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

Abstract

The 2019 southwest monsoon season is the first strong monsoon in the last 25 years and occurred concurrently with a strong positive Indian Ocean Dipole (pIOD). Given these unique circumstances, we examine the mesoscale eddy field in the Bay of Bengal (BoB) during 2019 and the interactions between these eddies and 3-7-day synoptic oscillations and tropical systems, which contribute to monsoon rainfall and modulate the active (wet) and break (dry) phases of the Indian monsoon. We find that there is a drastic difference in eddy characteristics between the western (80-90°E) and eastern (90-100°E) BoB, where eddies in the eastern BoB are dominated by coastal Kelvin wave (CKW) dynamics and topographical influences, and the eddies in the western BoB are more heavily modulated by East India Coastal Current dynamics. We further find that the pIOD increased the overall sea level in the BoB and modified CKW propagation in the BoB such that all eddy types had more positive sea level anomalies. We find that cyclonic eddies vary coherently with both upper ocean heat content and 3-7-day bandpass filtered precipitation and that both the number of cyclonic eddies and eddy amplitude increase in coherence with precipitation and ocean heat content following the passage of a tropical system. These results suggest a strong relationship between cyclonic eddies and 3-7-day atmospheric variability. Plain Language Summary The 2019 southwest monsoon is the first strong monsoon since 1994 and also occurred during a strong positive Indian Ocean Dipole event. We analyze the mesoscale eddy field in the Bay of Bengal during 2019 and investigate the connection between these eddies and 3-7-day synoptic oscillations, which control the active (flood) and break (drought) phases of the monsoon over India. We find that coastal Kelvin waves influence eddies in both the eastern and western Bay, but that ocean bottom topography and the western boundary current dominate each region's dynamics, respectively. The positive Indian Ocean Dipole heavily impacted the coastal Kelvin waves in 2019. 3-7-day synoptic events were most closely associated with cyclonic eddies, especially in the western Bay. The number of eddies and eddy amplitudes both increased following the passage of a tropical system. We also recognized a tendency for the tropical systems in 2019 to pass over cyclonic eddies and intensify afterward. Our results suggest a strong relationship between 3-7-day synoptic-scale variability and cyclonic eddies in the Bay of Bengal.

Lakshadweep High Propagation and Impacts on the Somali Current and Eddies During the Southwest Monsoon

“…As in Greaser et al (2020), we calculate the vertical velocity (w) using the zonal (u) and meridional (v) components of ocean currents from GLORYS12v1 by integrating the continuity equation:…”

Reviews and syntheses: Trends in primary production in the Bay of Bengal – is it at a tipping point?

“…Satellite data from 1998 to 2015 suggest a decrease in primary production in the global ocean (Gregg et al, 2003;Behrenfeld et al, 2006), and recent studies deducted a decrease in ocean primary production of 2.1 % per decade associated largely with a decrease in chlorophytes in the marine photic realm (Gregg et al, 2017;Gregg and Rousseaux, 2019). However, a recent study derived a nonlinear trend in primary production from a similar time episode, between 1998 and 2018 (Kulk et al, 2020).…”

“…However, a recent study derived a nonlinear trend in primary production from a similar time episode, between 1998 and 2018 (Kulk et al, 2020). Decreasing rates of primary production have been associated with high-latitude regions (Gregg et al, 2003), but also with the northern and equatorial Indian Ocean with a decrease of 9.7 % and 17.2 % per decade, respectively (Gregg and Rousseaux, 2019). These estimates, based on satellite imaging, were explained by a decrease in diatom and chlorophyte primary production of 15.4 % and 24.8 % per decade, respectively, for both the BoB and its sister basin, the Arabian Sea (Fig.…”

Impacts of the 2019 Strong IOD and Monsoon Events on Indian Ocean Sea Surface Salinity

“…It is however strongly impacted by remote forcing through Kelvin waves, which causes it to occasionally oppose local winds and flow in unexpected directions [12,19,36,51]. In addition, westwardtraveling Rossby waves, forced by variable winds in the interior of the BoB, impact the EICC [22,36,59]. A study by [12] suggests that the EICC is most unstable, disorganized, and weak during PIOD events.…”

“…Consequently, the IOD and ENSO can impact the Indian Ocean both collectively and independently. The 2019 PIOD event is the strongest IOD event thus far during the last four decades and co-occurred with an El Niño [22,60]. It is reported to have been caused by a strong interhemispheric pressure gradient between a stronger than usual pressure over Australia and a weaker than usual pressure over South China Sea/ Philippine Sea which led to a northward flow over the western Maritime Continent that generated a significant air-sea heat flux and thermocline feedback [16,34].…”