The 1997-1998 El Niņo was one of the most intense in the past century, and provides a rare opportunity to evaluate climate model response to forcing from a well-observed event. The large-scale circulation and associated cloud systems in the eastern and western Pacific exhibited a large response to changes in sea surface temperature. In contrast to observations, many models used in the IPCC 4th Assessment Report simulations archive do not show a credible, consistent atmospheric response when driven with observed sea surface temperatures. For example, observations reflect a significant decrease in the Walker Circulation, while many models fail to reproduce the observed changes in the tropical overturning circulation.

The geophysical signal of the 1997-1998 El Niņo was well observed by instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite, and can be detected in the cloud-object data set derived from CERES and VIRS. In this presentation, we compare simulated deep convective clouds in the Goddard Earth Observing System Model, Version 5 (GEOS-5) with deep convective cloud objects over the equatorial Pacific Ocean. We first examine the consistency between observed clouds and model output when it is used in the context of NASA's Modern Era Retrospective-analysis for Research and Application (MERRA). We then extend the comparison to free-running GCM simulations driven with observed sea surface temperature. In both comparisons, we exploit the fact that the convection and large scale circulation should exhibit a close relationship. This enables us to compare cloud objects with simulated clouds regardless of whether cloud systems were reproduced at the exact time and place they occurred.

Results from this investigation suggest that both the MERRA dataset and AMIP-style GEOS-5 simulations realistically represent the observed occurrence and properties of deep convection over the equatorial Pacific Ocean. However, the results of the comparisons are sensitive to the assumed subgrid distribution of cloud in each GCM grid box. We discuss the implications of this sensitivity for analysis of the output of simulations used in the forthcoming IPCC 5th Assessment Report.