Analysis of the Arctic Oscillation Simulated by AGCM

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Abstract
The annular mode in the Northern Hemisphere (NH) extratropical circulation, which has an equivalent barotoropic structure from the surface to the lower stratosphere, is called the Arctic Oscillation (AO), by Thompson and Wallace (1998). The AO is the leading empirical orthogonal function (EOF) mode of the sea level pressure field, and it can be seen throughout the year. It is more dominant in winter. It is characterized as a seesaw pattern of mass, between the Arctic region and the midlatitude belt. It is also a seesaw of the mean zonal wind, between the region poleward of 40°N and that equatorward of it. In this study, an analysis of the simulated AO is made, to confirm that the AO is an atmospheric internal mode, and for studying the transition mechanism to the high/low polarity of the AO. The seasonal and perpetual February runs were made by using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (AGCM). In both runs, the AO signature is dominant. Its magnitude is similar to the observed one. Thus, it is confirmed that the AO is an internal mode of the atmosphere. Using data from a perpetual February integration, a composite analysis of the transition to the high/low polarity of the AO is made. Based upon the transformed Eulerian mean equation for the mean zonal wind, it is investigated which term is responsible for the transition. The results indicate that the wave forcing term contributes to the transition and the term of the residual meridional circulation acts to restore the transition. A decomposition of the wave forcing to zonal wave component indicates that planetary-scale wavenumbers 2 and 3 contribute most to the transition. The synoptic-scale waves contribute partly to the low-latitude wind change. In the perpetual February run, a slowly propagating AO-related stratosphere-troposphere coupled mode, is detected. Its oscillation period is 4-6 months. The anomaly of the zonal-mean, zonal wind first appears in the subtropical stratosphere and propagates poleward. Once the anomaly reaches high latitude, it becomes large and propagates into the troposphere.