Seasonal and inter-annual variability of remotely-sensed sea surface height, visible light and chlorophyll in the Bay of Bengal

Abstract

Global warming and associated climate change is one of the immediate threats that the human being are confronted with and it is important to understand so that effective remedial measures could be put in place. The response of the earth-ocean-atmosphere system to human-induced climate change varies from region to region and hence studies of regional manifestation forms an important part. Since ocean is a reservoir of CO2, it is important to understand how oceans are responding to the enhanced warming regime. It is in this context that the present project was conceived which was aimed at understanding how the Bay of Bengal situated in the eastern part of the northern Indian Ocean is responding. The Bay of Bengal was chosen as the study area due to its unique characteristics such as very low salinity waters due to large river runoff, large sediment in put from the rivers, occurrence of cyclones, etc. For the present study, to understand how the Bay of Bengal is responding to global warming, we chose to study parameters such as sea level (1992-2006), chlorophyll pigment concentration, photosynthetically available radiation and diffuse attenuation using remote sensing data during the period 1997 to 2007. The sea level changes are directly linked to the global warming, as increased warming will lead to increased sea level rise due to melting of polar ice-caps. The chlorophyll pigment concentration indicates the biological productivity of the region and hence is important to understand its changes. The photosynthetically available radiation (PAR) will determine the quantum of light that is available at the ocean surface while the diffuse attenuation (kd490) determines how much of the light coming to the ocean surface will penetrate downward within the water column. Hence both Par and kd490 is needed to understand the ocean biological productivity. The SSH anomaly showed a dominant seasonal signal whose amplitude was about 8cm with maximum sea level occurring during May-June (~4cm) followed by November-December (~1.5cm). The lowest sea level occurred in the month of February (~ -4.5cm) followed by September (~ -1cm). The SSH anomaly showed a large inter-annual variability and the linear trend line showed an increasing sea level anomaly of ~2cm during the period 1992 to 2006. The chlorophyll pigment concentration showed a dominant seasonal signal with amplitude of about 0.2 mg/m3 with highest concentrations occurring in August (~0.57mg/m3) and lowest in April (~0.36 mg/m3). This seasonality was observed in the entire data set in a cyclic manner during 1997 to 2007 with some amount of inter-annual variability. The linear trend line showed that during the study period the chlorophyll pigment concentration showed a decrease of about 0.08mg/m3. The photosynthetically available radiation also showed a strong seasonality with amplitude of ~10 Einstein m-2 day with highest value in March and lowest in December. Though the data showed some inter-annual variability, there was no significant trend discernible during the period 1997 to 2007. The diffuse attenuation also showed a strong seasonality with minimum attenuation in the month of April (~0.051m-1) followed by a peak in August (~0.066m-1). Over the study period, diffuse attenuation showed a decreasing trend (about 0. 005m-1). In summary, the study noted an increasing trend in the sea level (1992-2006) while the chlorophyll pigment concentration showed a decline over the period 1997 to 2007. Though PAR did not show any significant trend, the diffuse attenuation showed a decline, implying that this change is due to the changes in the water column constituents. One of the factors may be the suspended sediment.