Seasonal thermocline in the China Seas and northwestern Pacific Ocean

Climatological seasonal variations of the thermocline in the China Seas and northwestern Pacific Ocean were studied using historical data from 1930 through 2001 (707,624 profiles). The quantitative roles of surface thermal buoyancy ( B q ), haline buoyancy flux ( B p ), and total buoyancy flux ( B ) against the wind‐induced mixing ( τ ) in different seasons and regions were also explored using the buoyancy ratio ( R = ∣ B q /B p ∣) and the Monin‐Obukhov depth ratio ( δ ), respectively. The thermocline has obvious seasonal variations in the study area north of 20°N. There is no thermocline along the west coast of the Bohai Sea (BS), Yellow Sea (YS), and northern East China Sea from December to March resulting from surface cooling and wind mixing. The significantly different variation of the thermocline strength on and off the Chinese shelf is mainly caused by the fact that the thermal stratification is enhanced by bottom tidal mixing on the shelf. The δ indicates that the thermocline depth on the Chinese shelf is mainly dominated by B in summer, while it is dominated by τ in winter. It reveals an opposite feature in the Kuroshio region; the dominating factor is B in winter, associated with the large heat buoyancy loss there. South of 20°N, the dominating factor is similar to that on the shelf, with the more obvious B dominant characteristic during the monsoon transition periods. The R demonstrates that B is mainly controlled by B q all year round, with some sporadically B p ‐dominated regions in the tropical area in winter and in the BS and eastern YS in September.