Seasonal evolution of the mixed layer depth in the Yellow Sea

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.Three distinct phases characterized the time evolution of the seasonal MLD: fully-mixed (deep) phase by late April, quasi-stable (shallow) phase from late April to late July, and gradual deepening phase after that. 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 a 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