Fundamental Mechanisms of Large-Scale Atmospheric Circulation Systems Related to the Decadal-Scale Climate Variability

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1) The presence of the two distinct modes of decadal-scale climate variability in the wintertime North Pacific has been confirmed, one of which is associated with the anomalous subtropical high. The anomalous high was found highly barotropic in its midlatitude portion, whereas its subtropical portion is distinctly baroclinic. This complex structure is found to be suited for the generation of anomalous subsidence, through which the surface and upper-level anomalous circulation can be tightly linked.2) The decadal-scale weakening of the Siberian High and the East Asian winter monsoon has been found to cause systematic enhancing of the storm track activity in the North Pacific despite the relaxed background baroclinicity. This enhancement is apparently opposite to what the linear theory of baroclinic instability predicts. In the presence of the excessively strong subtropical jet in association with the strong monsoon during the early through mid-1980s, synoptic-scale eddies propagating in … More to the North Pacific through the midlatitude tropopause over the Asian continent (9 km in altitude, 45°N) tend to be trapped into the jet core, while lifted up to 3 km and shifted equatorward by 1000 km away from the surface baroclinic zone at 40°N located along the boundary between the subpolar and subtropical oceanic gyres. The vertical coupling of the eddies with the surface baroclinic zone is thus weakened and thus unfavorable for the baroclinic growth of the eddies. In contrast, the eddy trapping has been less likely since the late 1980 s under the relaxed subtropical jet. The eddies were more likely to propagate through the midlatitude tropopause right above the surface baroclinic zone, resulting in their efficient baroclinic growth as observed. The decadal change observed in the eddy trapping is manifested as the structural changes of those eddies.3) A particular form of a wave-activity flux has been derived for the first time, which can represent three-dimensional group-velocity propagation of a Rossby wave packet, either stationary or migratory in the realistic westerlies. It has been revealed through our application of this flux that a stationary Rossby wave train emanated from each of the strong sub-seasonal tropospheric anomalies including blocking can reach into the lower stratosphere, forming a wave train along the polar-night jet. Those wave trains were found to contribute substantially to the observed sub-seasonal variability in the wintertime lower stratosphere. Less