Impact of stratification on internal waves and differential wearing of thermal inversions on the east coast of India

Abstract

The surface layers of the Bay of Bengal along the east coast of India exhibit intricate stratification owing to the differential distribution of freshwaters. The winter (January–February) cooling of the salinity-induced stable layers results in the development of thermal inversions that deteriorate toward the end of the season. The study focuses on the behavior of the thermal inversions in the light of the variable stratification and the monsoon imposed reversing coastal current. To address the associated processes, a three-dimensional Princeton Ocean Model is applied for the east coast of India, and numerical experiments carried out to study the means by which the thermal inversions tend to perish with the passage of winter. The model domain with variable curvilinear grid uses input fields that comprise realistic bathymetry and initial temperature/salinity conforming to winter/specified stratification. The surface forcing comprises wind stress and diurnal pattern air–sea heat fluxes. The body forcing is derived from the periodic tidal elevations at the open boundaries. It has been found that the thermal inversions tend to sustain as the equator-ward flowing East India Coastal Current (EICC) traps the cool low saline waters between Paradip and Kakinada. The current off Paradip is weak and variable and is not a part of EICC. Consequently, in the absence of replenishment of cool and freshsurface waters, the temperature/salinity gradients get eroded steadily. No thermal inversions are noticed south of Kakinada because of relatively weak current with diminished vertical salinity gradient. As the nature of stratification encountered in the bay is highly variable due to diverse reasons, the behavior of internal waves under different stratification scenarios is also addressed. Numerical experiments indicate that the energy/amplitude of the internal waves are comparable in the surface layers for any stratification, where as it is certain orders exalted in the deeper waters of the strong stratification scenario. Further, it is found that the energies and pattern of the temperature oscillations conform to the nature of mixed tide at the corresponding latitude. The underneath stratification is found to be more responsible for the generation of internal waves compared to the local stratification. This implies that the body forcing emanating from below is the cardinal contributor for the generation of internal waves. The numerical experiment with a flat and uniform bottom showing weak manifestation of internal waves endorses the same. This connotes that the continental slopes are an effective generator of the internal waves and the energy flux conversion of the barotropic tide to internal waves seems to be heavily dependent on the shoaling bottom.

Appendix

An overview of the relative importance/contribution of the boundary/body forcing is addressed here. Numerical experiments are carried out considering only tidal forcing (no heat flux) and only surface heat fluxes (no tide) with common wind stress forcing in the strong stratification scenario. Figure 15 depict the spectral estimates at 5 and 20 m depth, respectively, for the tide only experiment. The energy in the semidiurnal range (0.08 cph ≈ 12.5 h) at 5 m depth is weak, which has increased substantially at 20 m. There is no energy in the diurnal range (0.04 cph ≈ 24.0 h) associated with tidal forcing and the semidiurnal energy substantially increased with depth. Figure 16 depict the spectral estimates at 5 and 20 m depth, respectively, for the flux only experiment. Here also the energy in the semidiurnal range (0.08 cph ≈ 12.5 h) at 5 m depth is weak but unlike the tide only scenario, the crux of the energy here is in diurnal range (0.04 cph ≈ 24 h). The scenario at 20 m depth is again different. The semidiurnal component is missing all together and the diurnal one has substantially dropped. From these experiments, we may conclude that the influence of the surface heat fluxes are attenuated and weaken sharply with depth while on the contrary, the body forcing assumes strength with depth. As the internal wave activity is essentially in the deeper waters (below the mixed layer), we may conclude that the principal contributor to the generation of internal waves is only the tidal forcing.

Fig. 15

Energy spectra of temperature oscillations only with tide (no flux) at depths 5 and 20 m

Fig. 16

Energy spectra of temperature oscillations only with flux (no tide) at the same depths as in Fig. 15