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Winter Variability Of Mld, Blt And Associated Fluorescence Pattern: A Case Study In The Head Bay Of Bengal

Abstract: This study covers an area at the head Bay of Bengal (BOB) where no previous endeavors have yet been done to explore about the mixed layer depth (MLD), barrier layer thickness (BLT) and their association with the fluorescence pattern. It is well known to all that the BOB is a unique tropical basin in the north-eastern Indian Ocean. Geographically, the head BOB is situated at the northern most part of the basin. The BOB including the head Bay experiences seasonal reversing winds, causing heavy rain fall during the summer and a very weak rain pattern during winter monsoon. There are rivers at the north carrying fresh water in the form of river run-off into this basin, influencing the salinity. Besides, previous study illustrates the evidence of salinity intrusion into the BOB from Arabian Sea (AS). As such, there are inputs of fresh water in the form of seasonal participation, river run-off from north, and salinity intrusion from the south-west AS influencing overall hydrographic (temperature and salinity) parameters of the basin.  Fresh water and saline waters are transported south-ward and north- ward respectively by surface current depending on the seasons. During the summer, fresh water in this basin is much higher but it’s presence also evident during the winter. The presence of low surface saline water together with low temperature and seasonal wind pattern during the winter; cause haline stratification, temperature inversion, influencing to result varied MLD and forms thick barrier layers (BLT). MLD is a depth up to which the air-sea interaction is much stronger and made mixed layer most dynamic. Besides, the barrier layer is known to be a layer which resists bottom nutrients to come up to surface and effect the overall concentration of fluorescence (Chlorophyll A). In this study, the association of fluorescence with MLD and BLT, is tried to be investigated especially with BLT. Besides, rivers are carrying much of suspended sediment along with fresh water from land, making the head Bay water much turbid throughout year.  As such, the turbidity of the study area at the head Bay and it’s influence on the fluorescence pattern (chlorophyll A) has also been investigated. In the present study there has been continuous presence of freshwater at the surface of the study area. This caused haline stratification at top layers up to around 25m and formed thick barrier layers irrespective of temperature difference in winter (December 2014 and January 2015) months. The stable stability parameter gives the

evidence that water is well stratified in the study area. On the contrary, thermal stratification is also experienced in deeper depths (around 40m). Besides, inequality in MLD is observed due to the northeasterly varied wind speed. Comparatively deeper MLD is experienced during January 2015. It is further noticed that this dynamism in MLD and BLT is occurring in an environment of thermal inversion. Furthermore, a distinct relation between BLT and fluorescence pattern is observed. Fluorescence intensity reduces or retards at places where barrier layers are present. It is very interesting to observe that small scale eddies are trying to form at different locations of the study area to break these barrier layers. On the other hand, turbidity is observed generally hindering in the fluorescence pattern. But there is exception where high fluorescence presence is noticed despite high turbidity. This is observed at shallow depths during January 2015 where MLD is extended up to the bottom.  Mentionable, there are previous studies on the North BOB regarding the aforesaid affairs upto 20o N but beyond this latitude, there is no study yet. This paper tries to study the winter variability of MLD, BLT and associated fluorescence pattern of the head BOB covering an area between latitude 21o – 22o N and longitude 90o- 91.6oE .

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