ENSO connections and processes

The El Niño Southern Oscillation (ENSO) phenomenon is the largest source of year-to-year variability in the climate system. ENSO events are caused by natural ocean-atmosphere interactions in the tropical Pacific, but their impacts are transmitted globally through so-called teleconnections linking distant areas. Teleconnections act through changes in the atmospheric circulation, and they cause regional climate disturbances that can be very damaging, such as floods and droughts. Although Europe is far from the Pacific centre of action, ENSO is also a factor in UK/Europe climate variability. The ENSO cycle has warm (El Niño) and cold (La Niña) phases with largely opposite physical impacts. Recent studies have demonstrated that there is also more than one type of ENSO, depending on location within the equatorial Pacific region, and that teleconnection effects differ from one type to another. Further information about these differing effects, and the underlying processes, is required to improve the prediction of the consequences when an ENSO event arises.

The aim of the work in this proposal is to investigate ENSO and teleconnections by (a) analysing data from observations and realistic climate models, with the use of mathematical methods to determine various 'modes' for ENSO evolution and associated atmospheric changes, and by (b) carrying out numerical experiments with simplified atmospheric circulation models to study the dynamical interactions induced by ENSO-like forcing of the system. Although simplified models are less realistic, by reducing the physical mechanisms represented in such models attention can be focussed on particular dynamical processes and causes and effects which are nevertheless relevant to actual climate variability. The simplified model experiments also provide an opportunity to test the strengths and weaknesses of the analysis methods used in (a).

The results will help us to understand why teleconnection effects may differ from one ENSO event to another, and how processes in the tropics and extratropics are inter-related. An important potential benefit is that long-range forecast producers will be able to check whether the relevant pathways are properly represented in the systems used to predict ENSO and its impacts: accordingly the Met Office is a partner in this project through the monthly-to-decadal forecast group.

Several long-range forecast centres (Met Office and ECMWF within UK, and similar centres worldwide) produce and issue predictions of world-wide seasonal climate variations at lead times of several months, using complex numerical ocean-atmosphere-land models. The prediction systems are under continual development and assessment, with performance with regard to ENSO and ENSO-related variability - the dominant mode of year-to-year climate fluctuations - being a key indicator. These centres will benefit from the findings of the proposed research on ENSO connections through:
... new methodology for performance assessment, and new insight into the dynamics of ENSO connection processes, that could inform system development;
... new information on varying effects of different ENSO types that could affect the confidence placed in seasonal climate outlooks for specific regions, including UK/Europe.

Seasonal climate predictions are used by a diverse community of 'end-users' in for example energy, agriculture, and health sectors, for local and regional forward planning and risk management. Many of these uses relate to ENSO impacts. Actions are often based on 'typical' impacts estimated by combining all types of events, supplemented in some (but not all) cases by direct regional long-range forecast information. The results of this project will indirectly benefit this community of end-users, and through their actions the quality of life of the public at large, through better understanding of the effects of different ENSO types and through improved regional long-range forecasts.

The general topic of the proposal - ENSO and climate variability - is suitable for a 'popular science' presentation for the general public/schools, and an activity of this kind at the annual Cambridge science festival is proposed in 'pathways to impact'. By this means the project will help raise public awareness of a major climate feature that often has widespread 'headline' impacts.