Precursory Changes in Planetary Wave Activity for Midwinter Surface Pressure Anomalies over the Arctic

Abstract

The development of surface pressure anomalies observed in January over the Arctic is investigated based on composite analysis of observed data. The monthly anomalies in January with a high degree of zonal symmetry are found to extend from the surface into the lower stratosphere, in a manner similar to the Arctic Oscillation (AO). The formation of the AO-like signature in January tends to follow the strengthening/weakening of upward propagation of planetary waves in November over the western portion of Eurasia, where temperature anomalies associated with a train of external Rossby waves modify the thermal structure of the climatological planetary waves. In the lower stratosphere the external Rossby wavepacket also modulates meridional winds associated with the climatological planetary waves, augmenting upward propagations of the planetary waves. Thus, modulated upward propagation of the planetary waves subsequently changes the intensity of the stratospheric polar vortex, leading to the formation of the stratospheric AO-like signals and their downward extension. This study presents an example in which regional modifications in the structures of the upward-propagating planetary waves due to a zonally-confined external Rossby wave train can change the intensity of the stratospheric polar vortex, which may possibly prolong the predictability of seasonal climate anomalies in the Arctic and surrounding regions.