Formation Mechanism of the “Stability Gap” and the North Pacific Central Mode Water

Abstract

AbstractLocal feature of the formation region (165°E~160°W, 38°N~42°N) of the North Pacific Central Mode Water (NPCMW) is first put forward from data analysis, and for which, the external atmospheric forcing (solar shortwave radiation, net heat flux and wind stress curl) could not solely give acceptable explanation. Further analysis on the seasonal variability of the upper ocean stratification shows that a special weak zone of the ocean stratification in the upper ocean (<75 m)‐the “stability gap” is detected in the zone of (165°E~160°W, 38°N~42°N) in autumn (September‐October). As “Precondition Mechanism”, the “stability gap” provides a reliable answer for the “local feature” of the formation of the NPCMW. Based on a heat balance equation of the upper ocean mixed layer, diagnostic analysis suggests that the formation of the “stability gap” is the cooperative product of the surface heat flux forcing, vertical entrainment, Ekman advection and geostrophic advection. Among which, the latitudinal differences of the surface heat flux forcing, the cold Ekman advection and the warm geostrophic advection play the crucial roles on determining the critical eastern and western bound of the “stability gap”.