Upwelling phenomena in the southern coastal waters of Sri Lanka during southwest monsoon period as seen from MODIS

Abstract

MODIS (Moderate-resolution Imaging Spectroradiometer) aqua data for 2004 has been used to explore possible coastal upwelling areas around Sri Lanka during southwest monsoon. Two very important ocean parameters, ocean colour and sea surface temperature (SST), derived from MODIS data for the waters surrounding Sri Lanka are studied to understand the variability and the relationship of the two data sets. Remotely sensed data of 1.1 km resolution obtained from MODIS were mapped using seadas software within an area of latitudes between 2.0 N - 13.5 N and longitudes between 72.5 E – 88 E. The days with heavy clouds were excluded. Daily chlorophyll a and SST maps were used to make composites for each week and subsequently for each month. There were between 10 – 20 daily files for each month. The southwest monsoon period arrives roughly in May and generally lasts till September. The chlorophyll concentration varies roughly in the range 0.01 – 10.0 mg m −3 and the sea surface temperature varies between 25 – 32°C in the waters surrounding the island during the year. The monthly composites of chlorophyll a show high productive waters of over 5 mg m −3 average concentration in most of the southern and western coastal ocean during the months of June - August. The sea surface temperature drops by 2 to 3 degrees (centigrade) in coastal ocean waters in the south where high chlorophyll a concentrations are detected. Observation of chlorophyll a and SST maps indicate strong correlation (between 60% - 80% of negative correlation) between the two parameters in the southern coastal ocean region where high chlorophyll a and low SST values are detected in the months of June and July. This may be due to the upwelling phenomenon that occurs in the coastal waters. Upwelling brings cooler nutrient rich waters to the surface enhancing the production of phytoplankton near the surface waters and these high phytoplankton areas are generally attracted by large fish populations. Thus, correct identification of upwelling zones would be highly beneficial in the fishery industry. Key words : MODIS; sea surface chlorophyll a; SST; southwest monsoon; upwelling DOI: http://dx.doi.org/10.4038/sljp.v10i0.3831 Sri Lanka Journal of Physics, Vol. 10 (2009) 7-15