Observed Changes in the Activity of Baroclinic Waves and Their Feedbacks during the Life Cycles of Low - Circulation Anomalies in the Northern Hemisphere Winter.

Variations in the baroclinic wave activity and their feedback have been investigated, using a long data set of the tropospheric circulation in the Northern Hemisphere winter. Composite analysis based upon a large number of anomaly events has been performed relative to the onset and decay times of five anomaly patterns in the low-frequency circulation. The two polarities of these patterns correspond to the blocking and zonally-extended westerlies. A slowly-varying "envelope function" is defined locally to represent the amplitude modulation of baroclinic waves associated with the evolving low-frequency circulation anomalies. The most striking feature observed during the blocking onset is the anomalously high values of the envelope function upstream which, in most cases, correspond to the development of a deep surface low. As the blocking signature is fully established, a "sandwich structure" emerges, with intensified baroclinic wave activity along the northern and southern flanks of the blocking pattern and suppressed activity in between, consistent with the anomalies in the background flow. During the onset of the other polarity, baroclinic wave activity tends to be suppressed upstream, followed by the emergence of a sandwich structure with the reversed polarity as the midlatitude westerlies approach their full strength. In the composite maps which have been adjusted so as to minimize the cancellation between the individual eddy components, one or two strong anticyclonic baroclinic eddies appear to advect the low potential vorticity air systematically from the subtropics into the midlatitude where the blocking anticyclone is developing. These interactions tend to be nonlinear if the block is developing far north of the mean stormtracks. The vorticity flux feedback of baroclinic eddies appears to account for less than 30% of the observed growth of the low-frequency circulation anomalies during their onset, and to be even smaller during the decay stage. Anomalous vorticity advection by the low-frequency circulation anomalies during their onset, and to be even smaller during the decay stage. Anomalous vorticity advection by the low-frequency circulation can explain most of the observed evolution of these anomalies during their onset and decay stages. In contrast, the eddy feedback tends to be relatively important in maintaining those anomalies in their fully-developed stage.