Regional characteristics of winter cyclone activity around Hokkaido and its multidecadal trend asso
ciated with surface pressure patterns over Northeast Asia()
In East Asia, strong winter monsoon and high baroclinicity enhance cyclone activity. Winter cyclone
activity modulates the daily weather over Japan by inducing low temperatures and snowfall. Strong wind andheavy precipitation induced by the winter cyclone can cause disasters. It is well known that mesoscale cyclones
frequently occur in the Sea of Japan, causing locally but strong winds and heavy snowfall. Despite the highfrequency of genesis of mesoscale cyclones around the Sea of Japan, most previous studies on the cyclone
activity in the decadal timescale have focused only on synoptic-scale cyclones. Since mesoscale cyclones cause
local snowfall, the understanding of the decadal trends in winter cyclone activity, including mesoscale cyclones,
can provide new insights into regional responses to climate change, especially in Hokkaido where the frequencyof mesoscale cyclones is high. The purpose of this study is to clarify the decadal trends of winter cyclone activity
that include mesocyclones over and around northern Japan. In addition, the regional features of mesoscale
cyclogenesis in the northern part of the Sea of Japan were investigated by using numerical experiments.
The decadal trend of the passage of cyclones over and around Japan, including mesoscale cyclones, wasinvestigated during 62 winter seasons (DecemberMarch) from 1958/1959 to 2019/2020 using the long-term
reanalysis dataset JRA-55. During the study period, the passage of cyclones around northern Japan showed adecreasing trend. Most of the cyclones that pass around northern Japan are generated over the northern part of
the Sea of Japan where mesoscale cyclogenesis frequently occurs. It was also found that the duration of thecyclones generated in the northern part of the Sea of Japan is relatively short compared to the cyclones thatoriginated in the other regions.
Analyses based on the weather pattern classification revealed that the passage of upper-level troughs andlower-level cold air outbreaks over the northern part of the Sea of Japan are preferential conditions for thecyclogenesis in the northern part of the Sea of Japan. These conditions are consistent with the conditions for thedevelopment of mesoscale cyclones in this region. Statistical analysis indicated that the magnitude of the cold air
outbreak in this region was weakened. Hence, the changes in atmospheric conditions due to climate change may
have reduced mesocyclone genesis over the northern Sea of Japan, resulting in a significant reduction in thenumber of cyclones passing over and around Hokkaido over the past 60 years.

In this study, the role of the surrounding topography in the cyclogenesis over the northern part of the Sea ofJapan was also investigated. Long-term numerical experiments using the regional climate model WRF (TheWeather Research and Forecasting model) were conducted to investigate the effects of mountains located on theeastern coast of the Eurasian continent (i.e., the Sikhote-Alin mountain region) on the mesoscale cyclones
generated over the northern part of the Sea of Japan. Weather pattern classification clarified that the effects of themountains on the mesoscale cyclone vary depending on synoptic-scale atmospheric conditions. The difference
in sensitivity is due to the difference in the development mechanisms of the mesoscale cyclones. Mesoscale
cyclogenesis over the northern part of the Sea of Japan associated with the Siberian high and Aleutian low iscaused by the lower-level horizontal wind shear formed over the offshore region west of Hokkaido. This
horizontal wind shear is caused by the northwesterly winds that blow across the Sikhote-Alin mountain region.
Sensitivity experiment revealed that the removal of the Sikhote-Alin mountain region weakens the horizontal
shear of the lower-level winds over the northern part of the Sea of Japan and decreases this type of mesoscale
cyclogenesis. Therefore, the mesoscale cyclogenesis caused by lower-level horizontal wind shear is very
sensitive to the local topography. In contrast, the number of occurrences of mesoscale cyclogenesis induced bybaroclinic instability did not vary even after the removal of mountains. It is suggested that formation of themesoscale cyclones is not affected by the surrounding topography if their development is driven by upper-levelatmospheric conditions.
The analysis using reanalysis dataset and high-resolution datasets derived from the regional climate modelclarified the regional characteristics of winter cyclone activities around northern Japan. The results aresummarized as follows; 1) the passage frequency of winter cyclones around Hokkaido originated from thenorthern part of the Sea of Japan has decreased since the 1960s, 2) the weakening of the cold air outbreak overthe northern part of the Sea of Japan suppressed the mesoscale cyclogenesis in this region, and 3) the mesoscale
cyclogenesis over the northern part of the Sea of Japan associated with the Siberian high and Aleutian low iscaused by the lower-level wind shear formed by the mountains along the eastern coast of the Eurasian continent.
Decadal trend of cyclone activity over recent years and the factors behind that revealed in this study can provide
new insights into the response of regional climate in Hokkaido under ongoing climate change.