Intensification of the subpolar front in the Sea of Japan during winter cyclones

Abstract

The response of the subpolar front in the Sea of Japan (also known as the East Sea) to winter cyclones is investigated based on quantitative analyses of gridded and satellite data sets. Cyclone passages affecting the sea are detected using time series of spatially averaged surface turbulent heat fluxes. As the cyclones develop, there are strong cold‐air outbreaks that produce twice the normal heat loss over the sea. After removal of sea surface temperature (SST) seasonal trends, we found that cyclone passage (hence, cooling) mainly occurred over 3 days, with maximum SST reduction of −0.4°C. The greatest reduction was found along the subpolar front, where frontal sharpness (i.e., SST gradient) increased by 0.1°C (100 km)−1. Results of a mixed‐layer model were consistent with both temperature and frontal sharpness, and localized surface cooling along the subpolar front resulted from both horizontal heat advection and turbulent heat fluxes at the sea surface. Further analyses show that this localized cooling from horizontal heat advection is caused by the cross‐frontal Ekman flow (vertically averaged over the mixed layer) and strong northwesterly winds associated with the cold‐air outbreak during cyclone passage.

Decadal intensified and slantwise Subpolar Front in the Japan/East Sea

“…The changes in the intensity and position of the SPF may profoundly influence the overlying atmosphere and passing extratropical cyclones (Yoshiike and Kawamura, 2009;Yamamoto, 2013). Winter extratropical cyclones develop rapidly over the JES and enter into the northwestern Pacific (Zhao et al, 2016;Zhao et al, 2018). The decadal trend of the SPF in the JES may have far-reaching influences on the weather over Japan and the northwestern Pacific Ocean.…”

Modeled MABL Responses to the Winter Kuroshio SST Front in the East China Sea and Yellow Sea

“…Considering the air‐sea interaction, the changes of atmosphere caused by the SST front will give feedback to the ocean. For example, Zhao et al () proved that the development of cyclones will influence the structure and intensity of the subpolar SST front by heat flux transmission and turbulence. In our SST forcing experiment, to get the effect of SST forcing on the MABL, atmosphere‐to‐SST influence is ignored.…”

Modulation of Extratropical Cyclones by Previous Cyclones via the Sea Surface Temperature Anomaly Over the Sea of Japan in Winter

“…The FNL data set has a horizontal resolution of 1° in latitude and longitude and 27 vertical layers from the surface to 10 hPa. The period considered was used from November 2003 to February 2011, as in the work of Zhao et al (). In addition to the preprocessing of cyclone detection (see the next subsection), this data set was used to set initial and lateral boundary conditions in the numerical model.…”

“…Second, the same experiments were conducted, except that a negative SST anomaly (colored shading in Figure ) was set for the Sea of Japan (i.e., a low SST run, hereinafter, “LSST” runs). This anomaly was obtained from Zhao et al (), who computed the SST reduction caused by the passage of a single cyclone over the Sea of Japan. We therefore referred to this modeling as the sensitivity experiment, because the modeled difference between the CNTL and LSST runs showed the influence of a cyclone that had passed previously on the following cyclone via SST in the Sea of Japan.…”

“…Among such seas, the Sea of Japan, which is the focus of this study, may be the most active region of air‐sea interactions in East Asia, especially during winter when the strong monsoon conveys the cold air mass into the whole of East Asia (e.g., Nakamura, ; Nakamura et al, ). Zhao et al () showed that the Sea of Japan experienced strong surface cooling due to a single cyclone that reduced the SST by less than −0.5 °C around the subpolar front (∼0.25 °C averaged over the Sea of Japan) and that this SST reduction was sustained over the course of 4 days.…”