What ’ s the Representation of the 1 Moisture - Tropopause Relationship in CMIP 5 2 Models ?

5 A dynamical relationship that connects the extratropical tropopause potential tempera6 ture and the near surface distribution of equivalent potential temperature was proposed in 7 a previous study and was found to work successfully in capturing the annual cycle of the 8 extratropical tropopause in reanalyses. This study extends the diagnosis of the moisture9 tropopause relationship to an ensemble of CMIP5 models. 10 It’s found that, in general, CMIP5 multi-model averages are able to produce the one11 to-one moisture-tropopause relationship. However, a few biases are observed as compared 12 to reanalyses. First of all, ’cold’ biases are seen at both the upper and lower levels of the 13 troposphere, which are universal for all seasons, both hemispheres and almost all CMIP5 14 models. This has been known as the ’general coldness of climate models’ since 1990 but 15 the mechanisms remain elusive. It’s shown that, for Northern Hemisphere annual averages, 16 the upperand lower-level ’cold’ biases are, in fact, correlated across CMIP5 models, which 17 supports the dynamical linkage. Secondly, a large inter-model spread is found and nearly 18 half of the models under-estimate the annual cycle of the tropopause potential temperature 19 as compared to that of the near surface equivalent potential temperature fluctuation. This 20 implies the incapability of the models to propagate the surface seasonal cycle to the upper 21 levels. Finally, while reanalyses exhibit a pronounced asymmetry in tropopause potential 22 temperature between the northern and southern summers, only few CMIP5 models is able 23 to capture this aspect of the seasonal cycle due to the too dry specific humidity in northern 24 summer. 25