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Initial response of the winter stratospheric polar vortex to central and eastern Pacific warming.

The ubiquitous stratospheric polar vortex is an important feature of the Northern Hemisphere winter circulation. The vortex is in two-way communication with the troposphere. In one direction, fluctuations in the strength of the vortex are linked to important changes in mid- and high-latitude tropospheric flow patterns, and help to characterize key tropospheric modes of variability such as the Arctic Oscillation (AO). In the other direction, variability in the extratropical tropospheric planetary wave, driven by phenomena such as the El Nino-Southern Oscillation (ENSO), affects the strength of the vortex. Our study focuses on the effects of two types of ENSO variability [i.e., central and eastern Pacific warming events (CPW and EPW, respectively)] on the strength of the stratospheric polar vortex. We find that the initial response of the vortex to central and eastern Pacific warming can vary substantially in an individual winter, although on the average, the vortex initially weakens by a similar magnitude under the influence of both types of warming. What causes the different responses between individual winters is the interaction between the direct atmospheric response to the Pacific warming and the internal variability/initial condition of the extratropical atmosphere.

To obtain our results, we conducted an ensemble of model runs with the NCAR Whole Atmosphere Community Climate Model (WACCM) by varying initial atmospheric conditions. An idealized patch of sea surface temperature (SST) anomalies, created to mimic real CPW and EPW events, was introduced into each model run. Using this experimental setup, we found that the introduction of both CPW and EPW anomalies weakened the vortex in the ensemble mean. This weakening of the vortex was a result of negative zonal-mean zonal wind tendencies driven by changes in the mean meridional circulation (MMC) and eddy momentum fluxes. Despite the similarity of response in the ensemble mean, there was a large spread in vortex response between individual ensemble members. Interestingly, a few of the members instead exhibited an initial strengthening response to CPW and EPW. In these instances, the strengthening of the vortex was primarily a result of positive zonal wind tendencies due to changes in the MMC.

An immediate implication of this result is that in any individual year, knowledge about the tropical Pacific SST alone is not enough to make a skillful medium-range forecast for the future state of the stratospheric polar vortex. The initial state of the extratropical atmosphere must also be considered and accurately represented in the model in order to correctly predict the evolution of the vortex under the influence of CPW or EPW forcing.--Bradley M. Hegyi (Georgia Institute of Technology), Y. Deng, R.X. Black, and R. Zhou. "Initial Transient Response of the Winter Polar Stratospheric Vortex to Idealized Equatorial Pacific Sea Surface Temperature Anomalies in the NCAR WACOM," in a forthcoming issue of Journal of Climate.

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Title Annotation:PAPERS OF NOTE
Author:Hegyi, Bradley M.; Deng, Y.; Black, R.X.; Zhou, R.
Publication:Bulletin of the American Meteorological Society
Geographic Code:1USA
Date:May 1, 2014
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