Intensification of tidally generated internal waves in the north-central Bay of Bengal

Abstract

Flow of barotropic tidal currents over topographic features, such as continental slopes and submarine ridges, generates internal gravity waves at tidal periods known as internal tides. Amplitude of these waves are generally large near the generation regions. Analysis of Sea Surface Height (SSH) data, derived from satellite altimeter revealed the amplification of internal tides in the semidiurnal period in the north-central Bay of Bengal (BoB) (around 89  E, 16  N), which is about 450 km away from their generation sites. SSH signals found in the north-central BoB (~3 cm) were comparable to the maximum amplitudes (2.5 to 3.5 cm) observed near their potential generation sites in the BoB such as continental slopes in the head of the bay and Andaman-Nicobar (AN) Ridge. Simulations from a high-resolution regional ocean model also confirmed the presence of large internal tide amplitude in the north-central BoB. Our study revealed that convergence of internal tides, which were generated along the concaveshaped source (continental slopes in the head of the bay and the northern parts of AN Ridge), into its focal region caused their amplification in the north-central BoB. It was also found that internal tide energy dissipation rates in this focal region were about 10 times larger than those in other open ocean regions.Periodic flow of tidal currents over steep topography generates vertical oscillations of isopycnal surfaces in the stratified ocean. These internal oscillations propagate away as gravity waves, known as internal tides. Continental margins and submarine ridges are the main sources of this tidally-generated internal gravity waves in the ocean. Previous studies suggest that about 30% of the energy associated with internal tides, especially with high vertical modes, dissipates locally near the generation sites themselves and the remaining 70% of the energy radiates from the source, propagates long distances (O (1000 km)) and ultimately dissipates in deep ocean or in remote continental margins 1-3 . Dissipation of internal tides is one of the important sources of mechanical energy for the vertical mixing in the interior ocean 4 . Several mechanisms have been suggested for the dissipation of long-range propagating internal tides such as interactions with rough topography 5 , interactions with mean flows and eddies 6 , cascade to smaller scales via wave-wave interactions 7 etc. However, the pathways of internal tide energy in the ocean and regions of their energy dissipation are not completely understood.Generally, strong baroclinic currents and large isopycnal displacements at tidal frequencies are observed near their generation sites and their amplitude decreases away from the sources 8,9 . However, spatial variation of internal tides and associated energy dissipation in the ocean can be more complicated due to the interaction of waves from multiple sources 10 . For example, Wang et al. 11 reported that interference of internal tides from different sources results in large spatial inhomogeneity in the energy flu...

Formation of an Intrathermocline Eddy Triggered by the Coastal‐Trapped Wave in the Northern Bay of Bengal

“…Model‐simulated currents and temperature were validated with velocity measurements obtained from ADCP moorings along the western BoB and open ocean moorings in the BoB (Francis et al., 2020; Jithin et al., 2017). Simulations from this model were also used to study boundary currents (Francis et al., 2020), internal tides (Jithin & Francis, 2020; Jithin, Francis, et al., 2020, Jithin, Subeesh, et al., 2020), and coastal‐trapped waves along the boundaries of the BoB (Anup et al., 2021). Therefore, model validation in this paper is restricted to the comparison of observed and simulated ITE in the northern BoB using observations from RAMA moorings.…”

Role of internal tide mixing in keeping the deep Andaman Sea warmer than the Bay of Bengal

“…2c). Recently, Jithin et al 24,39 have reported that the internal tides, which radiate westward from the AN Ridge, propagate over large distances (1,000-1,400 km) across the BoB. They further showed that a part of these internal tides that shoal onto the continental margins of the east coast of India get further transmitted on to the inner shelf and eventually dissipate there.…”

“…Angle brackets ( ) indicate average over a tidal period 44 . Detailed discussion on the internal tide energy budget and its estimation can be found in Jithin et al 24 , 39 .…”

“…TUGOm Tidal ToolBox 38 is used to extract tidal signals from model output. Validation of the internal tides simulated by this model against observations from several Acoustic Doppler Current Profilers (ADCPs) deployed along the east coast of India 24 , and the data from the RAMA (Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction) moorings in the BoB and the deep ocean moorings from AS as well as the internal tide signals obtained from satellite altimetry data 39 have shown that the model simulates the observed features of internal tides in this region very well. Further details of the model configuration and validation of the subtidal circulation simulated by this model can be seen in Jithin et al 28 and Francis et al 29 .…”

A Seasonal Harmonic Model for Internal Tide Amplitude Prediction

“…Standing internal tides with seasonal shifts have been reported in other regions e.g., the Hawaiian Ridge (Rainville et al, 2010), the South China Sea (Ray & Zaron, 2011), and the Bay of Bengal (Jithin et al, 2020). Zaron (2019) reported the global distribution of seasonally modulated 2 E M baroclinic sea surface height variance (…”