An in situ gridded data of salinity, comprising Argo and CTD profiles, has been used to study the interannual variability of near-surface salinity (within 30 m from sea surface) in the Bay of Bengal (BoB) during the years 2005-2013. In addition to the broad agreement with earlier studies on the north-to-south gradient of surface salinity and general features of seasonal variability of salinity, the data also revealed few episodes of enhanced freshening in the BoB. The observations showed distinct anomalous low salinity
Composite analyses of mixed layer temperature (MLT) budget terms from near‐surface meteorological and oceanic observations in the central Bay of Bengal are utilized to evaluate the modulation of air‐sea interactions and MLT processes in response to the summer monsoon intraseasonal oscillation (MISO). For this purpose, we use moored buoy data at 15°N, 12°N, and 8°N along 90°E together with TropFlux meteorological parameters and the Ocean Surface Current Analyses Real‐time (OSCAR) current product. Our analysis shows a strong cooling tendency in MLT with maximum amplitude in the central and northern BoB during the northward propagation of enhanced convective activity associated with the active phase of the MISO; conversely, warming occurs during the suppressed phase of the MISO. The surface mixed layer is generally heated during convectively inactive phases of the MISO primarily due to increased net surface heat flux into the ocean. During convectively active MISO phases, the surface mixed layer is cooled by the combined influence of net surface heat loss to the atmosphere and entrainment cooling at the base of mixed layer. The variability of net surface heat flux is primarily due to modulation of latent heat flux and shortwave radiation. Shortwave is mostly controlled by an enhancement or reduction of cloudiness during the active and inactive MISO phases and latent heat flux is mostly controlled by variations in air‐sea humidity difference.
Consistent with earlier studies, this analysis reveals that net surface heat flux primarily controls the ML heat balance. The penetrative component of shortwave radiation plays a crucial role in the ML heat budget in the BoB, especially during the spring warming phase when the ML is thin. During winter and summer, vertical processes contribute significantly to the ML heat budget. During winter, the presence of a strong barrier layer and a temperature inversion (warmer water below the ML) leads to warming of the ML by entrainment of warm subsurface water into the ML. During summer, the barrier layer is relatively weak, and the ML is warmer than the underlying water (i.e., no temperature inversion); hence, the entrainment cools the mixed layer. The contribution of horizontal advection to the ML heat budget is greatest during winter when it serves to warm the upper ocean. In general, the residual term in the ML heat budget equation is quite large during the ML cooling phase compared to the warming phase when the contribution from vertical heat flux is small.
salinity stratification and negative temperature gradients between the base of the mixed layer and the thermocline form periodically on both short and long time scales and large and small spatial scales (Thadathil et al., 2002(Thadathil et al., , 2007Girishkumar et al., 2011;Agarwal et al., 2012). Precipitation and riverine input are the main sources of freshwater in barrier layers, and processes such as wind, local currents (Thadathil et al., 2007), and westward-propagating Rossby waves (Girishkumar et al., 2011) can modify and dissipate barrier layers. Consequent reversing temperature gradients result in mixing upward of water that can both warm and cool the sea surface (de Boyer Montégut et al., 2007). The alternating sign of the turbulent heat flux in part distinguishes the role of subsurface fluxes to sea surface modification between alternating monsoon seasons. Because of the influence of the monsoons on local weather, particularly precipitation, accurate prediction of the monsoon is a priority for BoB rim countries. However, the South Asian monsoon is a particularly difficult phenomenon for climate models to predict accurately (Syed et al., 2014). Good estimates of surface fluxes are considered a necessity in predicting large-scale air-sea interactions that contribute to coupled systems such as the South Asian monsoons, but these values can be difficult to constrain (Schott et al., 2009). It is even more challenging to estimate subsurface fluxes and their contributions to surface properties using observations. BoB heat budgets have been computed in the past. For example, Loschnigg and Webster (2000) used a model that parameterized vertical mixing, showing that lateral transport and storage balance surface heat fluxes. Shenoi et al. (2002) used climatological temperatures and surface heat fluxes and assumed a constant diffusivity at the base of a 50 m deep mixed layer, and found diffusive fluxes ranging between -35 W m -2 and -60 W m -2 . Sengupta et al. (2002) used data collected from a mooring in the central BoB to estimate a springtime heat budget of the mixed layer. They estimated residual cooling due to vertical mixing and advection to be about -25 W m -2 .de Boyer Montégut et al. (2007) use a global ocean general circulation model to highlight the importance of barrier layers in the BoB that allow negative temperature gradients in regions of strong salinity stratification. Girishkumar et al. (2013) also highlight the importance of barrier layers in their calculation of a wintertime heat budget using mooring data. They found subsurface heat fluxes using a constant diffusivity between November and February of -23 ± 15 and -10 ± 4 W m -2 in two subsequent years. Most recently, as part of the Air-Sea Interactions Regional Initiative (ASIRI; Goswami et al., 2016, in this issue), investigations of air-sea interactions (Weller et al., 2016, in this issue) and mixed layer heat budgets (Thangaprakash et al., 2016, in this issue) were carried out. In all of these studies, subsurface fluxes were either estima...
Physical and biogeochemical observations from an autonomous profiling Argo float in the Bay of Bengal show significant changes in upper ocean structure during the passage of tropical cyclone (TC) Hudhud (7–14 October 2014). TC Hudhud mixed water from a depth of about 50 m into the surface layers through a combination of upwelling and turbulent mixing. Mixing was extended into the depth of nutricline, the oxycline, and the subsurface‐chlorophyll‐maximum and thus had a strong impact on the biogeochemistry of the upper ocean. Before the storm, the near‐surface layer was nutrient depleted and was thus oligotrophic with the chlorophyll‐a concentration of less than 0.15 mg/m3. Storm mixing initially increased the chlorophyll by 1.4 mg/m3, increased the surface nitrate concentration to about 6.6 μM/kg, and decreased the subsurface dissolved oxygen (30–35 m) to 31% of saturation (140 μM). These conditions were favorable for phytoplankton growth resulting in an estimated increase in primary productivity averaging 1.5 g C·m−2·day−1 over 15 days. During this bloom, chlorophyll‐a increased by 3.6 mg/m3, and dissolved oxygen increased from 111% to 123% of saturation. Similar observations during TC Vardah (6–12 December 2016) showed much less mixing. Our analysis suggests that relatively small (high) translation speed and the presence of cold (warm) core eddy leads to strong (weak) oceanic response during TC Hudhud (TC Vardah). Thus, although cyclones can cause strong biogeochemical responses in the Bay of Bengal, the strength of response depends on the properties of the storm and the prevailing upper ocean structure such as the presence of mesoscale eddies.
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