Observational and Model Evidence for Positive Low-Level Cloud Feedback

Positve Feedback The uncertain effect of feedback between climate and clouds is one of the largest obstacles to producing more confident projections of global climate. Clement et al. (p. 460) examined how clouds, sea surface temperature, and large-scale atmospheric circulation vary in the Northeast Pacific region. Change in cloud coverage was the primary cause of sea surface temperature variations, and clouds provided a positive feedback to temperature variations. Furthermore, regional atmospheric circulation patterns were linked to patterns of cloudiness. One model produced realistic covariability between cloud cover, sea surface temperatures, and atmospheric circulation for the 20th century. Decreased low-level cloud cover in the Northeast Pacific region amplifies increases in sea surface temperatures. Feedbacks involving low-level clouds remain a primary cause of uncertainty in global climate model projections. This issue was addressed by examining changes in low-level clouds over the Northeast Pacific in observations and climate models. Decadal fluctuations were identified in multiple, independent cloud data sets, and changes in cloud cover appeared to be linked to changes in both local temperature structure and large-scale circulation. This observational analysis further indicated that clouds act as a positive feedback in this region on decadal time scales. The observed relationships between cloud cover and regional meteorological conditions provide a more complete way of testing the realism of the cloud simulation in current-generation climate models. The only model that passed this test simulated a reduction in cloud cover over much of the Pacific when greenhouse gases were increased, providing modeling evidence for a positive low-level cloud feedback.

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