Atmospheric response to the winter Atlantic Niño/Niña

Tropical Atlantic Variability (TAV) exerts a significant influence on the climate of different regions through distinct mechanisms that can be identified. Understanding these mechanisms and their impacts can improve the predictability of the North Atlantic-European winter climate. The Atlantic Niño (ATLN) or Equatorial Mode is known for being the dominant pattern of TAV, whose dynamics are similar to El Niño-Southern Oscillation in the tropical Pacific. This study aims at exploring the atmospheric response to positive/negative winter ATLN, as it has been much less documented than the summer ATLN. To this end, a set of sensitivity experiments has been performed with two global atmosphere models of different complexity (EC-EARTH and SPEEDY) to ensure that results are model-independent. The experiments have been designed as atmosphere-only simulations that comprise 150-year long integrations keeping the radiative forcing fixed at present-day climate conditions, covering the extended winter season from November to March: a control run with observed climatological SSTs over 1980-2010 (CTRL), and two sensitivity runs prescribing positive (ATLNiño) and negative (ATLNiña) SST anomalies with climatology elsewhere. The atmospheric response is evaluated by comparing the ensemble mean of each sensitivity run with CTRL. The results show a local Gill-type structure, symmetrically straddling the equator, whose amplitude is slightly larger in response to the heating than to the cooling, and increases from November-December to January-February. In the extratropics, the upper-tropospheric circulation displays a dipolar structure with cyclonic (anticyclonic) anomalies at mid-latitudes and anticyclonic (cyclonic) anomalies at subpolar latitudes, associated with ATLNiño (ATLNiña), which is different from the North Atlantic Oscillation (NAO). In addition to the impact on tropical convection, depicting a southward (northward) shift during ATLNiño (ATLNiña), the precipitation anomalies show a robust and approximately-linear signal on the European continent.