Contribution of cloud-fog transitions near the Kuroshio Extension to the fog-prone area in the northwest Pacific in summer

Abstract

Observations show that the northwest Pacific is a fog-prone area in summer. The frequency of fog increases rapidly north of the Kuroshio Extension (KE). Based on the observations and reanalysis results from 1979 to 2020, this study shows that there is a climatic correlation between the stratus clouds frequency over the KE area (33-39°N, 140-155°E) and the fog frequency over the fog-prone area (40-50°N, 145-165°E) (cor=0.47, p<0.05). This suggests that the stratus clouds over the KE area may be advected southward by southerly winds and transform into fog over the fog-prone area, contributing to the formation of the climatologically foggy area east of the Kuril Islands. The statistical results also indicate a significant negative correlation between the intensity of the sea surface temperature (SST) front over the KE area and the fog frequency over the fog-prone area (cor=-0.71, p<0.05), implying that the KE plays a key role in the cloud-fog transitions. A weak SST front over the KE area lowers the cloud base height by weakening the ascending branch of the secondary circulation on the warm flank of the SST front, thus facilitating the cloud-fog transitions on the north side of the KE area. This study enriches the understanding of sea fog formation in the North Pacific, proposes the contribution of cloud-fog transition to the climatological foggy area, and reveals the key role of air-sea interaction in it, which can provide theoretical support for sea fog forecast in this area.