East Asian cold outbreaks get a path index: the side of Mt. Baitoushan that cold air takes splits snowfall zones
Synopsis
Cold air outbreaks over East Asia are now sorted by which side of Mt. Baitoushan the cold airmass flux passes, not by intensity alone. Using JRA-3Q reanalysis for winters from 1979/80 to 2023/24, researchers computed the flux of cold airmass and applied self-organizing maps to group 2,052 outbreak cases into Western, Northern, and Intermediate groups, then defined a pathway index as the log ratio of flux west and east of the mountain. The index correlates negatively with 925 hPa wind convergence over the Sea of Japan, so the western route favors the Japan-Sea Polar airmass Convergence Zone and heavier coastal snowfall.
Interpretation
East Asian cold air outbreaks can now be split into three types by which side of Mt. Baitoushan the cold air descends, rather than by intensity or meridional extent alone. Earlier classifications either sorted outbreaks by meridional extent or examined individual severe events, leaving the horizontal two-dimensional pathway of the cold airmass flux unclassified. Across winters from 1979/80 to 2023/24, 2,052 outbreak cases fell into the Western, Northern, and Intermediate groups, with the Intermediate group most frequent, the Northern group next, and the Western group least frequent.
The classification rests on an index computed from the ratio of two rectangular area-averaged fluxes, so it does not require a prior synoptic typing step. The index takes the base-2 logarithm of the flux ratio between the west and east sides of Mt. Baitoushan, turning the ratio into a simple sign change. Self-organizing maps on the flux field produced 25 nodes; sorting them by ascending index and using peaks in the difference between adjacent ranks at positions 6 and 19 cut the 25 nodes into three groups.
When cold air takes the western route, horizontal wind convergence over the Sea of Japan is stronger, giving climatological support to the formation of the Japan-Sea Polar airmass Convergence Zone. The earlier judgment that the Changbai Mountains block cold air, strengthen the western flux, and develop the convergence zone came mainly from case studies, with limited climatological statistical evidence. The pathway index and area-averaged 925 hPa horizontal wind convergence over the central Sea of Japan correlate at r = -0.51 with p far below 0.01, and the three group means lie close to the regression line.
Perspective
The classification targets cold air outbreaks under the East Asian winter monsoon, using JRA-3Q reanalysis for winters from 1979/80 to 2023/24 at 1.25° × 1.25° horizontal and 6-hourly temporal resolution. The index is computed from area-averaged cold airmass flux in two rectangles west and east of Mt. Baitoushan, and applies to judging the likelihood of Japan-Sea Polar airmass Convergence Zone formation and the regional differences in snowfall along Japan. The authors propose applying the method to future climate projection data and note that the reanalysis resolution is insufficient for mesoscale convection, so higher-resolution data are needed to examine whether the convergence zone extends from the western Sea of Japan to the Tohoku coast.
The boundaries between the three groups come from local peaks in the difference between adjacent node indices, with an added requirement that each group hold a reasonable number of nodes, so the cut points carry some subjectivity. The Intermediate group is internally varied: some nodes show a large pressure gradient and strong cold air transport, others a small gradient and weak transport. The reanalysis resolution cannot resolve mesoscale convection, so whether the convergence zone truly reaches from the western Sea of Japan to the Tohoku coast is not directly verified. How outbreaks themselves change under future warming is left to a separate study.