Decadal Variability of North Pacific Central Mode Water

Abstract

An isopycnal model forced with wind stress and heat fluxes from 1965 through 1993 was used to examine the effects of variable atmospheric forcing on the ventilation of the North Pacific. During this time period, a climatic regime shift occurred that had significant impacts on heat fluxes, sea surface temperature (SST), and wind stress patterns. The climate shift, occurring in the winter of 1976/77, affected the formation rates and locations, and properties of the Central Mode Water (CMW) formed in the model. Three model runs were compared: one with variable buoyancy forcing and climatological wind forcing, one with variable wind forcing and climatological buoyancy forcing, and one with variability in both the buoyancy and the wind forcing. The comparison indicates that buoyancy forcing is of primary importance in the variability of mode water formation and properties surrounding the climate shift. One measure of the climate shift is the Pacific decadal oscillation (PDO), an index of SST variability in the North Pacific, which changed sign in 1976/77. A positive state for the PDO is associated with deeper model mixed layers, formation of denser varieties of CMW, and an anticyclonic circulation anomaly in the CMW density range.