El Niño‐Related Vertical Mixing Enhancement Under the Winter Mixed Layer at Western Subarctic North Pacific Station K2

Abstract

AbstractWe conducted a current velocity and hydrographic time series observation in the upper ocean by deploying a mooring buoy at station K2 (N, E) from July 2015 to June 2019 to understand the physical conditions that affect the acidification of the winter mixed layer (ML) water in the western subarctic North Pacific. Disturbances with substantial current velocity shear were excited by the wind changes remotely related to the 2015/2016 and 2018/2019 El Niño events. The density stratification beneath the ML was also weakened in winter, particularly the El Niño years. Based on the bulk Richardson number mixing scheme, we found that the El Niño‐related strengthening of the current velocity shear enhanced the vertical diffusivity beneath the winter ML driven by the near‐inertial current velocity variation. The weakening of the density stratification is ineffective in enhancing the vertical diffusivity. It is suggested that the El Niño‐related large vertical mixing accelerates the acidification of the winter ML water through the enhancement of the entrainment of the deep water that is rich in dissolved inorganic carbon.