Observed anomalous chlorophyll- a bloom in the northern Arabian Sea during winter 2008

Abstract The observed interannual variability of winter chlorophyll- a (chl- a ) bloom in the northern Arabian Sea (AS) using the remote sensing data of Sea-viewing Wide Field-of-View Sensor (SeaWiFS) and Aqua Moderate Resolution Imaging Spectro radiometer (MODIS) was analyzed for the period 1997–2010. We show that the strongest winter bloom in the 13-year record occurred in February 2008, with regional mean chl- a concentration in the north-central AS (14°N–20°N and 60°E–68°E) being 35% higher than any other winter peak, and open-ocean values exceeding 2 mg m -3 . The bloom was initiated during an episode of intense winter monsoon forcing, characterized by surface cooling exceeding 1 °C and deepening of the upper-ocean mixed layer and thermocline depth (D23). Analysis of aerosol optical depth (AOD), sea surface temperature, winds, and subsurface thermal structure indicates that the chl- a increase clearly preceded major dust events. This shows that dust deposition did not trigger the February 2008 bloom. Instead, convective mixing associated with anomalous winter cooling was the primary driver of bloom initiation, while atmospheric input, if any, was limited to a possible secondary role in sustaining and enhancing bloom event later in the month. The February 2008 event highlights the sensitivity of Arabian Sea winter productivity to episodic intensification of winter monsoon winds and associated convective mixing, with implications for future ecosystem responses under increasing climate variability.