The Role of Salinity in the Southeastern Arabian Sea in Determining Monsoon Onset and Strength

Abstract

In this study, we examine the role of freshwater transported from the Bay of Bengal into the southeastern Arabian Sea (SEAS) in determining both the timing of monsoon onset and the strength of the ensuing monsoon. To do this, we use a combination of satellite-derived salinity data and reanalysis products from 1980 to 2016 in order to discuss seasonal, interannual, subdecadal, and long-term decadal variability in various atmospheric and oceanic contributors such as ocean heat content, freshwater transport, ERA5 instantaneous moisture flux, mixed layer depth, and barrier layer thickness. The salt budget in the SEAS reveals the dominance of meridional freshwater advective transport that brings in variability in salinity and thus in salinity stratification, as well as in mixed layer processes. We find that these parameters exhibit the prevalence of a long-term decadal variability (with trends of increasing/decreasing phases over a 15-year duration) over which interannual (3-year trend) and intradecadal (7-year trend) variability trends are superimposed. The patterns of both the 3-and 7-year trends follow each other closely, and relatively high amplitudes are seen in the 3-year trends. The long-term decrease in moisture flux and freshwater transport into the SEAS along with a rise in ocean heat content over a 15-year duration after 1994 contributed to a lack of strong monsoons in recent years; the prevailing interannual and intradecadal variability in these parameters associated with the Indian Ocean Dipole and El Niño Southern Oscillation events favored weaker and normal monsoons after 1994. Plain Language SummaryThe timing and intensity of the southwest monsoon is difficult to forecast and has been the subject of a great deal of research. In particular, the genesis of a monsoon onset vortex over the southeastern Arabian Sea (SEAS) encompassing the Arabian Sea Mini Warm Pool has received increased attention for the role it plays on the onset of the southwest monsoon. In our study, we examine how the low salinity waters transported from the Bay of Bengal (both along the east coast of India and from the southern Bay of Bengal) into the SEAS region influence the strength and timing of the ensuing southwest monsoon. Our finding is that this low salinity water has a significant impact on barrier layer formation in the SEAS and the vertical thermal structure of the Arabian Sea Mini Warm Pool. Further examination of subdecadal, decadal, and long-period decadal trends from 1980 to 2016 shows that instantaneous moisture fluxes and zonal winds over the SEAS have decreased substantially, and barrier layer thickness has increased since 1994 in association with the increasing trend in ocean heat content and contributed to the lack of strong southwest monsoons after 1994.

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: where H is water depth (40 m in this study), z 0 is the ocean surface, ρ is the density of seawater calculated at each depth using the equation of state, C p is the specific heat capacity of seawater (3,930 J kg −1 K −1 ; Horii et al, 2009), and T is the mean temperature over dz , the depth thickness.…”

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

“…MLD was calculated using the 0.03-kg/m 3 density criterion as the threshold value, and both MLD and ILD calculations used an equivalent temperature difference of 0.2°C (de Boyer Montégut et al, 2004). The 0.03-kg/m 3 density criterion has been shown to be appropriate for near-equatorial regions with strong diurnal variability, such as the BoB, and is thus employed in this study (de Boyer Montégut et al, 2004;Roman-Stork et al, 2020a). monsoon rainfall was very low in June (Table 2).…”

“…The Indian Institute of Tropical Meteorology defines strong (weak) monsoons as those whose rainfall totals exceed (fall below) 10% of the long‐term mean. By this criterion, there were only three strong monsoons since 1980 until the strong monsoon of 2019 (Roman‐Stork et al, 2020a). The 2019 monsoon season was the first strong monsoon since 1994, after a gap of 25 years, and its occurrence motivates this research.…”

“…The 2019 monsoon season was the first strong monsoon since 1994, after a gap of 25 years, and its occurrence motivates this research. A recent study by Roman‐Stork et al (2020a) identified the prevalence of long‐term decadal variability over a 15‐year duration in the southeastern Arabian Sea in various oceanic and atmospheric parameters, such as moisture flux, barrier layer thickness, ocean heat content, and surface winds. It was found that a combination of long‐term changes in these parameters contributed to the 25‐year pause in strong monsoons since 1994 (Roman‐Stork et al, 2020a).…”

Ocean–Atmosphere Interactions during Hurricanes Marco and Laura (2020)

“…As of August 2019, ERA5 replaced the ERA-Interim reanalysis with heightened spatial and temporal resolutions, improvements in atmospheric parameters, and most importantly for this study, improved consideration of tropical cyclones [31,32]. Instantaneous moisture flux represents evaporation as an upward flux from the ocean surface to the atmosphere (negative values) and condensation as a downward flux from the atmosphere to the ocean surface (positive values) [33].…”