; Trott et al, ). Mixed Rossby Gravity (MRG) or Yanai waves in the Indian Ocean that also occur on biweekly timescales and have been shown to have the same structure as the 10‐20‐day mode (Chatterjee et al, ; Miyama et al, ; Sengupta et al, ).…”

“…The 3‐7‐day oscillations in the monsoon trough, or tropical convergence zone, are representative of the active (wet) and break (dry) cycles of the monsoon (Gadgil & Asha, ; Gadgil & Joseph, ; Goswami et al, ; Krishnamurti & Bhalme, ; Trott et al, ). The monsoon trough is primarily positioned over land around 25°N (Krishnamurti & Bhalme, ), with only a secondary mode over the ocean (Gadgil & Asha, ), and so the 3‐7‐day mode in the ocean is much weaker than the other ISOs.…”

“…These active and break phases are heavily influenced by other ISOs, both through the cascading of energy and through dynamic interactions of these ISOs with the monsoon trough (Gadgil & Asha, ; Trott & Subrahmanyam, ). While there have been numerous studies that have examined the impact of various oceanic and atmospheric processes on active and break cycles of the monsoon (Gadgil & Joseph, ), very few have directly focused on the 3‐7‐day mode in the ocean (Subrahmanyam et al, ; Trott et al, ; Trott & Subrahmanyam, ).…”