The oceanic mixed layer is defined as the vertically well?mixed near?surface layer that can be induced by both atmospheric forcing (surface cooling and/or wind stirring) and ocean processes (lateral advection, Langmuir circulation, wave?current interaction, internal wave). The mixed layer (ML) directly affects air?sea exchange of heat and momentum, thereby contributing to sea surface temperature, which is one of the most important oceanic variables for Earth?s climate. Major oceanic biological and chemical processes that play crucial roles in Earth?s climate also occur in ML. Numerous researchers have investigated variability of the mixed layer depth (MLD) over the global ocean or regional oceans, including the North Pacific Ocean. However, basin?wide distribution of climatological MLD in the East Sea has never been fully investigated before, although it is fundamentally important information for understanding spatio?temporal variability of bio?geochemical materials and for evaluating performance of numerical models. It has been suggested that MLD variability of the East Sea can be affected by basin?scale circulation patterns, boundary currents, mesoscale eddy activity.