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Predictability of monthly european precipitation and temperature based on ENSO-NAO interactions

Last winter precipitation and temperature anomalies over Europe have increased the interest in studying the influence on climate of ENSO-NAO interactions over this region. Episodes of cold waves and floodings have been observed in different areas of Europe affecting several socio-economic activities such as health or agriculture. More information about the physical processes involved is required to understand and evaluate the strong impacts of these events.
This work is focused on the NAO-ENSO interactions that affect European climate anomalies and the predictability associated to these phenomena at a seasonal time-scale. Monthly precipitation and 2-meter temperature observations from the gridded E-OBS [1] dataset and hindcasts from the ENSEMBLES STREAM2 [2] multi-model seasonal forecast experiment are jointly analyzed in the period 1960-2000. The study is based on terciles following the methodology introduced by Frías et al 2010 [3], extended here to Europe. Monthly NAO index and averaged SST in El Niño3.4 region are used to characterize the large scale circulation. These indices are divided into three categories according to their terciles: positive, negative and neutral NAO, and El Niño, La Niña and neutral ENSO. Monthly validations have been carried out for every possible ENSO-NAO combination, assessing the resulting multimodel skill. It is concluded that predictability is much higher during these situations than when comparing to the whole period. A study of teleconnections is also done in order to check whether or not this increase of skill is connected to the NAO-ENSO phenomena. It is observed that, although NAO is the dominant mode of variability in Europe, ENSO acts as a modulator factor. It occurs for instance with the classic teleconnection found in winter over Spain when the negative phase of the NAO takes place (more rain than usual) which is intensified when coupling El Niño to the negative NAO.

[1] Haylock, M.R., N. Hofstra, A.M.G. Klein Tank, E.J. Klok, P.D. Jones, M. New. 2008: AEuropean daily high-resolution gridded dataset of surface temperature and precipitation. J.Geophys. Res (Atmospheres), 113, D20119, doi: 10.1029/2008JD10201.
[2] Weisheimer, A., F.J. Doblas-Reyes, T.N. Palmer, A. Alessandri, A. Arribas, M. Déqué, N.Keenlyside, M. MacVean, A. Navarra and P. Rogel (2009), ENSEMBLES - a new multi-modelensemble for seasonal-to-annual predictions: Skill and progress beyond DEMETER in forecastingtropical Pacific SSTs, Geophys. Res. Lett., 36, L21711, doi: 10.1029/2009GL040896.
[3] Frías, M.D., Herrera, S., Cofiño, A.S., Gutiérrez, J.M. 2010: Assessing the Skill of Precipitationand Temperature Seasonal Forecasts in Spain. Windows of Opportunity Related to ENSO Events.Journal of Climate 23, 209-220, doi: 10.1175/2009JCLI2824.1.