The Structure and Maintenance of Stationary Waves in the Winter Northern Hemisphere

Abstract

Previous studies of extratropical stationary waves in the winter Northern Hemisphere (NH) often focused on effects of orography and land?ocean thermal contrast on the formation, structure, and maintenance of these waves. In contrast, research attention to tropical stationary waves was attracted by the summer monsoon circulations and the ENSO-related climate variability. Consequently, the structure and basic dynamics of tropical stationary waves and the relationship of these waves with those in mid?high latitudes have long been neglected. Thus, the following several distinct features of observed winter NH stationary waves have not been explained: 1) an abrupt change in the longitudinal phase across 30°N; 2) a transition from the vertical phase reversal of tropical stationary waves to the vertically westward tilt of extratropical stationary waves; and 3) a longitudinally quarter-phase relationship between stationary waves and east?west circulations, and a reversal of this relationship across 30°N. It is inferred from a spectral streamfunction budget analysis with the NCEP?NCAR reanalyses that these wave features are caused by the transition of wave dynamics from the Sverdrup regime in the Tropics to the Rossby regime in the mid?high latitudes. Based on the simplified vorticity equations of these two dynamic regimes, analytic solutions obtained with observed velocity potential fields (which were used to portray the global divergent circulation) confirm that the aforementioned distinct features of stationary waves are attributed to the dynamics transition across 30°N. Since east?west circulations are part of the global divergent circulation, it is revealed from a diagnosis of the velocity potential maintenance equation that this circulation component is maintained in the Tropics primarily by diabatic heating and in the mid?high latitudes by both horizontal heat advection and diabatic heating. Evidently, stationary waves are maintained by diabatic heating through the divergent circulation and the dynamics transition of these waves from the Sverdrup regime to the Rossby regime is attributed to strong midlatitude westerlies.