Decadal modulation of El Nino Southern Oscillation and its global impacts

This project will investigate the coupling between modes of climate variability and their effects on seasonal-to-decadal forecast skill. Teleconnections describe interconnections between climate anomalies at distant locations across Earth. Teleconnections frequently arise through the response of atmospheric circulation to changing oceanic conditions. If these patterns can be accurately forecast, the signals provide a major source of predictive skill in seasonal-to-decadal forecasts (Scaife et al., 2016). Hence, there is a need to identify predictable climate signals and understand their effect on the risk of global climate hazards to improve seasonal-to-decadal predictions.

Most previous research has considered teleconnections arising from modes of variability separately. However, there is growing evidence for coupling between modes of variability and their teleconnections on different timescales (Cai et al., 2019). Some recent studies have speculated Atlantic Multidecadal Variability - a fluctuating pattern in the North Atlantic ocean -- augments the response of the winter North Atlantic Oscillation to El Nino Southern Oscillation, the dominant pattern of year-to-year variability in the tropics, ENSO is a key driver of the NAO and this teleconnection is important for seasonal forecasting of European winter climate. However, questions remain about whether the AMV modulation of the ENSO-NAO teleconnection is robust and whether it can be distinguished from a local AMV impact on the NAO (Simpson et al., 2018). This PhD project will develop a new framework for assessing the AMV influence on the ENSO-NAO teleconnection using state-of-the-art large ensemble climate simulations. This will provide the most comprehensive assessment to date of decadal modulation of ENSO-NAO coupling by the AMV.

Another major research topic has been how ENSO affects climate in remote regions, including the North Atlantic. Few studies have considered teleconnections that alter ENSO itself. Recent work at the University of Leeds suggests the warm phase of AMV weakens ENSO on decadal timescales. This project will test this effect in a larger set of climate models and identify whether it is robustly simulated. New model simulations will also be performed to test hypotheses and isolate mechanisms. These experiments will exploit new 'nudging' capabilities within a climate model that enable specified regions to be constrained to observations to isolate remote impacts.

To close the loop on Pacific-Atlantic interactions, the third question addressed in the project will be: what are the impacts of the North Atlantic Oscillation on ENSO? The motivation comes from the observation that on seasonal timescales the NAO drives a tripolar pattern in North Atlantic sea surface temperatures through modified air-sea coupling. This raises the potential for longer-term multidecadal NAO trends to impact on climate outside the North Atlantic, but this is currently unexplored. This project will tackle this new question using a combination of statistical analysis of observations and a similar model experiment design where NAO anomalies are 'nudged' and the global response evolves.

The specific objectives of the project will be adapted to fit the interests of the student and to pursue the most promising avenues of enquiry. Specific research objectives could include:

Quantify how Atlantic Multidecadal Variability affects the ENSO-NAO relationship including the associated mechanisms; test whether this is robust in climate models.
Explore the impact of the AMV on ENSO and its mechanisms via Atlantic-Pacific interactions.
Investigate the global impacts of interannual to decadal NAO variability, including potential impacts on ENSO.