Remotely-Observed Early Spring Warming in the Southwestern Yellow Sea Due to Weakened Winter Monsoon

Abstract

The seasonal warming over the southwestern Yellow Sea (YS) in the spring is of vital importance to the local ecologic environment, especially to the massive green algae blooms of the YS in late spring and early summer. Based on daily optimum interpolation sea surface temperature (SST) data consisting of satellite derived SST from Advanced Very High Resolution Radiometer (AVHRR) and in situ measurements, this study analyzed the spring SST variation over the southwestern YS (SWYS) from 1982 to 2018. The results show that the recent warming trend of spring SST over the SWYS is four-to-six times that of the global average, and as a result, sea water over the Subei Shoal (SBS) shifts about 10-13 days earlier to reach 10 • C in early April. This implies that, accordingly, the micro-propagules of green algae over the SBS may have the chance to germinate earlier. SST variability in early April significantly correlates with northerly wind and exhibits a general warming over the SWYS with an intensified warming anchored along the axis of the submarine canyon off the Yangtze estuary. The Moderate Resolution Imaging Spectroradiometer (MODIS) red-green-blue composite images captured the intrusion of the Taiwan Warm Current (TWC) into the SWYS through the submarine canyon during northerly wind relaxation in early April. Ocean remote sensing provides important clues for understanding the regional SST variability in the SWYS. Following this clue, this study finds that the weakening of winter monsoon in the spring leads to northward migration of the TWC and results in enhanced spring warming over the SWYS. The attendant advanced warming in spring, resulting in a favorable temperature condition for early development of green alga, may have contributed to the green tide blooms in the Yellow Sea in the recent decade. affecting algae growth have been well studied and understood [6][7][8]. It is widely accepted that the optimal sea water temperature in the YS facilitates the formation of the green tide [6][7][8][9]. Song et al. [9] reported that micro-propagules of green algae in coastal waters of the SBS begin to germinate at 10 • C or a higher water temperature. Xiao et al. [7] found the growth rate and photosynthetic rate of the detached U. prolifera were significantly higher at moderate temperature levels. Hence, sea temperature plays a significant role in the development of the green tides, especially at the early stage. Xuan et al. [10] found that SST and wind in the spring played an important role in the development of the spring phytoplankton bloom. The variation of the spring SST is of vital importance to the ecologic environment over the southwestern Yellow Sea (SWYS, Figure 1). detached U. prolifera were significantly higher at moderate temperature levels. Hence, sea 48 temperature plays a significant role in the development of the green tides, especially at the early 49 stage. Xuan et al. [10] found that SST and wind in the spring played an important role in the 50 development of the spring phytoplankton bloom. The variati...

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