The upper-ocean response and adjustment to surface forcing in an oceanic general circulation model Ⅱ. Decadal-to-interdecadal variability

Decadal-to-interdecadal variability of the temperature and circulation in the North Pacific Ocean,as well as the mechanisms of temperature evolution in the 1976-77 regime shift are investigated with a high resolution oceanic general circulation model forced with COADS 1945~1993 monthly mean data.The model successfully captures the dominant mode of the SST anomalies on the decadal-to-inter decadal timescales,as well as the major feature of SST anomalies in the 1976-77 regime shift.The time series of the simulated SST anomalies in three key regions (central North Pacific,coast of California and KOE region) were found consistent well with the observations.The model also successfully reproduces two distinct subduction events for the mid-latitude temperature anomalies in 1970s and 1980s,respectively.For the surface ocean,the dominant decadal mode in the circulation anomalies displays an oceanic gyre that consists with the wind stress anomalies by Ekman relation.But for the whole upper-ocean,two oceanic gyres were found dominant,the subtropical and subpolar gyres.The subtropical gyre is more prominent than the subpolar gyre,and the anomaly of the subpolar gyre delays the subtropical gyre by about 3 years.Examination of the upper-ocean heat budget in three key regions reveals that the 1976-77 regime shift was caused by both the sustained heat flux input anomalies and the strong horizontal advection anomalies in the central North Pacific.In the Californian coastal region,only the heat flux input anomalies were found dominant,and the effect of the horizontal advection anomalies is negligible.In the KOE region,the vertical advection,heat flux and horizontal advection anomalies are all important in producing the regime shift and the horizontal advection anomalies were found crucial for the time of the regime shift.