Impact of tropical SSTs on the late-winter signal over the North Atlantic-European region

The North Atlantic-European (NAE) region is influenced by many climate factors that could be cooccurring and/or coinciding. These various influences, combined with the pronounced internal atmospheric variability, make separating the distinct impacts on the NAE area difficult. Besides the North Atlantic Oscillation (NAO), which is the strongest pattern of climate variability in the mid and high latitudes of the Northern hemisphere, the NAE region is also affected by the El Niño-Southern Oscillation (ENSO). ENSO is an atmosphere-ocean phenomenon involving SST changes in the tropical Pacific. Its remote influence can be manifested directly through the atmosphere, but also indirectly involving different parts of the ocean.
In this study, we use a set of sensitivity experiments to investigate the potential impact of tropical sea surface temperatures (SSTs) on the signal of geopotential heights at 200 hPa (GH200) over the NAE region. Using an intermediately complex atmospheric general circulation model (ICTP AGCM), we have designed five experiments based on 35-member ensembles of long integrations with SST anomalies prescribed in various regions acting as lower boundary forcing for the model. In the AGCM experiments, the SST forcing was prescribed globally, in the tropical zone of all oceans, only inside the tropical Atlantic area, in the tropical Indian Ocean and limited to the tropical Pacific. Additionally, an experiment containing only climatological SSTs was analysed.
The monthly GH200 signal was calculated based on the difference between the ensemble mean of each experiment and the climatological mean for the considered period. In addition, to separate the impact of the ENSO from other influences, the signal was calculated for ENSO and non-ENSO years, respectively. The influence of ENSO was analyzed in the context of the direct tropical Pacific SST forcing, but also from the aspect of the indirect contribution of other parts of the tropical oceans. The linearity of the response was analyzed with regard to the amplitude of the forcing, but also with regard to the contribution of individual parts of the tropical oceans.
Results have shown that the GH200 signal is the strongest in the late-winter months (January-March) in all experiments. The AGCM experiment with SST boundary forcing prescribed only in the tropical Atlantic consistently yielded the least amount of signal. The strongest signal linked to ENSO events was found in the experiment with the SST forcing prescribed only in the tropical Pacific. It is also demonstrated that the potentially predictable ENSO-related signal over the NAE region is projected onto the East Atlantic pattern.