Revisiting The Importance of a Southern Ocean Pattern Effect

Abstract

The spatial pattern of surface warming determines radiative feedbacks (“pattern effect”). The seminal work of Senior & Mitchell (2000) (SM00) argued that Southern Ocean clouds set the evolution of global radiative feedbacks in time and hence, determine climate sensitivity. We revisit their argument in current generation climate models: We quantify the time evolution of local atmospheric stability, clouds, and their local and global radiative effects as the Southern Ocean warming is catching up with global mean warming. SM00 argue that in the early period after a strong radiative forcing with CO2, the Southern Ocean’s warming is delayed compared to the global mean (I) and that the faster warming tropics set the free tropospheric temperature over the Southern Ocean (II), creating a relatively stable lapse rate (III). While overall cloud cover shrinks, the loss of stratiform clouds is suppressed (IV) and convection is unfavorable (V) during this fast timescale. As Southern Ocean warming catches up with the rest of the globe (VI), the local Southern Ocean lapse rate destabilizes (VII) which accelerates the loss of stratiform clouds and creates a more favorable environment for convection(VIII). SM00 argue that the time-evolution of Southern Ocean clouds controls the local Southern Ocean cloud feedback (IX) and that the processes I-VIII dominate the time-evolution of the global net radiative feedback and hence, climate sensitivity (X). We find that current generation models are able to confirm SM00’s arguments up to the Southern Ocean controlling the local Southern cloud feedback (I-IX). However, the time evolution of Southern Ocean clouds does not reflect in the time evolution of the global net feedback (X). We propose that SM00 mistakenly assumed a causal connection between climate sensitivity and the Southern Ocean cloud evolution, while the actual link to climate sensitivity is the global sensitivity of shortwave clouds to warming and the remote effects of Southern Ocean heat uptake.