Southern Hemisphere Cloud–Dynamics Biases in CMIP5 Models and Their Implications for Climate Projections

Abstract

This study quantifies cloud-radiative anomalies associated with interannual variability in the latitude of the Southern Hemisphere (SH) midlatitude eddy-driven jet, in 20 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two distinct model types are found. In the first class of models (type I models), total cloud fraction is reduced at SH midlatitudes as the jet moves poleward, contributing to enhanced shortwave radiative warming. In the second class of models (type II models), this dynamically induced cloud radiative warming effect is largely absent. Type I and type II models have distinct deficiencies in their representation of observed Southern Ocean clouds, but comparison with two independent satellite datasets indicates that the cloud-dynamics behavior of type II models is more realistic.Because the SH midlatitude jet shifts poleward in response to CO 2 forcing, the cloud-dynamics biases uncovered from interannual variability are directly relevant for climate change projections. In CMIP5 model experiments with abruptly quadrupled atmospheric CO 2 concentrations, the global-mean surface temperature initially warms more in type I models, even though their equilibrium climate sensitivity is not significantly larger. In type I models, this larger initial warming is linked to the rapid adjustment of the circulation and clouds to CO 2 forcing in the SH, where a nearly instantaneous poleward shift of the midlatitude jet is accompanied by a reduction in the reflection of solar radiation by clouds. In type II models, the SH jet also shifts rapidly poleward with CO 2 quadrupling, but it is not accompanied by cloud radiative warming anomalies, resulting in a smaller initial globalmean surface temperature warming.

External Influences on Modeled and Observed Cloud Trends

“…The L-only fingerprint F(L) is increasingly apparent in both observational datasets and is incompatible with internal variability. We note, however, that several authors have suggested that cloud shifts may not be trivially related to changes in the jet (Grise and Polvani 2014;Ceppi et al 2014;Kay et al 2014), and further study of the interaction between clouds and the circulation is necessary to understand this response. This is in agreement with previous studies that suggest models fail to capture the observed poleward expansion (Johanson and Fu 2009;Seidel et al 2008).…”

High Resolution Model Intercomparison Project (HighResMIP v1.0) for CMIP6

“…Yoshida and Ishikawa, 2013). Even for the dynamical storm track, which may be thought satisfactorily resolved by low-resolution climate models, its bias in latitudinal position is related to the cloud amount bias in CMIP5 models (Grise and Polvani, 2014). Existing high-resolution atmosphere simulations suggest that the characteristics of the jet stream (Hodges et al, 2011) and blocking (Jung et al, 2012) will be improved by higher resolution.…”

Clouds, circulation and climate sensitivity

“…But the myriad ways in which clouds couple to the storm tracks are just beginning to be appreciated, for instance through their radiative effects. As the clouds embedded within the storm tracks shift, there are systematic implications for the radiation budget and its influence on the temperature gradients that give rise to the storms in the first place 24,25 . The development of a hierarchy of modelling approaches is advancing understanding of how moist processes such as those embedded along frontal systems, interactions with ocean circulations, and cloud radiative effects influence both storm development and the structure of the storm tracks.…”