Abstract
Coupling between tropospheric and stratospheric leading modes in anomaly fields is investigated. By using daily data at many levels in addition to monthly mean data, the transition of spatial patterns and the direction and speed of the vertical propagation are examined in detail.
Results show that the North Atlantic Oscillation mode (NAO) and the Pacific–North American mode (PNA) dominating in the troposphere couple with the annular mode (AM) and a wavenumber-1 mode (W1) dominating in the stratosphere, respectively, with significant temporal correlations. The transition of the patterns occur at about 150 hPa.
The couple of NAO–AM (first mode) amplifies almost simultaneously from the surface to the 10-hPa level. Sometimes amplifications are repeated a few times, in which maxima of the amplification move to the lower atmosphere. Viewing these sequences from a relatively long time scale, the first mode slowly propagates to the lower atmosphere. The coupling of PNA–W1 (second mode) propagates from the troposphere into the stratosphere in about 1 week.
There is some relationship between the first and second modes. After the second mode propagates from the troposphere into the stratosphere, the first mode develops with a lag of 10–15 days. However, this relationship has different characteristics between positive and negative phases.
Since large-amplitude AM with one phase corresponds to the stratospheric sudden warming, the sudden warming is caused by the following sequence. The PNA with a negative anomaly over the Pacific amplifies in the troposphere, exciting the W1 with a positive anomaly over North America in the stratosphere, which causes the sudden warming. Thus, amplification of the PNA leads to the sudden warming.
Corresponding author address: Prof. Hisanori Itoh, Dept. of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan. Email: itoh@weather.geo.kyushu-u.ac.jp
