Understanding the anomalously cold European winter of 2005/06 using relaxation experiments

TitleUnderstanding the anomalously cold European winter of 2005/06 using relaxation experiments
Publication TypeTechnical memorandum
Date Published10/2009
Secondary TitleECMWF Technical Memoranda
AuthorsJung, T, Palmer, TN, Rodwell, M, Serrar, S

Experiments with the atmospheric component of the ECMWF Integrated Forecasting System (IFS) have been carried out to study the origin of the atmospheric circulation anomalies that led to the unusually cold European winter of 2005/06. Experiments with prescribed sea surface temperature (SST) and sea ice fields fail to reproduce the observed atmospheric circulation anomalies suggesting that the role of SST and sea ice was either not very important or the atmospheric response to SST and sea ice was not very well captured by the ECMWF model. Additional experiments are carried out in which certain regions of the atmosphere are relaxed towards analysis data thereby artificially suppressing the development of forecast error. It is shown that both tropospheric circulation anomalies in the Euro-Atlantic region and the anomalously weak stratospheric polar vortex can be explained by tropical circulation anomalies. Separate relaxation experiments for the tropical stratosphere and tropical troposphere highlight the role of the easterly phase of Quasi-Biennial Oscillation (QBO) and, most importantly, diabatic heating anomalies over South America and the tropical Atlantic. From these results it is argued that the relaxation technique is a very powerful diagnostic tool to understand remote origins of seasonal-mean anomalies.

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