The dependence of the winter stratospheric and Euro-Atlantic climate response on ENSO amplitude is investigated using the HadGEM3 model. Experiments are performed with imposed east Pacific sea surface temperature perturbations corresponding to Niño-3.4 anomalies of ±0.75, 1.5, 2.25, and 3.0 K. In the North Pacific, El Niño (EN) deepens and shifts the Aleutian low eastward, while the equivalent magnitude La Niña (LN) perturbations drive anomalies of opposite sign that are around 4 times weaker. The muted North Pacific response to LN can be traced back to the weaker response of tropical convection and the associated anomalous Rossby wave source. The EN perturbations weaken the Arctic polar vortex, with the winter mean zonal mean zonal wind at 60°N and 10 hPa decreasing approximately linearly with Niño-3.4 anomaly by around −3.6 m s−1 K−1. For the strongest EN case (+3 K), the frequency of sudden stratospheric warmings (SSWs) increases by ~60% compared to the control experiment. Hence the results do not support a saturation of the stratospheric pathway for strong EN as suggested in previous literature. The equivalent amplitude LN perturbations cause a weak strengthening of the polar vortex and no substantial change in SSW frequency, in contrast to some reanalysis-based studies. EN induces a negative North Atlantic Oscillation (NAO) index throughout boreal winter, which increases approximately linearly with the Niño-3.4 anomaly by around −0.6 standard deviations K−1. Only the response to the strongest LN perturbations projects onto a weak positive NAO in November, suggesting that the mechanism for the Euro-Atlantic response to LN may be distinct from EN.

El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the tropics. The spatiotemporal characteristics of ENSO exhibit variations (Dieppois et al., 2021;Wittenberg, 2009) driven by chaotic atmosphere-ocean dynamics (Fedorov et al., 2003;Zhang et al., 2021) and sources of variability from outside the tropical Pacific (Meehl et al., 2001). Given the global reach of ENSO and its impacts (Diaz et al., 2001), understanding and predicting changes in ENSO is a major challenge for the scientific community.In the last decade, it has become evident that climate variability and changes in other ocean basins can affect the tropical Pacific and ENSO (Cai et al., 2019). One example is the impact of Atlantic Multidecadal Variability (AMV) on the mean climate in the tropical Pacific (e.g., Ruprich-Robert et al., 2017). However, the extent to which AMV affects ENSO remains unclear (

<p>Model simulations show a robust increase in ENSO-related precipitation variability in a warmer climate, but there remains uncertainty in whether the characteristics of ENSO events themselves may change in the future. Our study aims to disentangle these effects by addressing how the global impacts of observed large El Niño events would change in different background climate states covering the preindustrial, present and future periods.</p> <p>Pacemaker simulations with the EC-Earth3-CC model were performed replaying the 3 strongest observed El Niño events from the historical record (1982/83, 1997/98, 2015/16). Model tropical Pacific sea surface temperature (SST) anomalies were restored towards observations, while imposing different background states, mimicking past, present and future climate conditions (following the SSP2-4.5). All variables outside the restoring region evolve freely in a coupled-atmosphere ocean transient simulation. For each start date, 30 ensemble members with different initial conditions were run for 2 years. Using this approach we ask ‘what impacts would arise if the observed El Niño occurred in the past or future’?</p> <p>In response to the same imposed El Niño SST anomalies, precipitation anomalies are shifted towards the Eastern equatorial Pacific in the future compared to the present day, leading to changes in the extratropical response to El Niño. Some examples are an amplification of the surface temperature response over north-eastern North America, northern South America and Australia in boreal winter. We link the changes of El Niño related tropical Pacific precipitation to a decrease in the climatological zonal SST gradient in the equatorial Pacific, as we move from past to future climatologies, which potentially leads to a higher convection sensitivity to SST anomalies over the Central and Eastern equatorial Pacific in the future. Interestingly, the simulations indicate there has already been an intensification of El Niño impacts between present day and preindustrial, which is comparable to the differences found between future and present. This nonlinear behaviour highlights the need to understand potential changes to convection thresholds in the tropical Pacific to be able to explain El Niño teleconnections under climate change scenarios. Ongoing work is exploring the changes in atmospheric circulation that lead to the overall intensification of El Niño impacts that we show in our study.</p>

<p>Variability in the Aleutian Low is a known contributor to North Pacific sea surface temperature (SST) variability, but its role in forcing the basin-wide SST anomalies that characterise Pacific Decadal Variability (PDV) is unclear owing to the difficulty of disentangling coupled atmosphere-ocean processes. Here we perform a large ensemble experiment with an intermediate complexity GCM where the winter-time Aleutian Low is nudged to an anomalously strong state during successive winters. This ensemble is compared to a free-running simulation to isolate the impacts of the anomalous Aleutian Low. The nudged experiment produces a basin-scale SST response that closely resembles PDV in the free running simulation, confirming that the Aleutian Low can force PDV-like variability. Tropical Pacific sea surface temperatures (SSTs) are significantly warmer in response to the strong Aleutian Low, demonstrating that extratropical atmospheric forcing can impart a signature in tropical SSTs. The largest tropical Pacific warming is manifest in the season following nudging (boreal spring), though anomalies persist year-round. We use the Bjerknes Stability Index to attribute the drivers of the tropical Pacific SST response and find that the thermocline feedback is key, which itself is most dominant in summer. The results lend new understanding to the potential for extratropical atmospheric forcing of tropical ocean variability.</p>