ON THE ORIGIN AND NATURE OF THE INTERANNUAL VARIABILITY OF THE WINTER STRATOSPHERIC CIRCULATION IN THE NORTHERN-HEMISPHERE

During the northern hemisphere winter, the stratospheric circulation exhibits quite large interannual variability. In the present study, the nature and origin of the interannual variation in the winter stratosphere is studied with two different approaches: one is composite analysis assuming a priori four possible causes: solar activity, equatorial quasi-biennial oscillation (QBO), volcanic aerosols, and unknown cause producing trends, while the other is an extended empirical orthogonal function analysis (E-EOF). The results of composite analysis show that in November, anomalous zonal-mean zonal winds and meridional temperature gradients are found in accordance with the assumed forcings in different regions in the stratosphere. As winter progresses, however, anomalous patterns in zonal wind become similar by forming a dipole-type pattern and extend down into the troposphere. The common features of the evolution of the anomalies during the winter can be well explained by the first mode of the E-EOF. The results of the present study suggest that interannual variability in the winter stratosphere in the northern hemisphere can be considered an internal mode of variability triggered by changes in forcing, such as solar activity, equatorial QBO, and volcanic aerosols. Although overall features of the evolution of zonal-mean zonal wind associated with trends are similar, the cause of the trends is still undetermined.