6533b82cfe1ef96bd128fd37

RESEARCH PRODUCT

Changes in the frequency of the Weather Regimes over the Euro-Atlantic and Mediterranean sector and their relation to the anomalous temperatures over the Mediterranean Sea.

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19 pages; International audience; An exercise has been carried out to assess to what extent the Euro-Atlantic Weather Regimes (WR),described from the ERA-interim Reanalysis in the summer season, projects onto a pool of AGCMAMIPsimulations in which sea surface temperatures (SST) are prescribed from observations.Although the model simulations present some biases in the spatial structure and seasonality of WRs,exhibiting also less variability, they are able to capture main WR over the region in summer season:+Middle East –Middle East, +NAO, -NAO. WR paradigm is used to quantify changes in theatmosphere under warmer/colder than normal conditions over the Mediterranean Sea. To addressthis problem, firstly, changes in the frequency and spatial pattern are evaluated versus the spread ofthe ensemble. A change in the spatial pattern of –NAO is found with higher (lower) pressurescentred over France when conditions over the Mediterranean are warmer (colder) than normal.Changes in frequency in the ensemble mean along the season are also evaluated and compared withthe signal to noise ratio over the whole season. When temperatures over Mediterranean Sea arewarmer (colder) than normal significant changes of WR frequency are: i) more (less) frequency forthe -Middle East in June/July ii) shift of the frequency of occurrence for the WR associated with+NAO and iii) less (more) occurrence of –NAO in September-October. Despite the limitation, theanalysis suggests that extreme conditions over the Mediterranean basin could modulate WR frequencies,which could have an impact on European weather conditions. Further analysis need to beperformed in order to isolate the atmosphere variability forced by the Mediterranean Sea.