THE INFLUENCES OF THE SEA ICE AND THE WIND FIELD ON THE WINTER AIR TEMPERATURE VARIATION IN HOKKAIDO

摘要

Variations of air temperature in Hokkaido during winter are characterized by the presence of two minima in ten-day normals; days 21-30 (end of January) and days 41-50 (middle of February). We name this tendency the "W-phenomenon", which is significant in the northern coastal area of Hokkaido facing the Sea of Okhotsk. The second minimum occurs when the ice cover is most extensive along the Okhotsk coast of Hokkaido. We examine this relation using rotated principal component analysis (RPCA). The RPCA results show that the second mode is a factor of the ice cover that is related to the second minimum. However, the correlation coefficient between rotated principal components of the second mode and ice concentration is rather low, -0.56. The RPCA results also show that the wind field is another factor that governs the "W-phenomenon". The relationship between the rotated principal components and wind directions shows that the drop in temperature in Hokkaido is associated with not only heavy ice cover but also northeasterly wind. Two mechanisms of the northeasterly wind system are expected : weak winds from an anticyclone over Okhotsk ice cover off Hokkaido and strong winds from a cyclone over the Pacific Ocean off the southeast coast of Hokkaido. Both these cases are possibly caused by the heavy ice cover in the Sea of Okhotsk. In the former mechanism, the wind directly carries cold air flow cooled over the sea ice. In the latter, the heavy ice cover prevents cyclones from moving into the Sea of Okhotsk.