The Role of Cloud Radiative Effects in North Atlantic SST Variability

Due to the relatively short observational record, efforts to reduce the cloud feedback uncertainty in global warming simulations have focused on studying the role of clouds in interannual to decadal atmosphere-ocean variability. Studies suggest that both low and high cloud radiative effects (CRE) reinforce interannual to decadal tropical SST fluctuations (e.g. ENSO, and the tropical branch of the AMO), and that large-scale extratropical weather patterns (e.g. the NAM) are associated with important CRE. It is unclear, however, if CRE play an important role in the pathway between extratropical weather and tropical atmosphere-ocean variability. This pathway involves the generation of subtropical SST anomalies by weakening of the trade winds during the negative phase of the NAM, and propagation of the subtropical SST anomalies into the deep tropics via a wind-evaporation-SST (WES) feedback. This work will therefore provide an interesting opportunity to investigate interactions between clouds, atmospheric circulation and SST in nature, as well to improve our understanding of the processes that contribute to tropical SST variability. This work will have two parts. In the first part, we will diagnose CRE associated with the modes of atmosphere-ocean variability that “bridge” large-scale extratropical weather to interannual to decadal tropical SST variability (e.g. the Meridional Modes). TOA and surface radiation observations will be acquired from remotely-sensed products that span multiple decades (e.g. ISCCP-FD and CERES-EBAF), and SST observations will be used from reanalysis products (e.g. MERRA-2). Since the typical measure of CRE (the difference between all-sky and clear-sky radiation) conflates the radiative effects of clouds and non-cloud variables, an adjusted CRE will be calculated by decomposing radiation fields into components from water vapor, temperature, and clouds using TOA and surface radiative kernels from CESM-CAM5 (Pendergrass et al., 2018). In the second part, the role of cloud feedbacks in the extratropical to tropical pathway will be directly investigated using MPI-ESM-LR output from a standard pre-industrial 250 year, coupled atmosphere-ocean global climate simulation with and without CRE that interact with the atmospheric circulation (Radel et. al., 2016). The strength of the cloud feedback in the extratropical to tropical pathway will be quantified by calculating the cloud feedback parameter (the change in adjusted CRE per change in SST) associated with the Meridional Modes in simulations with and without interactive clouds.