Abstract
Tropical oceanic regions are frequently prone to deep convections. Hence, it is very essential to understand the features of convection with the help of oceanic and atmospheric variables such as sea surface temperature (SST), outgoing longwave radiation (OLR), rainfall, relative humidity, columnar water vapour (CWV) etc. and the linkage among them. In our present study, we have divided the Bay of Bengal (BoB) into ten different subregions (SR) and have attempted to study the connection between the above-stated variables during convective and non-convective events in the southwest monsoon (SWM) season (June to September) for the period 1998–2010. The monthly behaviour of SST/OLR decreased by 0.5 °C/14 W/m2 from May to June and increased by 0.1 °C/7 W/m2 from September to October. Among the ten SRs, SR 5 and SR 10 are observed to be coldest and warmest, respectively, based on the SST variations. Intra-seasonal oscillations of the above-mentioned variables show the influences of quasi-biweekly oscillations (QBWO) and Madden-Julian oscillations (MJO). As the threshold values for SST, OLR and rainfall were already reported, we have drawn our attention to deduce a threshold value for water vapour in lower level troposphere (water vapour density (WVD) at 850 mb) which highly influences the convection. In arriving at a threshold of low-level water vapour, we have analysed the convective and non-convective events of each central 1 × 1° grid in all the SRs for the period from 1998 to 2010, along with water vapour scale height. Our analysis inferred that the low-level water vapour density at 850 mb varied above 12 g/m3during convective days and below 12 g/m3during non-convective days. We noticed that the variability in water vapour density is more in non-convective days than in convective days over BoB. The results of the study may be useful to understand the water vapour dynamics with SST, OLR and rainfall.
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