Response of the Bay of Bengal to 3‐7‐Day Synoptic Oscillations During the Southwest Monsoon of 2019

Abstract

The satellite‐derived precipitation over central India (73–78°E, 20–25°N) during the 2019 southwest monsoon season reveals that high‐amplitude 3‐7‐day synoptic oscillations dominated the other intraseasonal oscillations and contributed to the excess seasonal monsoon rainfall. In this paper, we address the contribution of 3‐7‐day synoptic oscillations in atmospheric and oceanic parameters during the 2019 southwest monsoon season over the Bay of Bengal (BoB). Our study reveals that both satellite observations and the Nucleus for European Modeling of the Ocean (NEMO) model simulations show high‐amplitude 3‐7‐day synoptic scale events during the 2019 monsoon. The upper 300 m of the water column responds to the basin‐scale 3‐7‐day synoptic oscillations as revealed in the vertical structures of NEMO model simulated temperature, salinity, and currents in the BoB. The synoptic scale wind stress drives strong currents in the shallow mixed layer due to salinity stratification in the northern BoB. The northward propagation of the synoptic scale oscillations is evident across the BoB but is capped to the base of the mixed layer, above which the propagation is affected by wind stress and the spread of freshwater flux. The 3‐7‐day synoptic scale oscillations in NEMO sea surface height appear to be related to synoptic scale wind‐driven convergence and divergence of waters through mesoscale eddy circulations that in turn contribute to thermocline variability, rather than the temperature and salinity in the mixed layer.

Interactions Between Mesoscale Eddies and Synoptic Oscillations in the Bay of Bengal During the Strong Monsoon of 2019

“…As in Roman-Stork, Subrahmanyam, and Murty (2020) and Nyadjro et al (2012), upper ocean heat content (OHC) in the 0-40 m layer is calculated using NEMO outputs in the equation:…”

“…The 10-20-day mode is a westward-propagating double-cell system in the equatorial Indian Ocean and BoB (Chatterjee & Goswami, 2004;Kikuchi & Wang, 2009;Murakami, 1976;. The 3-7-day mode, composed of synoptic disturbances and oscillations in the monsoon trough, is a primary contributor to the active and break cycles in the monsoon (Goswami et al, 2003;Subrahmanyam et al, 2020). With the monsoon system being a highly coupled phenomenon, a number of studies have focused on the atmospheric implications of these 3-7-day ISOs (Gadgil & Asha, 1992;Gadgil & Joseph, 2003;Goswami & Ajaya Mohan, 2001;Goswami et al, 2003;Krishnamurti & Bhalme, 1976;Murakami, 1976;Rajeevan et al, 2010;Singh & Nakamura, 2010), and some more recent studies have observed the oceanic response to these oscillations (Goswami et al, 2016;Roman-Stork, Subrahmanyam, & Trott, 2020;Subrahmanyam et al, 2020); however, there remains a gap in the knowledge as to how mesoscale eddies could be in turn influencing these synoptic oscillations.…”

“…Similar to the IOD and ENSO, there are a number of intraseasonal oscillations (ISOs) operating within the Indian monsoon system that impact the active (wet) and break (dry) phases of the monsoon (Rajeevan et al, 2010; Roman‐Stork, Subrahmanyam, & Murty, 2019; Subrahmanyam et al, 2020; Trott, Subrahmanyam, Roman‐Stork, et al, 2019; Vecchi & Harrison, 2002). The ISOs include the 30–90‐day Madden‐Julian Oscillation (MJO), the 10–20‐day quasi‐biweekly mode, and 3–7‐day synoptic events, with the 3–7‐day oscillations being the most understudied.…”

“…The 10–20‐day mode is a westward‐propagating double‐cell system in the equatorial Indian Ocean and BoB (Chatterjee & Goswami, 2004; Kikuchi & Wang, 2009; Murakami, 1976; Roman‐Stork, Subrahmanyam, & Murty, 2019). The 3–7‐day mode, composed of synoptic disturbances and oscillations in the monsoon trough, is a primary contributor to the active and break cycles in the monsoon (Goswami et al, 2003; Subrahmanyam et al, 2020).…”

“…With the monsoon system being a highly coupled phenomenon, a number of studies have focused on the atmospheric implications of these 3–7‐day ISOs (Gadgil & Asha, 1992; Gadgil & Joseph, 2003; Goswami & Ajaya Mohan, 2001; Goswami et al, 2003; Krishnamurti & Bhalme, 1976; Murakami, 1976; Rajeevan et al, 2010; Singh & Nakamura, 2010), and some more recent studies have observed the oceanic response to these oscillations (Goswami et al, 2016; Roman‐Stork, Subrahmanyam, & Trott, 2020; Subrahmanyam et al, 2020); however, there remains a gap in the knowledge as to how mesoscale eddies could be in turn influencing these synoptic oscillations.…”