Interannual Variations of Eastern Pacific Stratocumulus Cloud Cover: Observations vs. GCM Simulation

We have analyzed the interannual variations of marine stratocumulus clouds over the eastern Pacific Ocean, for the years 1983 - 1988, as observed by the International Satellite Cloud Climatology Project (ISCCP) and as simulated by the Colorado State University General Circulation, Model (CSU GCM). Variations of lower tropospheric temperature and vertical motion are also analyzed, using ECMWF analyses. The model results are taken from a ten-year simulation covering 1979 - 1988, with observed sea surface temperatures and sea ice distributions. We concentrate on July conditions.

Results show that the model is reasonably successful in reproducing the observed patterns of marine boundary layer cloudiness for a normal year, and the observed transition from non-precipitating stratus clouds to the deep convection of the ITCZ, as one moves westward and equator-ward from the coast of the Americas, Cloud-top entrainment instability, as parameterized in the model, plays an important role in limiting the area covered by stratocumulus clouds.

The results show that interannual variability of the large-scale sinking motion and lower tropospheric temperature essentially determine the year-to-year fluctuations of the marine boundary-layer cloudiness. The interannual variations simulated by the model, particularly in connection with El Ninõs, are fairly realistic. This suggests that the model is capable of simulating the response of low-level cloudiness to changes in the large-scale circulation of the atmosphere, perhaps including those that might accompany a climate change.