Daily to Seasonal Variability of the Mixed Layer Depth in the Central Yellow Sea: Effects of Atmospheric Forcing

Abstract

Daily to seasonal variability of the mixed layer depth (MLD) was investigated based on the CTD mooring time series during the April to October 2015 period, taken from the Ongjin Socheongcho Ocean Research Station in the central Yellow Sea. The time evolution of the seasonal MLD was characterized by three distinct phases: fully-mixed (deep) phase by late April, quasi-stable (shallow) phase from late April to late July, and gradual deepening phase thereafter. During the first phase, the entire water column was occupied by cold bottom waters, which seemed to impede the change of the MLD according to atmospheric forcing. Besides this fully-mixed phase, daily MLD fluctuations varied coherently with the surface wind speed with a lag of several hours on the time scale of few days. During the deepening phase, the MLD increased monotonically at a rate of 2.6 m per month, corresponding to gradually increased net heat losses to the atmosphere at a rate of 2.7 W/m(2) per day, suggesting the role of net heat flux in determining the seasonal variance of the MLD. The relationships of both the winds and net heat fluxes with the MLDs weakened to an insignificant level from late July to middle August. During this period, heavy rainfall associated with both typhoon and tropical depressions, and the attendant riverine runoff probably added buoyancy forcing, forming a barrier layer near the MLD base. Our results collectively suggest that atmospheric forcing plays a critical role in determining the variability of the MLD in the central Yellow Sea, where three-dimensional oceanic processes including tides and currents are relatively weak.