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Understanding the role of the atmosphere and the seasonal cycle in El Niño characteristics in CGCMs

PI: Eric Guilyardi
Institution: IPSL/Laboratoire des Sciences du Climat et de l'Environnement, CNRS / CGAM, Univ Reading
ENSO is a genuinely coupled phenomenon, the dynamics of which are still only partially understood. An improved understanding of the processes that set the frequency and amplitude of El Niño events is a key element underpinning the development of climate prediction on seasonal and longer timescales. Also imperative is a better understanding of how the processes critical to El Niño are represented in coupled climate models, and how systematic biases in modelled climate may impact upon them. Errors in the simulation of El Niño have been cited in the IPCC AR3 as a major source of uncertainty in projections of future climate change.

By swaping ocean and atmosphere components in coupled GCMs, recent work has shown that the atmosphere component has a dominant role in defining El Niño frequency and base amplitude (Guilyardi et al. 2004). In particular, higher atmosphere resolution dramatically improves the low frequency spectra of El Niño. Even though this emphasis may be due to shortcomings in present-day atmosphere GCMs, more work is needed to elucidate the reasons behind this result.

The IPCC AR4 coupled simulations database provides a unique opportunity to further document this issue in the most recent coupled GCMs of the community. In addition, the analysis will focus on a number of key diagnostics (like the coupling strengh) likely to shed some light on the dominant role of the atmosphere component.

Model data required:
- all monthly means, over 100 years minimum (CF/netCDF preferably)

Atmosphere: MSLP, zonal/meridional wind stress, downward heat fluxes
(SW, LW, turbulent), downward mass fluxes (E, P, R), CRF (LW, SW),
cloud cover(s)

Ocean: SST, SSS, T (3d down to 500 m), S (3d down to 500 m), U(3d), V(3d),
W(3d), mixed-layer depth (density criterion), heat content of first 300m,
depth of 20C, sea-level, [and equation of state used]

Guilyardi E., S. Gualdi, J. M. Slingo, A. Navarra, P. Delecluse, J. Cole, G. Madec, M. Roberts, M. Latif, and L. Terray (2004): Representing El Niño in coupled ocean-atmosphere GCMs: the dominant role of the atmosphere model. J. Clim. In press
  • Guilyardi, E., 2006: El Niño – mean state – seasonal cycle interactions in a multi–model ensemble. Clim Dyn, 26, 329-348, DOI:10.1007/s00382-005-0084-6. Abstract. Full Article. Edit.
  • Guilyardi, E. and A. Wittenberg and A. Fedorov and M. Collins and. C. Wang and A. Capotondi and G. J. van Oldenborgh and T. Stockdale, 2009: Understanding El Niño in Ocean-Atmosphere General Circulation Models: progress and challenges. Bull. Amer. Met. Soc.. In press. Abstract. Edit.

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