Stratospheric sudden warmings and slowly propagating zonal-mean zonal wind anomalies

Abstract

The relationship between stratospheric sudden warming (SSWs) and slowly propagating zonal wind anomalies during the northern hemisphere cold season (November to March) is investigated using two leading empirical orthogonal functions (EOFs) of 15-day mean zonal-mean zonal wind. Poleward and downward propagation of the zonal-mean zonal wind anomalies can be translated as rotation of state vector in a plane spanned by the two EOFs. Major SSWs occur in phase with the slowly propagating zonal wind anomalies, which implies a "conditioning" of the atmosphere by the slowly varying state. Movement of the state vector suggests quasi-periodicity of the order of 4 months, although generally only one cycle is achieved during a major warming winter. This characteristic feature can be compared to the stratospheric vacillation cycle in a mechanistic model. It is also shown that changes in the tropospheric planetary waves associated with the slowly propagating zonal wind anomalies coincide with changes in the Arctic Oscillation. Copyright 2000 by the American Geophysical Union.