Historical and projected changes in the Southern Hemisphere Sub-tropical Jet during winter from the CMIP5 models

We present projected changes in the speed and meridional location of the Subtropical Jet (STJ) during winter using output of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. We use the ERA-Interim reanalysis dataset to evaluate the historical simulations of the STJ by 18 of the CMIP5 models for the period 1979–2012. Based on the climatology of the STJ from ERA-Interim, we selected the area of study as 70°E–290°E and 20°S–40°S, which is over the Indian and Southern Pacific Oceans, and 300–100 hPa to reduce altitude-related bias. An assessment of the ability of the CMIP5 models in simulating ENSO effects on the jet stream were carried out using standardized zonal wind anomalies at 300–100 hPa. Results show that 47 % of the CMIP5 models used in this study were able to simulate ENSO impacts realistically. In addition, it is more difficult for the models to reproduce the observed intensity of ENSO impacts than the patterns. The historical simulations of the CMIP5 models show a wide range of trends in meridional movement and jet strength, with a multi-model mean of 0.04° decade−1 equatorward and 0.42 ms−1 decade−1 respectively. In contrast to the ERA-Interim analysis, 94 % of the CMIP5 models show a strengthening of the jet in the historical runs. Variability of the jet strength is significantly (5 %) linked to the sea surface temperature changes over the eastern tropical Pacific. The CMIP5 model projections with Representative Concentration Pathways (RCPs) 4.5 and 8.5 were used for analysis of changes of the STJ for the period 2011–2099. Based on the RCP 4.5 (RCP 8.5) scenario the multi-model mean trend of the 18 CMIP5 models project a statistically significant (5 % level) increase in jet strength by the end of the century of 0.29 ms−1 decade−1 (0.60 ms−1 decade−1). Also, the mean meridional location of the jet is projected to shift poleward by 0.006° decade−1 (0.042° decade−1) in 2099 during winter, with the only significant (5 %) trend being with RCP 8.5.

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