Modulation of ENSO by tropical intraseasonal air-sea fluxes in the tropical Pacific and the impact of climate change – METRO

The project will be organised in 3 main tasks (Fig. 3), detailed in section 3.3, with each of these tasks
addressing several of the 7 scientific questions above:
Task 1. Characterisation of ISO (momentum, heat fluxes and Kelvin waves) variability and links with ENSO in observations (Task 1.a) and in coupled models (Task 1.b) (Q1, Q2, Q3, Q4, Q5, Q6). Definition of standard metrics to characterize ISO/ENSO relationship (Task 1.c).
Task 2. Understanding the physical mechanisms of the ISO/ENSO relation in coupled models (Q1, Q2, Q3, Q4, Q6).
Task3. Performance analysis of IPCC-class CMIP3 and CMIP5 models using Task 2 diagnostics and assessment of the climate change impact (Q5, Q6, Q7).

Work achieved M1-M18 :
• Documentation of source of WWB. Only one third are related to MJO (Postdoctoral fellow, paper in preparation, contribution to D1.1)
• Physical mechanisms explaining the impact of WWBs on the displacement of the eastern edge of WP (postdoctoral fellow, paper in preparation, contribution to D1.1)
• Analysis of IKW in observations and models (M2 training period, contribution to D1.2)
• Development of a new surface fluxes product in the tropics (Tropflux, contribution to D1.1) and analysis of its intraseasonal variability

N/A

1. Bellenger et Duvel, 2012, The event-to-event variability of the boreal winter MJO, Geophys. Res. Lett., 39, L08701.
2. Dufresne and coauthors Climate change projections using the IPSL-CM5 Earth System Model : from CMIP3 to CMIP5, in revision for Climate Dynamics
3. Bellenger, Guilyardi, Leloup, Lengaigne and Vialard 2012 : El Niño in climate models : from CMIP3 to CMIP5 in preparaition for Climate Dynamics.
4. Praveen Kumar B., J. Vialard, M. Lengaigne ,V.S.N. Murty and M. J. McPhaden, 2012: TropFlux: Air-Sea Fluxes for the Global Tropical Oceans – Description and evaluation against observations, Clim. Dyn., 38, 1521-1543.
5. Praveen Kumar B., J. Vialard, M. Lengaigne ,V.S.N. Murty M. J. McPhaden, M. Cronin, F. Pinsard and K. Gopala Reddy, 2012: TropFlux wind stresses over the tropical oceans: evaluation and comparison with other products, Clim. Dyn., online.
6. Vialard, J., K. Drushka, H. Bellenger, M. Lengaigne, S. Pous and J-P. Duvel, Processes of Madden-Julian sea surface temperature signature in the North Western Australian Basin, Clim. Dyn., submitted.

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