The impact of the extratropical North Atlantic SSTs on the ENSO signal in the North Atlantic-European region

Tropical sea surface temperatures (SSTs) play a crucial role as a source of boundary-forced predictability for the atmosphere in the extratropics, influencing atmospheric dynamics characterised by significant internal variability and limited predictability. One of the most influential events determining global climate variability is the El Niño-Southern Oscillation (ENSO). Over the North-Atlantic European region (NAE), many processes obscure the manifestation of ENSO. In this study, we analyse the ENSO-induced signal over the NAE region and investigate the contributions of the individual tropical basins and the mid-latitude North Atlantic SSTs. Using an intermediate-complexity atmospheric general circulation model (ICTP AGCM), we performed a set of targeted experiments using 35-member ensembles of long integrations with SST anomalies in different regions that served as a lower boundary forcing for the model. This experimental framework facilitated the separation of the influences from individual basins and allowed the estimation of their respective contributions to the overall signal. Our analysis shows a recognisable atmospheric response occurring primarily in the late winter months, with the strongest signal associated with ENSO events. The competing influences emanating from individual tropical basins are highlighted. At the same time, the superposition effect of the extratropical North Atlantic SSTs is demonstrated through the modulation of the storm tracks. Although the atmospheric response to the midlatitude SST forcing is weak, extratropical air-sea interaction still can be an important modifier of local atmospheric variability.  Both the model results and the NOAA-CIRES-DOE 20th Century Reanalysis variance of geopotential heights (GH200) show the presence of an ENSO signature, which manifests itself as a distinct pattern in the North Atlantic and projects onto the East Atlantic pattern.