熱帯域東西気圧振動(南方振動)の冬期北半球大気循環に及ぼす影響

Abstract

An analysis is made of dynamical features of planetary waves and zonal mean winds in the troposphere during the Northern winter from January 1963 to December 1982, by paying a special attention to the differences between the years of negative and positive extremes of the Southern Oscillation Index (SOI).The difference is more evident in Late winter (January-February-March) than in Early winter (November-December). Mean zonal winds in the Late winter are closely related to the SOI; they are in positive correlation at extratropical latitudes and in negative correlation at subtropical latitudes. Consequently, the difference of mean zonal wind speeds between the negative and positive extremes of the SOI shows a barotropic seesaw pattern in meridional cross section with a node around 40°N. This pattern is associated with the strong (weak) subtropical jet and weak (strong) mid-latitude westerlies in the negative (positive) extremes.Wave quantities averaged over the Late winter show clear differences between the two categories. In the mid-latitude troposphere, wave driving (DF) has larger negative values in the negative extremes than in the positive extremes; this is due mainly to the vertical derivative of heat flux (F(z)). On the other hand, around the subtropical jet region, DF has smaller negative values in the negative extremes than in the positive extremes; this is due to horizontal derivative of the momentum flux F(y). Thus the weaker DF due to both F(y) and F(z) is related to the stronger mean zonal winds.The height fields over the Northern Hemisphere are also classified into two patterns: Wave 2 in the negative extremes with two troughs over the western Pacific and western Atlantic, and Wave 3 in the positive extremes with another major trough lying over the Black Sea. The differences in the wind field between the two categories are mainly attributed to the development of Atlantic anticyclones. Notable wave-like and teleconnective patterns are found especially in the positive extremes. It is speculated that the differential heating at low latitudes could modulate mid-latitudes circulations.