METRO

Many observational and modelling studies suggest a strong role of intraseasonal atmospheric variability (ISO) in influencing the development of the El Niño - Southern Oscillation (ENSO). There is however, a large diversity in the representation of this ISO variability and of its response in coupled General Circulation Models (CGCM), that limit our ability to understand and predict El Niño. The goal of this proposal is to investigate the influence of intraseasonal wind and heat fluxes variability on the predictability of the coupled ocean-atmosphere system in the tropical Pacific. A first objective of this proposal will be to describe the ISO air-sea flux properties in both observations in in-house CGCMs. Previous studies have shown that changes in the background state of the ocean-atmosphere itself (may it be due to ENSO itself, the seasonal cycle or climate change) influence the intensity and spatial impact of air-sea flux perturbations. We will also examine this relation between ISO perturbations and the background state in model and observations, in order to determine if this control of stochastic atmospheric forcing by ENSO itself brings additional predictability. A second objective of this proposal is to examine how variations of the background state of the tropical Pacific influence the response of the coupled system to ISO variability, as suggested by the very diverse response to ISO perturbations in CGCM experiments. Whereas some aspects of these questions have already been explored for the dynamical response to ISO perturbations in the central Pacific, an innovative aspect of this project will be to also investigate response to heat fluxes, and response in the eastern Pacific. The interannual heat flux feedback in the eastern tropical Pacific ocean was indeed recently shown to have a key role in ENSO amplification in coupled GCMs but the impact of intraseasonal heat and momentum fluxes in the eastern Pacific on ENSO remains largely unknown. The last objective of this proposal is to examine more particularly how changes in the ocean-atmosphere background state induces by climate change (as diagnosed from IPCC-scenarios) can affect ISO variability and its impact on ENSO. The present project aims at exploring these scientific questions using both observations and models. Dynamical impacts, heat flux feedbacks and ENSO evolution will be analysed in ensembles of simulations which will differ by the models used, their initial ocean heat content, by their initial ISO forcing, and by the bulk formulation used in the atmospheric model to derive the heat and momentum turbulent air-sea fluxes. The evolution of these feedbacks in climate change scenario simulations will then be assessed. Besides improving our knowledge of tropical multi-scale interactions and our understanding of ENSO seasonal predictability, the expected benefit of the proposal is a precise and systematic guidance for CGCM improvements by providing a framework to analyse the ISO influence in IPCC-class coupled GCMs and process-based metrics to the wider community