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.
Résumé
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 AGCMAMIP
simulations 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 the
atmosphere under warmer/colder than normal conditions over the Mediterranean Sea. To address
this problem, firstly, changes in the frequency and spatial pattern are evaluated versus the spread of
the ensemble. A change in the spatial pattern of –NAO is found with higher (lower) pressures
centred 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 with
the signal to noise ratio over the whole season. When temperatures over Mediterranean Sea are
warmer (colder) than normal significant changes of WR frequency are: i) more (less) frequency for
the -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, the
analysis 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 be
performed in order to isolate the atmosphere variability forced by the Mediterranean Sea.