Diagnosis of water mass transformation and formation rates in a high‐resolution GCM of the North Pacific

Abstract

AbstractWe studied the relationship between assessments of water mass formation estimated by thermodynamic and kinematic methods by applying both methods to the Subtropical Mode Water (STMW) and Central Mode Water (CMW) formation regions of the North Pacific. We used long‐term outputs of a high‐resolution general circulation model under climatological normal year forcings to calculate the water mass transformation rate and its density derivative (the net formation rate) and to examine the roles of mixing, shortwave penetration, and mesoscale eddies. Diagnosis of the thermodynamic relationship indicated that the transformation rate associated with surface density flux largely explained the net transformation rate. The difference of these rates, the total mixing effect, was relatively small compared with previous estimates. Explicit and implicit mixing largely explained the total mixing effect. Diagnosis of the kinematic relationship indicated that the annual subduction rate could be calculated from the net transformation rate and the open boundary volume flux over the analysis region. Comparison of both methods implied that the source of subducted STMW was determined mainly by the transformation associated with surface density flux, but this conclusion was not true for CMW. The modeled effects of internal mixing and volume flux from the north (Oyashio Current) also became important in the formation of CMW. Omission of shortwave penetration led to an error in the air‐sea transformation and formation rates in the STMW and CMW regions. We also found the important roles of mesoscale eddies in the transformation and formation of mode waters.