Observed evidence of the atmospheric response to the shift in the sea surface temperature front of the Oyashio Extension

This study aims to investigate the effect of the north-south displacement of the Oyashio Extension (OE) SST front on the atmosphere mainly based on satellite-derived observations. An index is used for quantifying the time-varying location of the SST front. After removing ENSO signals, linear regression is applied to interpret the connection between the OE index and a number of climate variables. The variables include surface precipitation rate, rain water, and latent heat at different layers obtained from Tropical Rainfall Measuring Mission, vertical profiles of air temperature and geopotential height from the Atmospheric Infrared Sounder (AIRS) along with vertical pressure velocity and other atmospheric variables from the ECMWF Interim Reanalysis.

It is found that when the OE makes a northward displacement, there are warm SST signals in the vicinity of the OE region, the Kuroshio Extension region, and the central North Pacific. Over the OE region, increasing surface precipitation and penetration of upward air motion to 500mb are located overlying warm SST anomalies. Over the Kuroshio Extension region, warm SST and upward air motion are collocated with the two ridges of the Kuroshio Extension current meanders. Noticeable but modest meso-scale characteristics of ocean meanders are found in precipitation when a spatial filter is conducted prior to linear regression to isolate small-scale signals. The varying Kuroshio Extension current meanders with the OE displacement leave imprints on air temperature and geopotential height in free atmosphere on the frontal scale. The magnitude of small-scale air temperature and geopotential height response is small compared to the large-scale response, which displays a dipole structure with positive values in the north and negative values in the south of the North Pacific according to the AIRS data. Unlike the AIRS data, the frontal-scale responses of air temperature and geopotential height using ERA-Interim data are confined to the atmospheric boundary layer, implying the limitation of reanalysis data to interpret small-scale variations associating with ocean fronts. This study uses observations longer than 10 years to highlight the impact of SST front variability on the atmosphere not only within the boundary layer but also in free atmosphere.