One approach to estimating vertical heat diffusivity (K T) is to compute it from the residual of the mixed layer (ML) heat budget. Based on this approach, we use moored buoy data at 15°N, 12°N, and 8°N along 90°E over the period of 2007-2018 to estimate the seasonal average of K T at the base of the mixed layer in the Bay of Bengal (BoB). We find that K T is lower during spring and higher during winter compared to summer and fall at the mooring locations. Moreover, K T is generally higher in the southern BoB compared to the northern BoB. The present study also shows that the seasonal and spatial variability of K T is modulated both by stratification at the base of ML and by seasonal and spatial heterogeneity in atmospheric forcing, most notably wind stress and buoyancy flux. The availability of information on the spatial and seasonal variability of K T in the BoB will facilitate evaluation and validation of turbulent mixing parameterization schemes incorporated into ocean models.
The observed seasonal and intraseasonal evolution of near‐surface meteorological and oceanographic variables in the Andaman Sea for the period March 2014 to December 2017 are examined using moored buoy observations at 10.5°N, 94°E. The amplitude of temperature inversions is very weak (0.2 to 0.4 °C), and they appeared primarily during winter (November–January) and latter part of summer (May–August). The net surface heat flux plays a primary role, and vertical processes term contributes secondarily to determine the seasonal mixed layer (ML) heat storage variability. Consistent with the seasonal variations of formation and strength of temperature inversion, vertical processes term shows a positive tendency during winter. The sea surface salinity shows large amplitude intraseasonal variability during fall and winter, and it is attributed to the variability of horizontal circulation in the presence of large lateral sea surface salinity gradients at the mooring location. The sea surface temperature shows the presence of strong intraseasonal variability between 20 and 80 days, though its amplitude of oscillation is distinctly higher during May–October than November–April. Band‐pass filtered (20–80 days) time series of different components of the ML heat budget shows that the net surface heat flux primarily determines the intraseasonal ML heat storage variability. Our analysis further shows that during May–October, both net shortwave radiation and latent heat flux together determine the modulation of the intraseasonal net surface heat flux. In contrast, latent heat flux acts as the sole factor to determine the modulation of the intraseasonal net surface heat flux during November–April.
Microstructure measurements from two cruises during winter and spring 2019 documented the importance of double-diffusion processes for small-scale mixing in the upper 400 m of the open-ocean region of the eastern Arabian Sea (EAS) below the mixed layer. The data indicated that shear-driven mixing rates are weak, contributing diapycnal diffusivity (Kρ) of not more than 5.4 × 10−6 m2 s−1 in the EAS. Instead, signatures of double diffusion were strong, with the water column favorable for salt fingers in 70% of the region and favorable for diffusive convection in 2%–3% of the region. Well-defined thermohaline staircases were present in all the profiles in these regions that occupied 20% of the water column. Strong diffusive convection favorable regime occurred in ∼45% of data in the barrier layer region of the southern EAS (SEAS). Comparison of different parameterizations of double diffusion with the measurements of vertical heat diffusivity (KT) found that the Radko and Smith salt fingering scheme and the Kelley diffusive convection scheme best match with the observations. The estimates based on flux law show that the combination of downward heat flux of approximately −3 W m−2 associated with salt fingering in the thermocline region of the EAS and the upward heat flux of ∼5 W m−2 due to diffusive convection in the barrier layer region of the SEAS cools the thermocline.
Sea surface temperatures (SSTs) simulated by almost all models in Coupled Model Inter‐comparison Project phase five are consistently colder (∼−1°C to −4°C) than the observation in the northern Arabian Sea (AS) during winter and spring. These biases significantly weaken the seasonal and extended summer monsoon prediction skills and climate change projection's reliability in this region. To understand the relative contribution of diapycnal heat flux (Jh) compared to other terms in the mixed layer (ML) temperature (MLT) budget during these two seasons, we use time series of vertical profiles of microstructure shear measurements collected in the northeastern AS (NEAS) at 18.4°N and 67.4°E during January 10–17, 2019 (W19) and May 7–21, 2019 (S19). It is found that the vertical processes term and net surface heat flux together determine the bulk of MLT tendency during S19 and W19, and the contribution of the horizontal advection and Jh is relatively smaller. The mean value of Jh (∼−2 W m−2) at the base of the ML shows a comparable magnitude during W19 and S19, and the median values of diapycnal diffusivity (Kρ) at the ML base are not significantly different between W19 (∼3.1 × 10−6 m2 s−1) and S19 (∼5.2 × 10−6 m2 s−1). Besides, Kρ and Jh values were estimated using different Kρ‐Richardson number (Ri) based interior ocean parameterization schemes at the ML base in the NEAS overestimated with respect to observation. The alleged role of overestimation of Kρ and Jh in parameterization schemes on model simulation of cold SST bias in the AS is also discussed.
Due to strong turbulent mixing, the ocean surface boundary layer region is generally not conducive to double diffusion. However, vertical microstructure profiles observations in the northeastern Arabian Sea during May 2019 imply the formation of salt fingers in the diurnal thermocline (DT) region during the daytime. In the DT layer, conditions are favorable for salt fingering: Turner angle values are between 50 and 55° with both temperature and salinity decreasing with depth; shear-driven mixing is weak with a turbulent Reynolds number of about 30. The presence of salt fingering in the DT is confirmed by the presence of staircase-like structures with step sizes larger than the Ozmidov length and by the dissipation ratio that is larger than the mixing coefficient. The unusual daytime salinity maximum in the mixed layer that supports salt fingering is primarily due to a daytime reduction in vertical entrainment of fresh water along with minor contributions from evaporation and horizontal advection and a significant contribution from detrainment processes.
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