The Influence of Cloud Feedbacks on Tropical Climate Variability

Abstract:

Prior studies have provided observational evidence that cloud feedback over the subtropical stratocumulus regions is positive because cloud cover is anti-correlated with local Sea Surface Temperature (SST) anomalies. Moreover, regressions of observed atmospheric fields on equatorial SST anomalies indicate that cloud feedbacks over the southern hemisphere stratocumulus regions (i.e., Peruvian and Namibian) co-vary with the respective equatorial modes of variability: El Niño Southern Oscillation (ENSO) and Atlantic Niño. However, from observations alone it is not possible to quantify the influence of regional cloud feedbacks on large-scale climate variability. To address this question, a set of sensitivity experiments are conducted using an atmospheric general circulation model (ECHAM6) coupled to a slab-ocean in which the strength of positive cloud feedback is enhanced over the Peruvian and Namibian regions. Enhanced positive cloud feedback increases the variance and the persistence of local as well as equatorial SST anomalies, enhancing ENSO and Atlantic Niño variability. We explore the role of cloud feedbacks over other regions in the South Atlantic and find that they increases local SST variability, but exhibit negligible responses at the equator. Our results indicate that the subtropical stratocumulus regions play a central role in enhancing equatorial SST variability because they are located where the SST anomalies have the largest growth rates, that is, where the variance of SST is largest and the damping rate of SST is weakest. These results highlight the important role of the regional coupling of cloud cover over the subtropical stratocumulus regions with local SSTs, and have implications for the response of equatorial climate variability to climate change.