Previous studies have suggested the importance of blocking high (BH) development for the occurrence of stratospheric sudden warming (SSW), while there is a recent study that failed to identify their statistical linkage. Through composite analysis applied to high-amplitude anticyclonic anomaly events observed around every grid point over the extratropical Northern Hemisphere, the present study reveals a distinct geographical dependence of BH influence on the upward propagation of planetary waves (PWs) into the stratosphere. Tropospheric BHs that develop over the Euro-Atlantic sector tend to enhance upward PW propagation, leading to the warming in the polar stratosphere and, in some cases, to major SSW events. In contrast, the upward PW propagation tends to be suppressed by BHs developing over the western Pacific and the Far East, resulting in the polar stratospheric cooling. This dependence is found to arise mainly from the sensitivity of the interference between the climatological PWs and upward-propagating Rossby wave packets emanating from BHs to their geographical locations. This study also reveals that whether a BH over the eastern Pacific and Alaska can enhance or reduce the upward PW propagation is case dependent. It is suggested that BHs that induce the stratospheric cooling can weaken the statistical relationship between BHs and SSWs.

The relatively warm 2009–2010 Arctic winter was an exceptional one as the North Atlantic Oscillation index attained persistent extreme negative values. Here, selected aspects of the Arctic stratosphere during this winter inspired by the analysis of the international field experiment RECONCILE are presented. First of all, and as a kind of reference, the evolution of the polar vortex in its different phases is documented. Special emphasis is put on explaining the formation of the exceptionally cold vortex in mid winter after a sequence of stratospheric disturbances which were caused by upward propagating planetary waves. A major sudden stratospheric warming (SSW) occurring near the end of January 2010 concluded the anomalous cold vortex period. Wave ice polar stratospheric clouds were frequently observed by spaceborne remote-sensing instruments over the Arctic during the cold period in January 2010. Here, one such case observed over Greenland is analysed in more detail and an attempt is made to correlate flow information of an operational numerical weather prediction model to the magnitude of the mountain-wave induced temperature fluctuations. Finally, it is shown that the forecasts of the ECMWF ensemble prediction system for the onset of the major SSW were very skilful and the ensemble spread was very small. However, the ensemble spread increased dramatically after the major SSW, displaying the strong non-linearity and internal variability involved in the SSW event

The western Pacific (WP) pattern, characterized by north–south dipolar anomalies in pressure over the Far East and western North Pacific, is known as one of the dominant teleconnection patterns in the wintertime Northern Hemisphere. Composite analysis reveals that monthly height anomalies exhibit baroclinic structure with their phase lines tilting southwestward with height in the lower troposphere. The anomalies can thus yield not only a poleward heat flux across the climatological thermal gradient across the strong Pacific jet but also a westward heat flux across the climatological thermal gradient between the North Pacific and the cooler Asian continent. The resultant baroclinic conversion of available potential energy (APE) from the climatological-mean flow contributes most efficiently to the APE maintenance of the monthly WP pattern, acting against strong thermal damping effects by anomalous heat exchanges with the underlying ocean and anomalous precipitation in the subtropics and by the effect of anomalous eddy heat flux under modulated storm-track activity. Kinetic energy (KE) of the pattern is maintained through barotropic feedback forcing associated with modulated activity of transient eddies and the conversion from the climatological-mean westerlies, both of which act against frictional damping. The net feedback forcing by transient eddies is therefore not particularly efficient. The present study suggests that the WP pattern has a characteristic of a dynamical mode that can maintain itself through efficient energy conversion from the climatological-mean fields even without external forcing, including remote influence from the tropics.

ABSTRACT:A diagnostic framework is introduced in which anomalous zonally averaged Rossby wave-activity injection into the stratosphere is decomposed into a contribution solely from zonally confined upward-propagating Rossby wave packets and another from interaction of the wave packets with the climatological planetary waves. To pinpoint the tropospheric sources of the wave packets, a particular form of wave-activity flux is evaluated for the associated circulation anomalies. The framework is applied to reanalysis data for the period prior to a stratospheric sudden warming (SSW) event in January 2006, which was associated with two successive events of above-normal wave-activity injection from the troposphere. In the earlier event, a pair of wave packets that emanated from tropospheric anomalies over the North Pacific and over Europe enhanced the upward wave-activity injection, which was augmented further by their interaction with the climatological planetary wave. In contrast, in the later period a wave packet that emanated from an anticyclonic anomaly over the North Atlantic is found to be the primary contributor to the enhanced planetary wave-activity injection, while its interaction with the climatological planetary wave contributed negatively. The predominant importance of the sole contribution from a single wave packet is also found in a major SSW event observed over Antarctica in September 2002. These results indicate that the diagnostic framework presented in this study is a useful tool for understanding the interaction between anomalies associated with zonally confined wave packets and climatological-mean planetary waves in the study of stratosphere-troposphere dynamical coupling.