Observed Changes in Baroclinic Wave Activity during the Life Cycles of Low-Frequency Circulation Anomalies

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Abstract
Anomalous baroclinic wave activity during the onset and decay of blocking patterns in the Northern Hemisphere winter is investigated based upon a 30-year record of 500 mb height and sea-level pressure. A slowly-varying “envelope function” is defined at each gridpoint in order to represent the modulation of the amplitude of baroclinic wave activity by the low-frequency circulation anomalies related to blocking. The envelope function fields and the low-frequency circulation fields are composited relative to the onset and decay times of four persistent dipole patterns in low-frequency circulation anomalies located downstream of the climatological-mean stormtracks. One polarity of the anomaly patterns corresponds to dipole or modon-like blocking patterns over the eastern oceans, and the other to strong zonal flow patterns. Each of the eight composites is comprised of more than 50 persistent blocking or zonal flow episodes. The most striking common feature observed during the onset of blocking over the midlatitude eastern oceans is the enhancement of baroclinic wave activity along the upstream stormtrack. This enhancement tends to occur about five days before the blocking pattern is fully established. In most cases, the enhanced baroclinic wave activity depicted by the envelope function corresponds to the development of a deep surface low. This anomalously strong baroclinic wave activity then splits into two branches: one spreading northeastward along the western flank of the developing anticyclone, and the other southeastward around the cyclonic low-frequency circulation anomaly developing to the south of the block. As the dipole structure in the low-frequency circulation anomalies associated with the block becomes well established, baroclinic wave activity upstream returns to normal, and a ‘sandwich structure” emerges, with intensified baroclinic wave activity in the enhanced westerly flow along the northern and southern flanks of the dipole and suppressed baroclinic wave activity in between, where the westerlies are weaker than normal.

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