Role of the mean state for the Southern Hemispheric jet stream response to CO₂ forcing in CMIP6 models

Abstract

Global climate models indicate that the Southern Hemispheric (SH) jet stream shifts poleward in response to CO 2 forcing, but the magnitude of this shift remains highly uncertain. Here we analyse the SH jet stream response to 4×CO 2 forcing in Coupled Model Intercomparison Project phase 6 (CMIP6) simulations, and find a substantially muted jet shift during winter compared with CMIP5. We suggest this muted response results from a more poleward mean jet position, consistent with a strongly reduced bias in jet position relative to the reanalysis during 1980-2004. The improved mean jet position cannot be explained by changes in the simulated sea surface temperatures. Instead, we find indications that increased horizontal grid resolution in CMIP6 relative to CMIP5 has contributed to the higher mean jet latitude, and thus to the reduced jet shift under CO 2 forcing. These results imply that CMIP6 models can provide more realistic projections of SH climate change.

An Evaluation of the Large‐Scale Atmospheric Circulation and Its Variability in CESM2 and Other CMIP Models

“…For the models and time period considered here, which differ slightly from that in Curtis et al 2020, we indeed find that the CMIP6 (including BCAM6 and BWACCM6) ensemble mean equatorward bias in the annual mean is only around 1.1°compared to around 2.6°in CMIP5. However, in the JJA season alone, not considered by Curtis et al (2020), we still find an ensemble mean equatorward bias in the CMIP6 models of around 3.3°, which can be compared with a value of around 4.3°for the CMIP5 models. Thus, while there are some improvements, a substantial equatorward bias still remains in CMIP6, in JJA in particular, and there are still some models that place the SH westerlies about 16°too far equatorward in this season (Figure 2j).…”

Intermodel CMIP5 relationships in the baseline Southern Ocean climate system and with future projections

“…For example, there is a reduction in the intermodel spread of seasonal sea ice variations, and the regional distribution is improved, compared to CMIP5 (Roach et al, 2020). The Southern Hemisphere jet stream and storm tracks are also less biased in CMIP6, exhibiting higher mean jet latitude (Bracegirdle et al, 2020;Curtis et al, 2020;Goyal et al, 2021;Priestley et al, 2020), and therefore reduced jet shift under future warming (Curtis et al, 2020). The reduced jet stream bias is likely due to increased horizontal atmospheric resolution (Curtis et al, 2020).…”

“…The Southern Hemisphere jet stream and storm tracks are also less biased in CMIP6, exhibiting higher mean jet latitude (Bracegirdle et al, 2020;Curtis et al, 2020;Goyal et al, 2021;Priestley et al, 2020), and therefore reduced jet shift under future warming (Curtis et al, 2020). The reduced jet stream bias is likely due to increased horizontal atmospheric resolution (Curtis et al, 2020). Along with improvements to the representation of surface wind stress forcing, the simulated strength of the Antarctic Circumpolar Current and associated density gradients have improved in CMIP6 (Beadling et al, 2020).…”

Increasing risk of another Cape Town “Day Zero” drought in the 21st century

“…To facilitate the improvement of water-management practices and the infrastructure necessary to make the system more resilient, it is critical to first determine how likely a meteorological drought like the one in 2015-2017 might be in the coming decades. Increased aridity is expected in most of southern Africa (12)(13)(14) as a consequence of the Hadley Cell poleward expansion (4,(15)(16)(17)(18) and southward shift of the Southern Hemisphere jet stream (19). Second, the risk of more extreme droughts should be quantified to understand the potential for emerging risks that could make a Day Zero event in Cape Town unavoidable.…”

“…Such estimates make use of statistical models of the probability distribution's tail (e.g., the generalized extreme value) applied to observations and previous-generation [i.e., as those participating to the Coupled Model Intercomparison Project Phase 3 (CMIP3) (20) and 5 (21)] climate models. CMIP3 and CMIP5 models have been shown to have a systematically biased position of the Southern Hemisphere jet stream toward the Equator, due to insufficient horizontal resolution (19). This produces a large uncertainty in model projections of jet-stream shifts (22,23), thus hindering realistic projections of Southern Hemisphere climate change.…”