This dataset documents the trends and variability in the latitude and strength of the belt of lower-atmosphere westerly winds over the Southern Ocean, referred to as the ‘westerly jet’. Time series of annual mean and seasonal diagnostics are available for the period 1979-present are documented, specifically time series of seasonal and annual mean jet latitude and strength. The diagnostics are derived from the European Centre for Medium Range Weather Forecasts (ECMWF) ERA-Interim reanalysis (Dee et al., 2011), which is an observationally-constrained reconstruction of atmospheric conditions. The broad characterisation of the westerly winds into these simple diagnostics has been found to be useful for understanding long-term climate change due to contrasting drivers of change and impacts on other aspects of the climate system.
The jet indices shown here were calculated following the definition detailed in Bracegirdle et al. (2018). This definition is based on westerly wind in the lower atmosphere (850 hPa, which is approximately 1.5 km above mean sea level). At each latitude the all-longitude (zonal) average was calculatd between 75°S and 10°S. The maximum in this diagnostic is defined as the jet strength and its location defines the jet latitude. This is a very similar approach to that used in a number of other studies, e.g. Kidston and Gerber (2010), Son et al. (2010) and Swart and Fyfe (2012).
More detailed information can be found on this topic at the NCAR/UCAR Climate Data Guide: https://climatedataguide.ucar.edu/climate-data/southern-hemisphere-westerly-jet-strength-and-position
- Bracegirdle, T. J., P. Hyder, C. R. Holmes, in press: CMIP5 diversity in southern westerly jet projections related to historical sea ice area; strong link to strengthening and weak link to shift. J. Climate, doi:10.1175/jcli-d-17-0320.1.
- Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553-597, doi:10.1002/qj.828.
- Kidston, J., and E. P. Gerber, 2010: Intermodel variability of the poleward shift of the austral jet stream in the CMIP3 integrations linked to biases in 20th century climatology. Geophys. Res. Lett., 37, L09708, doi:10.1029/2010gl042873.
- Son, S. W., and Coauthors, 2010: Impact of stratospheric ozone on Southern Hemisphere circulation change: A multimodel assessment. Journal of Geophysical Research-Atmospheres, 115, D00M07, doi:10.1029/2010JD014271.
- Swart, N. C., and J. C. Fyfe, 2012: Observed and simulated changes in the Southern Hemisphere surface westerly wind-stress. Geophys. Res. Lett., 39, L16711, doi:10.1029/2012gl052810.
The European Centre for Medium Range Weather Forecasting is thanked for providing the ERA-Interim datasets.