Mechanisms of Long‐Term Variability and Recent Trend of Salinity Along 137°E

Abstract

AbstractTo investigate the mechanisms for observed features along 137°E, such as freshening trends since the 1990s and decadal variability, the spatial pattern of salinity anomalies at σθ = 25.4 kg m−3 isopycnal surfaces and their variability were investigated based on ocean general circulation model (OGCM) outputs. While sea surface salinity (SSS) is restored to its climatology in the OGCM, the model captures recent salinity trends and meridionally coherent interannual to decadal variability at 137°E. The trend signal seems to be traced back over the eastern North Pacific (around 30°N, 160°W). Despite the limited SSS variability, the meridional shift of the outcrop line caused by sea surface temperature variation is found to determine the decadal spiciness variability subducting on the isopycnal surface at σθ = 25.4 kg m−3, that is, warm‐saltier anomalies with a southward shift and cold‐fresh anomalies with a northward shift as colder‐fresher water distributes to the north at the sea surface. Furthermore, we conducted tracer experiments with/without mesoscale eddies to examine possible roles of mesoscale eddies in the propagation of spiciness anomalies. Tracers diffuse across the mean stream line by mesoscale eddies and spread from 10°N to 25°N. However, in the tracer experiment with steady mean flow (i.e., without mesoscale eddies), the center of the tracer propagates along the mean stream line and does not diffuse across the mean stream line. This suggests that mesoscale eddies are important for the synchronized salinity trend and variability found from 10°N to 30°N.