Modulation of El Niño Southern Oscillation and its impacts by the mean climate state

The El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability in the tropics and affects regional climates around the globe. During ENSO events, changes to atmospheric circulation and precipitation patterns in response to perturbed sea surface temperature lead to remote climate effects that vary regionally and by season. Internal modes of interannual climate variability such as the Pacific Decadal Oscillation (PDO) and the Atlantic multidecadal variability (AMV), along with anthropogenic sources of radiative forcing (eg. greenhouse gases emission and aerosol concentration) alter the global climate circulation on interannual and decadal timescales and therefore must be included in the study of ENSO impacts under future climate conditions.

A largely unexplored research area is whether the effects of ENSO events on climate extremes may change in a future, warmer climate. Climate change may affect both the frequency of climate extremes, increasing the variance, and the mean, changing the shape of the distribution of a certain variable in the future (Lavell et al., 2012). In a warmer climate, the increase in atmospheric moisture may intensify the variability in precipitation associated with ENSO (Christensen et al., 2013). However, generally few attempts have been made to examine the specific contribution of ENSO to changing extremes.
The latest climate model simulations encompassed in the Climate Model Intercomparison Model Phase 6 (CMIP6) will mean significant improvements in the representation of ENSO (Collins et al., 2014). A especial focus of the project will be on evaluating the influence of the North Atlantic mean state on ENSO and its climate impacts. This will be addressed by contrasting ENSO impacts between simulations in which the North Atlantic SST will be constraint to warm or cold conditions. Recent literature suggests that Atlantic multidecadal variability (AMV) can modify the tropical Pacific mean state (McGregor et al., 2014, Li et al., 2016, Ruprich-Robert et al., 2017). However, it is still not clear how those changes in the Pacific mean state translate into changes of ENSO behaviours (e.g. Wang et al., 2017). In addition, recent studies suggested that the AMV can also influence ENSO teleconnections even without changing ENSO itself (Lopez-Parages et al., 2015). A better understanding of the AMV influences on ENSO and its teleconnections will increase the level of confidence of extreme impacts assessments and seasonal forecasts.
The specific objectives of the project are:

1. Investigate ENSO and its impacts from different model resolutions. In particular, contrasting these in low and high resolution simulations, including those performed within the High Resolution Model Intercomparison project (HighResMIP) and other very high resolution simulations performed by BSC. This approach represents a great advantage in terms of high-resolution runs that will allow to assess small-scale climate impacts.
2. Evaluate the influence of different mean states on ENSO and its impacts (including precipitation and temperature extremes) by comparing simulated ENSO impacts from present day with end of 21th century simulations.
3. Assess ENSO impacts with different North Atlantic background conditions using the Decadal Climate Prediction Project (DCPP) Component C experiments of CMIP6.
4. Determine relationships between ENSO and different anthropogenic forcing levels from step change perturbation experiments.
5. Determine the most vulnerable regions to future ENSO events and make an impact assessment of extremes.