Circulation Regime-Dependent Nonlinear Interactions during Northern Hemisphere Winter

Abstract

Average statistics for periods of large positive 500 mb height anomalies are compared to statistics for all other situations using NMC data for the 15 Januaries from 1963 to 1977. The 500 mb heights and geostrophic streamfunctions are represented as surface spherical harmonics, and energy and enstrophy spectra along with nonlinear wave-wave interaction statistics are computed. Differences in 500 mb geopotential height variance, kinetic energy and enstrophy spectra occur between large positive anomaly events and other days in the two-dimensional spectral index band from roughly n=6 to n=9, where n is the degree of the associated Legendre function. The same index band experiences a reversal of both the usual kinetic energy and enstrophy cascades during large positive anomaly events. That is, the 6≤n≤9 band gains energy and enstrophy from wave-wave interactions during the anomaly events and loses energy and enstrophy by the same process at other times. The source of this energy and enstrophy is higher index (smaller two-dimensional scale) waves. The indication is that the Atlantic cases are more Subject to this cascade reversal than are Pacific events. Our results suggest that the smaller scale, transient eddies may play a regime-dependent role in interactions with atmospheric circulation modes on the scale of the persistent anomalies. When interacting with larger-scale features, the role of smaller-scale transients may not always be dissipative.