Downward-Propagating Temperature Anomalies in the Preconditioned Polar Stratosphere

Abstract

Dynamical links of the Northern Hemisphere stratosphere and troposphere are studied, with an emphasis on whether stratospheric changes have a direct effect on tropospheric weather and climate. In particular, downward propagation of stratospheric anomalies of polar temperature in the winter?spring season is examined based upon 22 years of NCEP?NCAR reanalysis data. It is found that the polar stratosphere is sometimes preconditioned, which allows a warm anomaly to propagate from the upper stratosphere to the troposphere, and sometimes it prohibits downward propagation. The Arctic Oscillation (AO) is more clearly seen in the former case. To understand what dynamical conditions dictate the stratospheric property of downward propagation, the upper-stratospheric warming episodes with very large anomalies (such as stratospheric sudden warming) are selected and divided into two categories according to their downward-propagating features. Eliassen?Palm (E?P) diagnostics and wave propagation theories are used to examine the characteristics of wave?mean flow interactions in the two different categories. It is found that in the propagating case the initial wave forcing is very large and the polar westerly wind is reversed. As a result, dynamically induced anomalies propagate down as the critical line descends. A positive feedback is that the dramatic change in zonal wind alters the refractive index in a way favorable for continuous poleward transport of wave energy. The second pulse of wave flux conducts polar warm anomalies farther down. Consequently, the upper-tropospheric circulations are changed, in particular, the subtropical North Atlantic jet stream shifts to the south by ?5 degrees of latitude, and the alignment of the jet stream becomes more zonal, which is similar to the negative phase of the North Atlantic Oscillation (NAO).