| ABSTRACT | El Niño Southern Oscillation (ENSO) is one of the strongest sources of climate variability all over the world. Although it has been examined extensively and its impact on climate variability of certain regions is clear and well explained, there are still a lot of questions regarding its effect on European and Mediterranean region. The ENSO-related signal is detectable there, but is generally weak as well as spatially and seasonally dependent. On the other hand, Europe is dominantly influenced by North Atlantic Oscillation (NAO), a mode of natural variability which impact is well established.
Since both phenomena may exert a certain influence on European climate at the same time, it is not simple to distinguish this two effects. Firstly, ENSO impact may be masked with a stronger effect of NAO. Secondly, there is also possibility that NAO signal is modulated by ENSO, particularly during strong ENSO events. Therefore, we may pose a question whether is possible to separate ENSO and NAO signals. One way to answer this question is examination of climate anomalies considering the values of NAO and ENSO indices and categorizing years (seasons) into corresponding categories. This approach enables detection of years with different combinations of ENSO and NAO impacts which may be strong, weak and neutral.
Due to periodicity of ENSO which is 2-7 years and the fact that there is no particular periodicity of NAO, it may be hard to obtain categories with a large enough sample size to achieve analysis with sufficient statistical power. Therefore, here we examine relative ENSO and NAO impact on European climate using a large ensemble of long numerical simulations. We analyze 35-member ensemble of numerical simulations for 1855-2010 period performed with ICTP AGCM, an atmospheric general circulation model (AGCM) of intermediate complexity developed at International Centre for Theoretical Physics (ICTP). Positive, negative and neutral ENSO years are determined according to the strength of wintertime (JFM) Nino3.4 index, while PC-based NAO index is used for classification of NAO years. Composite analysis is applied to the anomalies of different meteorological parameters (geopotential height, tempertaure etc.) with the aim to analyze atmospheric patterns associated with specific ENSO-NAO combinations. Additionally, signal-to-noise analysis is also performed. |