Department Seminar: Margaret Duffy

Title: Atmospheric mechanisms of the pattern effect

Abstract: Climate feedbacks amplify over time in climate model simulations due to the ``pattern effect," which describes the influence of the pattern of sea surface temperature (SST) warming on global-mean feedbacks. In particular, delayed warming in the tropical East Pacific, as emerges several decades into abrupt 4xCO2 simulations, is associated with amplifying climate feedbacks. Several mechanisms for the relationship between the pattern of SST warming and global-mean radiative feedbacks have been proposed. Previous literature has favored a “tropospheric stability” mechanism, which described changes in low cloud amount due to changes in lower-tropospheric stability with warming. On the other hand, a “circus tent” model posits that in regions of deep atmospheric convection, this convection communicates local SST anomalies vertically. The pattern of SST response also influences the “Walker circulation." Here we evaluate these three proposed mechanisms in CAM6 warm-SST-patch experiments. We find that the circus tent model dominates in CAM6. We further find evidence that the circus tent model better explains interannual variations in feedbacks in observations than the prevailing tropospheric stability mechanism, though we note that the two mechanisms are related to one another.