Impacts of salinity on mixed-layer temperature variations in the Kuroshio-Oyashio confluence region

Abstract:

The Kuroshio-Oyashio confluence region (KOCR) in the western North Pacific, exhibits well-defined low-frequency variations on interannual to decadal time scales, and serves as a key region of midlatitude climate variability. While significant progress has been made in understanding dynamical and thermodynamical aspects of the KOCR variations, less attention has been paid to salinity variability there, owing to scarcity of in-situ salinity observation. Since salinity potentially exerts significant impacts on various physical processes in the upper ocean, it is important to properly describe its features and assess its possible impacts. For these purposes, we investigate salinity variations in the KOCR through analysis of available observational datasets and an ocean reanalysis product, as well as sensitivity experiments using a one-dimensional mixed layer model (1-D ML model).

By analyzing observational datasets, a coherent low-frequency fluctuation of temperature and salinity is detected in the KOCR region related to the stability of Kuroshio Extension (KE): anomalous warming and saltening (cooling and freshening) are observed in the KOCR when the upstream KE becomes unstable (stable). It is found that they are primarily caused by modulation of the mesoscale eddy activity and associated changes in heat and freshwater transport. Then, we decompose density anomalies into contribution from temperature and salinity anomalies to assess their relative importance. It turns out that salinity contribution has comparable, or even larger magnitude than that temperature contribution, particularly in the northern part of the KOCR region, where background temperature is low and hence the dependence of density on temperature is weak. To further quantify impacts of salinity, we have carried out a series of sensitivity experiments based on the 1-D ML model. The results from these experiments reveal that salinity anomalies significantly alter the strength of vertical mixing and eventually lead to difference of sea surface temperature about 1.0 °C. Our results suggest that salinity variations in the KOCR play a role in climate variability via an extratropical air sea interaction, and they may be one of the essential factors for accurate climate predictions.