Radiosonde Observational Evidence of the Impact of an Extremely Cold SST Spot on a Mesoscale Anticyclone

Radiosondes were launched in August 2006 from a research vessel above a spot around the Kuril Islands between the North Pacific and the Okhotsk Sea characterized by an extremely cold sea surface temperature (SST) and a large horizontal temperature gradient caused by strong tidal mixing. Because this cold SST was caused solely by this oceanic process, the observed data chiefly reflect the oceanic influence on the atmosphere. In this area, where the summertime SST is typically lower than in other seas in the same latitude belt, a minimum SST of 2 °C and a horizontal gradient of 7 °C/10 km were observed in August 2006. During the cruise, the area was covered by a layer of fog up to about 1000 m thick. The following observations reveal the presence of an atmospheric mesoscale anticyclone and a cold dome caused by ocean cooling of the atmosphere. The wind direction over the cold spot and the strong-gradient region was northeasterly below the altitude of about 500 m, in contrast to the southwesterly geostrophic wind, and the area of northeasterly wind was characterized by a cold air mass and a high-pressure anomaly. This ageostrophic wind distribution indicates mesoscale anticyclonic circulation below 500 m. The fog layer was also thinner in the northeasterly wind area than in other areas, suggesting vertical dry-air advection (i.e., a subsidence) associated with the mesoscale anticyclone. In addition, our observational results suggest that the pressure adjustment mechanism can explain this mesoscale circulation.