Spatiotemporal Variations in Upper‐Ocean Salinity Over the North Pacific in 2004–2021

Abstract

Interannual variations in upper ocean salinity in the North Pacific Ocean (NP) were examined comprehensively over a period of 18 years (2004–2021) with gridded data sets considering vertical profiles of salinity. The salinity in nearly half of the basin varied in phase with a slight lag, and the rest of the basin exhibited the opposite phase on a decadal scale. The average salinity above 400 dbar prominently increased in the eastern and western mid‐latitude regions from approximately 2010 to the latter half of the 2010s, and decreased in the central subarctic and subtropics during the same period. This spatial pattern was similar to that of temperature change. While the freshening in the subtropics was remarkably above 100 dbar, salinity changes in the subarctic were large even in the subsurface. The first mode of the complex empirical orthogonal function analysis indicated the eastward (westward) propagation of salinity anomalies in the subarctic (subtropics), that is, a clockwise shift. Salinity budget analysis revealed that the changes in the 2010s were mainly caused by geostrophic advection in zonal regions around 40°N and 20°N, and the eastern NP. Precipitation also contributed to freshening in the subtropics, but not in the central subarctic. The meridional migration of salinity fronts in the subarctic and subtropics induced by wind stress, which is meridional contraction of the subtropical gyre, contributed to freshening in the central NP. It is unclear whether salinification in the eastern and western regions inevitably synchronizes and should be further examined.