Evidence of a chlorophyll “tongue” in the Malacca Strait from satellite observations

Abstract

Remotely sensed surface chlorophyll-a concentration from the Moderate Resolution Imaging Spectroradiometer (MODIS) unveils signatures of a “tongue” like feature extending into the Malacca Strait from the west of Malaysia. A threefold increase of chlorophyll-a amplitudes is observed in the strait during the northeast monsoon (December–February) compared to the lowest values during the southwest monsoon (May–September). This seasonal variability of chlorophyll-a is linked with the seasonal variabilities of the meridional component of Ekman mass transport (M y ), sea surface temperature (SST), wind stress curl, and the sea surface height anomaly (SSHA). A multiple regression analysis indicates that the monsoonal winds and SST are the dominant contributors to the chlorophyll-a variability in the strait. The role of SSHA is limited to the presence of a seasonal cyclonic eddy in the strait during December–February and its associated upwelling. The monsoonal winds associated with the active phase of Madden Julian Oscillation (MJO) events during November induced strong coastal upwelling along the west coast of Malaysia. Later, strong upwelling associated with both wind-induced coastal upwelling and open ocean upwelling due to wind stress curl is observed in the strait during December–February, which is also well supported by the entrainment of cold and saline waters from the subsurface and advection of colder SST within the strait. The advection of higher chlorophyll-a in the north-westward extension of the “tongue” off the west coast of Malaysia during December – February is attributed to the Ekman mass transport due to higher wind stress. On the other hand, lower chlorophyll-a during May–September is due to the negative wind stress curl and downwelling. Rectified wavelet analysis shows higher correlations of chlorophyll-a variability with the active phases of the MJO events than with the ENSO and IOD events, indicating substantial impacts of the MJO events in the Malacca Strait. • First study to link Chl-a seasonality in Malacca Strait with winds, SST and SSH. • Highest Chl-a values during December–February manifest as a “tongue.” • Winds drive open ocean and coastal upwelling processes to generate the “tongue”. • MJO events influences chlorophyll-a variability more than ENSO and IOD events.