Influence of northeasterly trade winds on intensity of winter bloom in the Northern Arabian Sea

Abstract

Chlorophyll and wind pattern retrieved from remote sensing data have been used to study biological activity in the oceanic waters of Northern Arabian Sea (NAS) during February–March 2002–05. Occurrence of algal bloom in these waters during this period was noticed with the help of ship observations in the past. The same was detected from OCEANSAT I/OCM with time series chlorophyll images for January–March 2000. Occurrence of this bloom was later re-confirmed using OCM data in the subsequent years also. The time series chlorophyll images established that the bloom develops every year during February–March. This period happens to coincide with the presence of northeasterly trade winds over the NAS. Two ship cruises were conducted with the help of research vessels FORV Sagar Sampada (SS-212 during 26 February–7 March 2003 and SS-222 during 21 February–11 March 2004) during this period at the bloom site. The aim was species identification of the bloom and to study various environmental parameters associated with the bloom. Two diverse situations in the context of biological activity were observed while collecting in situ data in 2003 and 2004. Distribution of the bloom was found uniform over a large area and concentration of phytoplankton was relatively higher in 2003. Compared to this, it was observed during the same period in 2004 that phytoplankton was distributed in scattered and small patches and its concentration was relatively less. Corresponding to this observation, it was noticed from the ship data that wind strength was significantly weaker and the oceanic waters were less turbulent in 2004 compared to the same in 2003. In light of this elementary observation, an attempt was made to observe variations in the wind pattern during 2003 and 2004 using QuikSCAT/SeaWinds scatterometer data. It could be established that occurrence of the bloom as well as the observed inter annual variability in chlorophyll pattern were coupled with prevailing trade winds. It was found that density of surface water increased (inversion) during this period, which could result in convective action and the observed bloom. The vertical density gradient revealed an increasing pattern with increase in wind speed. Moreover, it was observed that response of chlorophyll to acting wind force is delayed by one to two weeks. This led to an important inference that wind can be treated as a precursor to predict variations in chlorophyll pattern in the context of the observed event of the bloom.