Modulation of the stratosphere-troposphere coupling process of the Northern Hemisphere annular mode associated with the ENSO cycle

Abstract

近年, 季節内変動スケールの北半球環状モードが対流圏で観測される際, 成層圏からの西風偏差下方伝播が先立って観測されることが, いくつかの観測的研究によって示されてきた. 本研究では, 長期間の観測データを用いることで, この種の成層圏―対流圏結合過程がENSO サイクルのwarm フェーズとcold フェーズでどのように変調するかを調べた.成層圏での西風偏差下方伝播過程はENSO サイクルによらずに観測されたが, その際に主に寄与する惑星波は異なり, WN 2(cold)とWN 1 (warm)であった. 上向き波活動度フラックス偏差が観測される経度帯もcold フェーズではユーラシア領域, warm フェーズではPNA 領域であった. 成層圏での西風偏差の後に対流圏で西風偏差が観測されるのは，cold フェーズだけたっだ. この時, WN 2 と総観規模擾乱の運動量輸送偏差が対流圏偏差場の形成に寄与した.

Recent observational studies reveal some evidence that, in intraseasonal time scales, thedownward propagation of zonal mean zonal wind anomalies in the stratosphere precedes theNorthern Hemisphere annular mode (NAM) signal in the troposphere. In this study, interannualvariability of this type of stratosphere-troposphere dynamical coupling in winter was examined byusing NCEP/NCAR reanalysis and Met Office HadISST datasets. In particular, we focused uponthe dependence of the coupling on the ENSO phase (warm/cold phase).The downward propagation of zonal wind anomalies in the stratosphere is evident irrespective ofthe ENSO phase, while the zonal wavenumber (WN) of planetary waves responsible for the zonalwind anomalies is different: WN 2 (WN 1) component plays a primary role in the cold (warm)phase. The region where anomalous upward propagation of wave activity is observed also dependson the ENSO phase: the Eurasian (Pacific-North Atlantic) sector for cold (warm) phase. On theother hand, large tropospheric zonal wind anomalies followed by the stratospheric zonal windanomalies appear only for the cold phase. Anomalous poleward propagation of wave activity ofplanetary waves with WN 2 as well as synoptic waves acts to reinforce the tropospheric zonal windanomalies.