Intermodel variability of the poleward shift of the austral jet stream in the CMIP3 integrations linked to biases in 20th century climatology

Abstract

[1] Future climate predictions by global circulation models in the Coupled Model Intercomparison Project Phase 3 (CMIP3) archive indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st century. Here it is shown that differences in the projected magnitude of the trend in the Southern Hemisphere are well correlated with biases in the latitude of the jet in the simulation of 20th century climate. Furthermore, the latitude of the jet in the models' 20th century climatology is correlated with biases in the internal variability of the jet stream, as quantified by the time scale of the annular mode. Thus an equatorward bias in the position of the jet is associated with both enhanced persistence of the annular mode, and an increased poleward shift of the jet. These relationships appear to be robust throughout the year except in the austral summer, when differences in forcing, particularly stratospheric ozone, make it impossible to compare the response of one model with another. These results suggest that the fidelity of a model's simulation of the 20th century climate may be related to its fitness for climate prediction. The cause of this relationship is discussed, as well as the implications for climate change projections. Citation: Kidston, J., and E. P. Gerber (2010), Intermodel variability of the poleward shift of the austral jet stream in the CMIP3 integrations linked to biases in 20th century climatology, Geophys. Res. Lett., 37, L09708,

Relationship between eddy‐driven jet latitude and width

“…[23] There is a strong relationship between the latitude of the jet and the time scale of the leading mode of variability, or the so called annular modes [Kidston and Gerber, 2010]. A number of authors [Gerber and Vallis, 2007; have argued that the persistence of zonal index and the annular modes is due to the self-maintenance of eddydriven jets.…”

Changes in the width of the tropical belt due to simple radiative forcing changes in the GeoMIP simulations

“…Increases in global-mean surface temperature can explain up to 79 % of the total inter-model variation in tropical expansion, noteworthy because it occurs within the inter-model space of fully coupled climate models. Different mean states (Kidston and Gerber, 2010), the representation of parameterized processes (Frierson, 2007), the strength of cloud feedbacks (Feldl and Bordoni, 2016), and model design choices such as horizontal resolution (Landu et al, 2014;Lorant and Royer, 2001;Davis and Birner, 2016) can all influence the circulation and its response. Tropical belt width changes are thus part and parcel of global climate change.…”

Projected changes of wintertime synoptic‐scale transient eddy activities in the East Asian eddy‐driven jet from CMIP5 experiments

“…In the following subsections, we will focus on the magnitude increase of EKE from the perspectives of the low-level static stability and its historical mean state. [Kidston and Gerber, 2010] and current CMIP5 [Barnes and Polvani, 2013] found that the future response of jet location is strongly related to its historical position. Similarly, the relationship between ΔEKE and the historical intensity of EKE, ΔEKE versus EKE, is then examined (Figure 2b).…”

“…Similar with the relationship between poleward shift of jet and its historical location [Kidston and Gerber, 2010], the correlation between ΔEKE and EKE can be interpreted in the term of model internal mechanisms. Since the historical EKE is underestimated in CMIP5 models, the strong enhancement of EKE in future warming climate is favorable to happen in the model with weak EKE.…”

“…Compared with the zonal symmetry and intermodel consensus in the Southern Hemisphere, however, the location displacement and magnitude change of the jet responding to global warming exhibit not only intermodel spread but also spatial inhomogeneity in the Northern Hemisphere [Yin, 2005;Kidston and Gerber, 2010;Barnes and Polvani, 2013]. Due to the complexity of the topography-induced thermodynamic condition in East Asia, the eddy-driven jet varies distinctively in this region.…”