Seasonal Variability of the Observed Barrier Layer in the Arabian Sea

The formation mechanisms of the barrier layer ( BL) and its seasonal variability in the Arabian Sea ( AS) are studied using a comprehensive dataset of temperature and salinity profiles from Argo and other archives for the AS. Relatively thick BL of 20-60 m with large spatial extent is found in the central-southwestern AS ( CSWAS), the convergence zone of the monsoon wind, during the peak summer monsoon ( July-August) and in the southeastern AS ( SEAS) and northeastern AS ( NEAS) during the winter ( January-February). Although the BL in the SEAS has been reported before, the observed thick BL in the central-southwestern AS during the peak summer monsoon and in the northeastern AS during late winter are the new findings of this study. The seasonal variability of BL thickness ( BLT) is closely related to the processes that occur during summer and winter monsoons. During both seasons, the Ekman processes and the distribution of low-salinity waters in the surface layer show a dominant influence on the observed BLT distributions. In addition, Kelvin and Rossby waves also modulate the observed BL thickness in the AS. The relatively low salinity surface water overlying the Arabian Sea high-salinity water ( ASHSW) provides an ideal ground for strong haline stratification in the CSWAS ( during summer monsoon) and in NEAS ( during winter monsoon). During summer, northward advection of equatorial low-salinity water by the Somali Current and the offshore advection of low-salinity water from the upwelling region facilitate the salinity stratification that is necessary to develop the observed BL in the CSWAS. In the SEAS, during winter, the winter monsoon current ( WMC) carries less saline water over relatively high salinity ambient water to form the observed BL there. The winter West India Coastal Current ( WICC) transports the low-salinity water from the SEAS to the NEAS, where it lies over the subducted ASHSW leading to strong haline stratification. Ekman pumping together with the downwelling Kelvin wave in the NEAS deepen the thermocline to cause the observed thick BL in the NEAS

Recent salinity intrusion in the Bengal delta: Observations and possible causes

Salinization stands among the most prominent environmental hazards of the largest delta on Earth, the Bengal delta. It has significant impacts on the local societies and the economy. Using an unprecedented collection of in situ river salinity records over the Bengal delta, extending from the Hooghly estuary in the west to the Meghna estuary in the east, we report a sudden salinization of the central part of the delta that occurred in 2006-2007. This results in a sudden landward shift of the seasonal march of the salinity front by about 20 km, taking place in the pre-monsoon season. Such a regime shift was never reported before. We investigate the various drivers of this sudden change and identify three possible forcing factors: the decrease in Ganges freshwater discharge, the rise of sea level and the depletion of the groundwater level. These factors may act independently, or in concert. Given the threat of the ongoing climate change and its cohort of adverse effects expected in the course of the 21st century in the Bengal delta, our study contributes to set the observational basis for the development of the next generation of salinization modeling platforms

VALIDATION OF ARGO DATA IN THE INDIAN OCEAN

ARGO-salinity data from the Indian Ocean are validated using salinity from float-versus-float match-ups and also using CTD observations from three cruises. Validation data sets are selected in such a way that the float and the match-up data are collocated with tolerable space-time difference. For validation a time difference of less than 10 days and space difference less than 100 Km have been considered. The evaluation is done using salinity data on theta (potential temperature) surfaces from deeper observations. One of the significant observation of float-versus-float validation is the large random error observed in the initial profiles of the float-salinity compared to later profiles. While the float salinity data is found to be largely in good agreement with the ship based CTD observations, there are cases where the salinity error exceeds the desired 0.01 PS

Quality of temperature and salinity data from Argo profiling floats in the Bay of Bengal

1870-1878<span style="font-size:9.0pt;font-family: " times="" new="" roman";mso-fareast-font-family:"times="" roman";mso-bidi-font-family:="" mangal;mso-ansi-language:en-gb;mso-fareast-language:zh-cn;mso-bidi-language:="" hi"="" lang="EN-GB">In the present study, temperature and salinity from APEX -Argo floats with reported SPB (Argo-SPB) and salinity from <i style="mso-bidi-font-style: normal">normal floats without any reported SPB (Argo-N) in the BoB have been subjected to quality check (QC). Method used for QC depends on time-space de-correlation scales (TSD-scales) of temperature and salinity in the BoB at selected potential temperature (θ) surface (10 oC). High quality shipboard CTD observations in the BoB have been used to identify TSD-scales of temperature and salinity. Observed TSD scales for salinity (temperature) at θ surface of 10 oC are 5 days and 60 km (8 days and 80 km). QC has been performed on matchups between Argo and shipboard CTD observations falling within the identified TSD- scales. QC on Argo-SPB could not identify any significant systematic bias/error, except for a single profile (cycle No. 48) of float-4900675. In the case of Argo-N, significant error is found in most of the salinity profiles from the float-2900268.</span

Improved bathymetric dataset and tidal model for the head Bay of Bengal

International audienceThe Bengal Delta is a highly complex and vulnerable environment where key dynamical features such as tides, storm surges, salinity, and sediment transport strongly depend on the accurate bottom topography representation. To curb the lack of accuracy of widely used global bathymetry databases in this shallow region, we produced an improved coastal bathymetry for the Bengal Delta through an exhaustive and thorough compilation of 70 nautical charts and river surveys. We merged this bathymetry dataset with a high-resolution digital evaluation model, resulting in a novel, consistent product with unprecedented coverage, from the deep parts of the ocean basin (north of 20°N) to the far upstream areas of the delta (around 24°N). This new bathymetry reduces errors in the tidal residuals by a factor of 2–3 for most coastal gauges as compared to global tidal models. The best results are obtained for the Hooghly River, where errors are as low as 6 cm. The improvement in the mouth of the Meghna is moderate, suggesting the possible rapid evolution of the bathymetry due to strong sediment transport. This new bathymetry is expected to significantly improve the modeling of coastal processes such as storm surges in the northern Bay of Bengal