Sea Surface Cooling Induced by Extratropical Cyclones in the Subtropical North Pacific: Mechanism and Interannual Variability

The influence of extratropical cyclones on the ocean is examined with a focus on the North Pacific subtropical front (STF), which is a zone of high sea surface temperature (SST) gradients located approximately 25-30 degrees N. The STF becomes most pronounced and susceptible to frequent passages of cyclones in spring. Satellite observations reveal a significant decrease in SST concurrent with cyclone passage along the STF. The SST decreases for approximately 2days and then gradually recovers over approximately 5days. The magnitude of the cooling is approximately 0.4 degrees C. The ocean reanalysis data show a similar surface cooling with a deepening of the mixed layer. The cooling seems to reach the seasonal thermocline and persist there for more than a week. A cyclone over the STF is a subsynoptic cyclone accompanied by a larger synoptic high to the northwest of the low. The surface cooling is mainly attributable to the decrease in shortwave radiation due to the cloud band of the low and the increases in the latent heat and the ocean entrainment, which are associated with intensified northerly winds between the low- and high-pressure systems. The STF varies in strength on interannual timescales in response to the western Pacific pattern, changing the baroclinicity of the overlying atmosphere and the activity of the cyclones. The intense cyclone activity weakens the seasonal SST warming in spring. It is suggested that the cyclones affect the variability of not only the mixed layer but also the seasonal thermocline. Plain Language Summary An extratropical cyclone is a low-pressure system that affects daily weather and climate. It is well known that intense tropical cyclones such as typhoons and hurricanes leave a cold sea surface wake beneath and behind them. However, little is known about extratropical cyclones. In this study, we examine how extratropical cyclones change the ocean, with a focus on the North Pacific subtropical front (STF), which is a zone of high sea surface temperature gradients located approximately 25-30 degrees N. The STF becomes most pronounced and susceptible to frequent passages of cyclones in spring. We found that extratropical cyclones cause sea surface cooling along the STF, mainly due to the cloud shielding of incoming solar radiation and the enhanced evaporative cooling and ocean mixing. The cooling is not limited to near the surface but seems to reach subsurface depths of approximately 100m and remains longer at this depth than at the surface. The year-to-year variability of the STF alters the occurrence and growth of cyclones over the front, affecting the seasonal warming trend in spring. A large number of cyclone passages weakens the seasonal warming.