Understanding the Dynamical Controlling Factors on Southern Ocean Clouds

It is well known that midlatitude clouds, particularly over the Southern Ocean, are misrepresented in global climate models. In particular, previous studies have pinpointed the cold-air sector of extratropical cyclones as the origin for these biases. In this study, we examine the relationship between clouds and midlatitude dynamics on both monthly and daily timescales and show that three dynamical “cloud-controlling factors” can explain the bulk of midlatitude cloud variability: large-scale vertical motion, lower tropospheric stability, and near-surface temperature advection. Model errors may thus arise from biases in representing cloud-controlling factors (atmospheric dynamics), in representing how clouds respond to those cloud-controlling factors (cloud parameterizations), or some combination thereof.

We examine the relationship between Southern Ocean clouds and the dynamical controlling factors in three cases: 1) a poleward shift of the eddy-driven jet (on monthly timescales), 2) a composite of an extratropical cyclone (on daily timescales), and 3) a composite of an extratropical anticyclone (on daily timescales). In all three cases, observations and global climate models agree well on the magnitude and distribution of vertical velocity, lower tropospheric stability, and near-surface temperature advection. However, the models and observations differ on the relative importance of these factors in controlling the cloud field. In particular, models appear to overestimate the sensitivity of cloud cover to variations in vertical velocity and underestimate the sensitivity of cloud cover to variations in lower tropospheric stability and near-surface temperature advection. As a result, clouds are too bright along the cold front of an extratropical cyclone and are underestimated in extratropical anticyclones and in the region equatorward of a poleward jet shift.