The anti-cyclonic gyre around the Qingdao cold water mass in the China marginal sea

Abstract. The circulation structure surrounding the Qingdao cold water mass in 2019 was investigated via three-dimensional numerical simulations. The study reveals that a cold pool appears in early spring and reaches its peak in late May, and this pool is accompanied by a local anticyclonic gyre. Momentum diagnostics are utilized to determine the relationship between the cold pool and this gyre. The momentum results reveal that vertical friction cannot be ignored due to the shallow topography and surface wind stress; as a result, the geostrophic balance is no longer applicable in the Qingdao cold water mass region. Consequently, the temperature and salinity gradients induced by the cold pool structure do not directly result in the anticyclonic gyre. Additional numerical experiments are conducted in which tidal forcing or wind forcing is excluded to understand the formation mechanism of this anticyclonic gyre. The t test results show that the difference between the control run and the ensemble experiments without tidal forcing (or without wind forcing) is statistically significant. Thus, both tidal forcing and wind forcing have a significant influence on the anticyclonic structure, although their impacts are different. Without tidal forcing, an unrealistic strong current appears throughout the domain. Moreover, the direction of the eastern side of the anticyclonic circulation is reversed. On the other hand, the wind forcing contributes to the magnitude of the anticyclonic circulation, especially in the western portion of the anticyclonic circulation. Additionally, subcirculation occurs vertically around the Qingdao cold water mass and is influenced by both wind and tidal forcings.