Responses of Polar Mesocyclone Genesis to Topographic Forcing along the Eastern Coast of Eurasian Continent

Polar mesocyclones (PMCs) occur frequently over the northern Sea of Japan. In the present study, topographic effects on PMC genesis in this region were investigated using long-term numerical simulations extending over 36 winter seasons. Sensitivity experiments showed that PMC genesis decreases in the part of the northern Sea of Japan when the mountain region at the eastern end of the Eurasian continent is removed. For instance, the generation of PMCs over offshore west of Hokkaido decreases significantly when the mountain range is removed, whereas the generation of PMCs over the Strait of Tartary remains unchanged. According to a composite analysis, this result can be attributed to the different responses of subregional oceanic surface wind to the removal of the mountains. In the experiment without mountains, cold air outbreaks from the continent blow directly over the Sea of Japan causing strong westerly winds over the offshore west of Hokkaido. Consequently, PMCs tend to make landfall earlier and before reaching maturity. The uniformly distributed westerly wind also has a negative impact on PMC genesis because of weakened horizontal wind shear and meridional temperature gradient. By contrast, the low-level wind over the Strait of Tartary before PMC genesis is unaffected by the mountains, and thus, topographic effects are not required for PMC genesis in this region. These results indicate that the responses of PMCs to topographic forcing have a regional variability.