Surface layer temperature inversion in the Bay of Bengal

Abstract

Hydrographic and XBT data archived in the Indian Oceanographic Data Centre (IODC) are used to understand the process of temperature inversions occurring in the Bay of Bengal. The following aspects of the inversions are addressed: i) annual distribution of the layer thickness, layer gradient, starting depth and their inter-annual variability, ii) static stability of the layer and iii) generating mechanism/s. It is seen that organized inversions in the bay occur during winter (from November to February) and the occurrence is mainly in the coastal/offshore waters. By January, the occurrence region shifts from the coastal waters of western bay towards the central and eastern region (Andaman Sea). Inversions of large temperature gradient (Delta T) of the order of 1.6-2.4 degrees and thin layer thickness (Delta D) (10-20 m) are located adjacent to the three major fresh water zones (Krishna, Godavari; Ganges, Brahmaputra and Irrawady). Inter-annual variability of the inversion occurring in the bay is examined utilizing XBT data collected along Andaman-Calcutta section during winter months of different years. Inversion layers with strong Delta T and Delta D22 are present in all the years with significant inter-annual variability. The vertical stability in temperature inversion layers are addressed based on the Delta S - Delta T (Delta S is the salinity gradient corresponding to Delta D) relationships derived from inversion profiles. The Delta S - Delta T plot revealed the existence of two clusters with Delta T values greater than 2 degrees C and less than 2 degrees C located north and south of 18 degrees N, respectively. While the stability of inversions north of 18 degrees N is 3600 x 10 super(-8) m super (-1), south of 18 degrees N, it is 3000x10 super(-8) m super (-1). The role of net surface heat flux in generating the observed inversions are examined from the climatological monthly estimates of the same derived from ISCCP (International Satellite Cloud Climatology Program), SRB (Surface Radiation Budget) radiational and COADS turbulent fluxes. In November and December, the surface heat loss in the coastal region cools the surface waters and because of the high stratification (caused by fresh water) the cool surface layer stay stagnated in the first few meters favoring the formation of temperature inversion. The organized inversions occurring in the central and northeastern bay (north of Andaman Sea) in January and February appears to be generated due to the horizontal advection. Since the net surface heat flux shows heat gain in this region, the organised inversions in this region appears to be generated by the cold fresher water advection (from the head of the bay and Irrawadi basin) over the remnant of the warm saline water in this region advected by the southwest monsoon drift current.