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

K K A S Yapa

doi:10.4038/sljp.v10i0.3831

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

Highlights

Satellite remote sensing has become increasingly helpful in studying world oceans and their many biological and physical processes
During the months of May – September the waters in the south are influenced by strong southwest winds that blow across the southern part of the country and by the strong western boundary current (Somali current) that flows northeastward via southern coastal waters
Ocean colour is used as a tool to locate as well as a measure of areas of enhanced biological production

Summary

Introduction

Satellite remote sensing has become increasingly helpful in studying world oceans and their many biological and physical processes. Though the scanner was able to record data only in few wavelength bands and there were many technical limitations, oceanographers around the world had the opportunity to study [1,2] vast scale oceans with greatest ease. Since CZCS, ocean remote sensing technology has advanced in a rapid pace with many countries launching their own satellites such as OCTS (Japan), SeaWiFS (USA), IRS-P4 (India), etc. The most recent one by United States, MODIS (Moderate-resolution Imaging Spectroradiometer) was launched in 2002. It carried most sophisticated detectors capable of measuring in 36 spectral bands, providing vast amount of information on land, ocean as well as atmosphere.

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