Cross-Frequency Coupling, Skewness, and Blocking in the Northern Hemisphere Winter Circulation

Variability in daily wintertime [December-February (DJF)] 500-hPa heights on low [L: <(30 day)(-1)], intermediate [M: (6-30 day)(-1)], and high [H: >(6 day)(-1)] frequencies is examined using 40-yr ECMWF Re-Analysis (ERA-40) data. Leading EOFs of L correspond to planetary-scale teleconnection patterns; those of M to retrograding, eastward-dispersing long waves oriented along great circle routes; and those of H to baroclinic waves in the climatological-mean storm tracks. In the Atlantic sector, EOF 1 of M appears to be embedded in EOF 1 of L. Cross-frequency coupling between L and Mexhibits distinctive patterns. In the Atlantic sector the negative polarity of the North Atlantic Oscillation (NAO) with above-normal heights over Greenland is associated with enhanced M variability over Greenland. An analogous relationship is observed in the Pacific sector between an NAO-like pattern and the variance of Mover Alaska. Cross-frequency coupling between L and H in both sectors is indicative of a reinforcement of the background flow by the baroclinic waves. Cross-frequency coupling between L and M is responsible for most of the skewness of the anomalies in the 500-hPa height field. Linear wave dynamics evidently play an important role in M. Composites of high amplitude anomalies of contrasting signs over Baffin Bay exhibit similar spatial structures (apart from the sign reversal) and they exhibit a similar evolution, with westward phase propagation and downstream development characteristic of the behavior of Rossby waves. It is argued that teleconnection patterns exhibit memories much longer than the 7-10-day decorrelation time of daily indices formed by projecting unfiltered daily fields onto their spatial patterns.