Physical processes and their role on the spatial and temporal variability of the spring phytoplankton bloom in the central Yellow Sea

The spatial and temporal variability of the spring phytoplankton bloom (SPB) in the central Yellow Sea is studied, using SeaWiFS surface chlorophyll remote-sensing data, AVHRR sea surface temperatures (SST), QuikSCAT sea surface wind speed (SSW) from 1998 to 2009 and the cruise survey data in 2007 and 2009. The influences of the hydrological conditions on the SPB are significant. (1) The rising SST and low SSW in spring play an important role in the development of the SPB. The SPB in the central Yellow Sea occurs primarily in April (from April 3 to April 24) and at this period the mean SST is generally greater than 10 °C, and 24 h averaged SSW is less than 5.4 m/s. The 99% of the SPB occurs when the SST is 9–14 °C and SSW is 0–7.9 m/s. (2) Specifically, the development of the SPB is from April 4 to April 7 and from April 4 to April 22 in 2007 and 2009 respectively. The longer duration of the SPB in 2009 than that in 2007 is related to the high SST in 2009, which is approximately 2 °C greater than that in 2007, and the weak SSW in 2009, which is much lower than that in 2007, which further indicate that these two factors are critical to the duration of the SPB. (3) The horizontal distribution of surface chlorophyll a in 2007 is found to be greater and located more northward than that in 2009. Comparing the spatial coverage of the SPB, the path of the Yellow Sea warm current, which is warmer and salty, and the location of maximum SST reveals that the spatial coverage of the SPB locates in the warmer temperature (>9 °C) and higher salinity (>33) waters in March and this warm and salty water was much more evident in 2007 than that in 2009 because a stronger warm current of the Yellow Sea in winter. (4) The vertical depths of the maximum chlorophyll layer (MCL) appear at three levels: surface, 10 m and 30 m in 2007, whereas the MCL occurs only at a sub-surface level in 2009 which is related to vertical stability of the water column, i.e., the water is vertically mixed in 2007 while vertically stratified in 2009.