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. Keywords polar mesocyclone; Sea of Japan; Strait of Tartary; topographic forcing; long-term numerical simulations Citation Tamura, K., and T. Sato, 2020: Responses of polar mesocyclone genesis to topographic forcing along the eastern coast of Eurasian Continent.

This study investigated the impact of land surface heterogeneity on Mesoscale Convective System (MCS) initiations in East Asia, using geostationary satellite data during June-August from 1996 through 2018. The detected MCSs over land exhibited clear diurnal variation with the lowest existence frequency at 10:00 and highest initiation frequency during 12:00−17:00 local time. To quantify land surface heterogeneity, the spatial standard deviation of equivalent Black-Body Temperature (TBB) within a cloud-free 0.35° × 0.35° box (σLST BB : Land Surface TBB) was computed for 10:00 each day. A comparison of the σLST BB and MCS databases revealed that the probability of MCS initiations increased with increasing σLST BB in East Mongolia while the probability was not sensitive to σLST BB in East China. This indicated that MCSs tend to form over heterogeneous land surface conditions in the semiarid region. We found that the impact of land surface heterogeneity on MCS initiations was highest over flat terrain in East Mongolia, where the convection trigger due to topographically-induced circulation was absent. These results suggest that the impact of land surface heterogeneity on MCS initiations during the warm season varies with climate zones and terrain complexities in East Asia, with strongest impact in semiarid and flat regions.

In East Asia, a strong winter monsoon and high baroclinicity cause enhanced cyclone activity, which has been the subject of intensive study. Despite the high frequency of genesis of mesoscale cyclones in this area, most of the studies on the decadal timescale have focused only on synoptic‐scale cyclones. To clarify the interannual variation of the passage of winter cyclones including synoptic‐scale and mesoscale cyclones over and around Japan, a tracking algorithm was applied to 62 winter seasons (December–March) from 1958/1959 to 2019/2020. During the study period, the passage of cyclones around northern Japan showed a decreasing trend. Most cyclones that pass around northern Japan are generated over the northern part of the Sea of Japan, which is an area where cyclogenesis has decreased during the study period. Weather pattern classification based on synoptic scale atmospheric conditions revealed that passage of upper‐level troughs and occurrence of lower‐level cold air outbreaks are the preferential conditions for cyclogenesis in this region. In addition, the intensity of the cold air outbreak in this region was weakened during the study period. These results suggest that changes in atmospheric conditions due to climate change have reduced mesocyclone genesis over the northern Sea of Japan, resulting in the reduction in the number of cyclones passing over and around northern Japan.

<p>Polar mesocyclones (PMCs) are mesoscale, maritime cyclones that occur around the high latitudes in the cold seasons. Over the northern Sea of Japan, PMC frequently occurs with cold air outbreaks from the east of the Eurasian Continent. In this study, effects of the mountains on the eastern end of the Eurasian Continent (Sikhote-Alin mountain range) on the PMCs genesis were examined by 36-years long-term numerical experiments. The sensitivity experiment, in which the Sikhote-Alin mountain range is removed, shows that the number of PMC genesis decreases and the duration between PMCs genesis and landfall becomes shorter compared with realistic experiment. These differences arise only in the southern part of the sea. This result suggests that the effect of the orographic forcing on PMC's behavior varies with the location of the PMCs genesis.</p>

East Asian winter weather has altered dramatically in response to progressing climate change. However, analysis of time‐averaged atmospheric fields might obscure climate change signals that could relate to specific types of events. This study aimed to detect climate change signals for various background atmospheric fields by applying classification to daily surface pressure patterns. We found a pronounced trend of increase in surface air temperature over Japan since the 1980s, stronger than the seasonal mean trend, when dominating cold air advection from the Sea of Okhotsk. This cold air was warmed and humidified by the retreat of sea ice in the Sea of Okhotsk. In contrast, the warming trend was mitigated when warm air advection was dominant. These results highlight the importance of daily background atmospheric conditions in amplifying and localizing the impact of regional climate change.