A snapshot of internal waves and hydrodynamic instabilities in the southern Bay of Bengal

Abstract

Measurements conducted in the southern Bay of Bengal (BoB) as a part of the ASIRI‐EBoB Program portray the characteristics of high‐frequency internal waves in the upper pycnocline as well as the velocity structure with episodic events of shear instability. A 20 h time series of CTD, ADCP, and acoustic backscatter profiles down to 150 m as well as temporal CTD measurements in the pycnocline at z = 54 m were taken to the east of Sri Lanka. Internal waves of periods ∼10–40 min were recorded at all depths below a shallow (∼20–30 m) surface mixed layer in the background of an 8 m amplitude internal tide. The absolute values of vertical displacements associated with high‐frequency waves followed the Nakagami distribution with a median value of 2.1 m and a 95% quintile 6.5 m. The internal wave amplitudes are normally distributed. The tails of the distribution deviate from normality due to episodic high‐amplitude displacements. The sporadic appearance of internal waves with amplitudes exceeding ∼5 m usually coincided with patches of low Richardson numbers, pointing to local shear instability as a possible mechanism of internal‐wave‐induced turbulence. The probability of shear instability in the summer BoB pycnocline based on an exponential distribution of the inverse Richardson number, however, appears to be relatively low, not exceeding 4% for Ri < 0.25 and about 10% for Ri < 0.36 (K‐H billows). The probability of the generation of asymmetric breaking internal waves and Holmboe instabilities is above ∼25%.

Satellite-Observed Multi-Scale Variability of Sea Surface Chlorophyll-a Concentration along the South Coast of the Sumatra-Java Islands

“…Nevertheless, lag correlation and composite analyses suggest that propagation of the coastal trapped intraseasonal Kelvin waves is responsible for the intraseasonal SSC anomalies along the south coast of Sumatra and Java. In addition, the intraseasonal SSC anomalies south of the Sunda and Lombok straits are attributed to the active mesoscale eddies in the southeastern tropical Indian Ocean, which is reminiscent the phenomenon that high SSC co-occurred with the Sri Lanka dome [5][6][7].…”

“…2021, 13, x FOR PEER REVIEW 2 of 20 deeper layer and runoff, respectively (Figure 1a). The global SSC is found subject to multiscale variability associated with ocean dynamical processes such as upwelling, mesoscale eddies, and sea ice melting [4][5][6][7]. Previous investigations show no significant trend in global annual median chlorophyll-a from 1998 to 2015 [8].…”

“…High sea surface chlorophyll-a concentration (SSC) areas generally are identified in the upwelling and coastal areas feeding by nutrient-rich water from deeper layer and runoff, respectively (Figure 1a). The global SSC is found subject to multi-scale variability associated with ocean dynamical processes such as upwelling, mesoscale eddies, and sea ice melting [4][5][6][7]. Previous investigations show no significant trend in global annual median chlorophyll-a from 1998 to 2015 [8].…”

Study on Internal Waves over the Bay of Bengal Using ALOS PALSAR L band SAR Imageries

“…Phaniharam et al 10 studied IW characteristics along the northwestern BOB using SAR imagery. In addition, using in situ observations in the southern BOB, Lozovatsky et al 11 studied highfrequency internal wave activity, and Wijesekera et al 12 studied their characteristics and dynamics. Later, modeling studies by Jensen et al 13 revealed that the IWs generated off the Andaman and Nicobar…”

Turbulence and Mixing in Energetic Environment of the Coastal Waters of Sri Lanka

“…Formation of mesoscale eddies and inertial waves [8,9] in the BoB stir water in the interior of the Bay, leading to a cascade of smaller scale local instabilities. Turbulence so generated produces mixing and vertical exchange of physical properties as well as biogeochemical fluxes, influencing the BoB ecosystem and strong near surface stratification in the region restricts turbulence to a thin surface boundary layer [10].…”