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ENSO related signal and predictable components of atmospheric circulation in the North Atlantic - European region

Tropical sea surface temperatures (SSTs) can act as a source of boundary-forced predictability for the atmosphere in the extratropics, which is characterized by its strong internal variability. If the strength of atmospheric response to this remote SST forcing is large enough to overcome the chaotic intrinsic variability of the extratropical atmosphere, then the boundary-forced circulation can be established and potential predictability increased. One of the strongest phenomena with such an influence on the climate variability throughout the world is the El Niño-Southern Oscillation (ENSO). Using an intermediately complex atmospheric general circulation model (ICTP AGCM), five experiments were conducted to detect the potential impact of tropical sea surface temperatures (SSTs) on the late-winter atmospheric circulation in the North Atlantic- European (NAE) region. Each of the AGCM experiments is a 35-member ensemble of 156-year long simulations forced with observed SST anomalies prescribed in different ocean areas: globally, in the entire tropical zone, tropical Atlantic region, and tropical Pacific. Additionally, an experiment containing only climatological SSTs was analysed. The late-winter 200-hPa geopotential heights signal was estimated by the difference between the ensemble mean of each experiment and the climatological mean of the ensemble mean for the considered period, following. To further inspect the impact of ENSO, the signal was calculated for subsets containing only ENSO or non-ENSO years, sorted according to the strength of the late-winter Niño3.4 index. We also compared the monthly signal averaged over the NAE region in differentAGCM experiments. Results have shown that the signal is the strongest in the late-winter months (January- March) in all experiments. Among the experiments containing lower-boundary forcing, the experiment with SST anomalies prescribed only in the tropical Atlantic consistently yielded the least amount of signal. Overall, the geopotential height signal is more pronounced over the NAE region in ENSO years, than in non-ENSO years. Alongside the “classical”analysis of the signal, the signal-to- noise optimal patterns method was applied to the geopotential heights at 200 hPa. The optimal patterns method seeks patterns which maximize the signal-to-noise ratio. Results have shown that the lower-boundary forcing from the tropical Pacific increases potential predictability of the late-winter atmospheric circulation in the North Atlantic-European region.

ENSO, predictable components, North-Atlantic European region

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