Cabbeling and the density of the North Pacific Intermediate Water quantified by an inverse method

Abstract

North Pacific Intermediate Water (NPIW), defined as the main salinity minimum in the subtropical North Pacific, at a density of 26.7-26.8σq, is denser than the winter surface water in the Oyashio which is the source of the salinity minimum. We showed previously that cabbeling and double diffusion during mixing between the Oyashio water and more saline Kuroshio water can account for the density increase from the surface source water to the salinity minimum. An inverse method is employed herein to quantify the effect of cabbeling, using CTD data from the western North Pacific. The difference between proportional mixing between parcels of Oyashio and Kuroshio waters and mixing along isopycnals is exploited to compute the convergence of water into density layers. The diapycnal transport convergence associated with cabbeling into the NPIW density layer is estimated to be 0.56 Sv for an assumed turnover time of 1 year in the region between 142°E and 152°E. Diapycnal transport convergences in the regions 152°E-165°E, 165°E-175°W, and 175°W-136°W are similarly estimated by assuming longer turnover times. We estimate that the total diapycnal transport convergence into the NPIW density layer may be up to 2.3 Sv in the entire NPIW region.