Monthly and Interannual Variations in Winter Positive Surface-Bottom Temperature Difference in Northeastern Coastal Waters of the Shandong Peninsula in the Yellow Sea

Traditionally, the Yellow Sea (YS) is believed to be well mixed throughout the water column or to experience significant temperature inversions in winter. Recently, we found a significant positive surface-bottom temperature difference (SBTD) as large as 3.7 degrees C in northeastern coastal waters of the Shandong Peninsula in the YS in winter. By analyzing 4 years (June 2016-September 2020) of online in situ observations, this study examined monthly and interannual variations in winter positive SBTD and the underlying dynamics. The winter positive SBTD generally formed in November, peaked in December/January, and disappeared in March. At the interannual time scale, this SBTD was strongest in 2018 and weakest in 2020. The underlying dynamics involved the combined effects of prevailing winter monsoon winds and resultant surface heat loss and current heat advection. When the surface northerly wind was strong, strengthened southward advection of cold air over the ocean cooled the water column intensely, whereas wind-enhanced southward advection of warm surface water partly offset the sea surface cooling and resulted in surface temperature cooling that was considerably slower than bottom temperature or even surface temperature warming. These processes together generated strong positive SBTDs. When the surface northerly wind was weak, all of the above processes became weak and resulted in weak positive SBTDs. The relationship with Yellow River runoff was also discussed. This study enriches our knowledge of winter thermal structure in the YS. More observations and coupled physical-biogeochemical simulations are needed to further clarify the impacts on climate and marine ecology.