The Role of Ocean Dynamics in Interannual to Decadal Variability and Predictability of Atlantic Sea Surface Temperatures

The goal of our work is to disentangle the roles of atmospheric forcing and various ocean dynamical processes in Atlantic sea surface temperature (SST) variability and predictability on interannual-to-decadal time scales. In order to achieve this goal, we develop a hierarchy of coupled models using the Community Earth System Model version 2 (CESM2). The model hierarchy includes ocean model components of varying complexity and comparing model pairs enables us to quantitatively determine the roles of specific aspects of ocean dynamics, including one-dimensional processes (vertical mixing, interannual mixed layer depth variations, entrainment) and three-dimensional ocean dynamics (including wind and buoyancy-driven processes), on driving SST variability. We apply a rigorous statistical technique, called covariance discriminant analysis (CDA), to diagnose the leading differences in Atlantic SST variance between model pairs, thus elucidating the impact of specific ocean processes on Atlantic SST variability. Additionally, we compare predictability of Atlantic SST between models in the hierarchy, to elucidate the role of oceanic processes in predictability.

We find that one-dimensional ocean processes, primarily entrainment damping, are responsible for reducing the variance of SST variations forced by atmospheric variability (e.g., tripolar SST anomalies associated with the North Atlantic Oscillation) throughout the basin. However, one-dimensional ocean processes are also responsible for enhancing the predictability of these variations, primarily due to the seasonal reemergence process. Three-dimensional ocean dynamics are responsible for enhancing both SST variability and predictability, primarily in the Tropical Atlantic and subpolar North Atlantic. Variations in wind forcing enhance SST variance, particularly in the Tropical Atlantic, where wind variations are important for Atlantic Nino.