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Introduction

It has been a controversy whether, and how, stratospheric changes have a direct effect on tropospheric weather and climate. Recently, Baldwin and Dunkerton (1999) have found downward propagation of AO (Arctic Oscillation) anomaly in low-pass-filtered data with a propagating time of three weeks from 10 hPa to surface. They also noticed that not every AO anomaly in the upper stratospheric propagated down, only those with large amplitude and persistence could have a clear signature through the troposphere. The purpose of this study is to find out under what conditions an upper stratospheric anomaly can propagate down to the troposphere, and when this happens, how the tropospheric circulations are affected. We use polar temperature anomaly (70N-90N) as an indicative feature of downward propagation, which is calculated at different pressure levels from 1000 hPa to 10 hPa based on 22 years (1978-1999) of NCEP/NCAR Reanalysis daily data (Kalnay et al., 1996). We mainly focus on winter-spring seasons because in summer stratosphere temperature anomaly is relatively small. The anomaly is normalized by its standard deviation at different level to minimize the density effect. Then we select those warming episodes with large amplitudes in the upper stratosphere, and divide them in two categories: a propagating feature is defined as the case in which temperature anomaly is greater than 2-standard deviation at 10 hPa and followed by a temperature anomaly greater than 1.5-standard deviation at 200 hPa, and a non-propagating feature as that temperature anomaly is also greater than 2-standard deviation at 10 hPa but followed by a temperature anomaly smaller than 1-standard deviation at 200 hPa.