Editing IPCC:AR6/WGI/Chapter-10 (section)

==== 10.6.2.5 Model Simulation and Attribution Over the Historical Period ====

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Due to the small scale of the Cape Town region, robust comparison of CMIP simulations to observations is difficult. However, in general, CMIP5 models capture the seasonality well, such as the dominance of austral winter rains, although they overestimate the peak and underestimate the shoulder season rainfall ( [[#Mahlalela--2019|Mahlalela et al., 2019]] ). Trends in rainfall are particularly difficult to assess as they are generally weak and depend strongly on the time period and dataset adopted for the analyses ( [[#10.6.2.3|Section 10.6.2.3]] ). A multi-method attribution study ( [[#Otto--2018|Otto et al., 2018]] ) estimates the probability of the 2015–2017 drought to have increased by a factor of three since pre-industrial times (with a wide 95% confidence interval of 1.5 to 6). However, throughout the 20th century, a substantial portion of the global models (about 36% of CMIP5 and 44% of CMIP6 models, as well as many of the MIROC SMILE members) simulate a statistically significant (95% level) decline in total annual rainfall, while there is no robust long-term trend in observations (Figure 10.18). [[#10.4|Section 10.4]] offers a more detailed assessment of attribution challenges.

Global models capture the overall behaviour of the observed main hemispherical processes, such as the expansion of the tropics, a positive trend in SAM and the poleward shift of the westerly jet. However, they fail to capture details of their observed climatology and variability ( [[#Simpson--2016|Simpson and Polvani, 2016]] ), and the magnitudes of simulated trends vary, though the models typically underestimate observed trends in these processes ( [[#Purich--2013|Purich et al., 2013]] ; [[#Staten--2018|Staten et al., 2018]] ). In general, CMIP5 models do capture the SAM-regional rainfall association, although not consistently across all seasons ( [[#Purich--2013|Purich et al., 2013]] ; E.-P. [[#Lim--2016|]] [[#Lim--2016|Lim et al., 2016]] ).

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