Trends in Austral jet position in ensembles of 1 high - and low - top CMIP 5 models

4 Trends in the position of the DJF Austral jet have been analysed for 5 multi-model ensemble simulations of a subset of highand low-top models 6 for the periods 1960-2000, 2000-2050, and 2050-2098 under the CMIP5 his7 torical, RCP4.5, and RCP8.5 scenarios. Comparison with ERA-Interim, 8 CFSR and the NCEP/NCAR reanalysis shows that the DJF and annual 9 mean jet positions in CMIP5 models are equatorward of reanalyses for 10 the 1979-2006 mean. Under the RCP8.5 scenario, the mean jet position 11 in the high-top models moves 3 degrees poleward of its 1860-1900 position 12 by 2098, compared to just over 2 degrees for the low-top models. 13 Changes in jet position are linked to changes in the meridional tem14 perature gradient. Compared to low-top models, the high-top models pre15 dict greater warming in the tropical upper troposphere due to increased 16 greenhouse gases for all periods considered: up to 0.28 K/decade more 17 in the period 2050-2098 under the RCP8.5 scenario. Larger polar lower18 stratospheric cooling is seen in high-top models: -1.64 K/decade compared 19 to -1.40 K/decade in the period 1960-2000, mainly in response to ozone 20 depletion, and -0.41 K/decade compared to -0.12 K/decade in the period 21 2050-2098, mainly in response to increases in greenhouse gases. 22 Analysis suggests that there may be a linear relationship between the 23 trend in jet position and meridional temperature gradient, even under 24 strong forcing. There were no clear indications of an approach to a geo25 metric limit on the absolute magnitude of the poleward shift by 2100. 26