Abstract
The tropics are expanding poleward at about 0.5 ∘ per decade in observations. This poleward expansion of the circulation is consistently reported using Hadley cell edge metrics and lower-atmospheric tropical edge metrics. However, some upper-atmospheric tropical metrics report smaller trends that are often not significant. One such upper-atmospheric metric is the subtropical jet latitude, which has smaller trends compared to the Hadley cell edge. In this study we investigate the robustness of the weak trends in the subtropical jet position by introducing a new method for locating the subtropical jet, and examining the trends and variability of the subtropical jet latitude. We introduce the tropopause gradient method based on the peak gradient in potential temperature along the dynamic tropopause. Using this method we find the trends in the subtropical jet latitude are indeed much smaller than 0.5 ∘ per decade, consistent with previous studies. We also find that natural variability within the subtropical jet latitude would not prevent trends from being detected if they were similar to the Hadley cell edge, as trends greater than 0.24∘ per decade could reliably be detected using monthly data or 0.09∘ per decade using daily data. Despite the poleward expansion of the tropics, there is no robust evidence to suggest the subtropical jet is shifting poleward in either hemisphere. Neither the current diagnostic methods nor natural variability can account for the small subtropical jet trends. The most likely explanation, which requires further investigation, is that the subtropical jet position is not tied dynamically to the Hadley cell edge.
| Original language | English |
|---|---|
| Pages (from-to) | 1741-1759 |
| Number of pages | 19 |
| Journal | Climate Dynamics |
| Volume | 54 |
| Issue number | 3-4 |
| DOIs | |
| State | Published - Feb 1 2020 |
Funding
The code for the tropopause gradient method has been made open-source, Kelleher and Maher (2019), available at https://github.com/mkstratos/stj_pv and the documentation can be found at https://stj-pv.readthedocs.io. The Natural Environment Research Council and Met Office fund PM as part of the ParaCon project (Grant NE/N013123/1), MK and PS as part of the EuroClim project (Grant NE/M006123/1). We especially want to thank Gloria Manney for their kind and thorough review of the manuscript and our two anonymous reviewers who together helped to greatly improve our study. PM would like to thank Hanh Nguyen and Chris Lucus from the Bureau of Meteorology for interesting discussions on this work and tropical expansion more generally. ERA-Interim courtesy of ECMWF, CFSR/CFSv2 and JRA-55 were obtained from UCAR RDA, courtesy of NCEP and JMA respectively, and MERRA-2 courtesy of NASA. This research used resources of the Compute and Data Environment for Science (CADES) at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. The authors would also like to acknowledge the numpy (Oliphant 2006), scipy (Jones et\u00A0al. 2001), and xarray (Hoyer and Hamman 2017) Python libraries used in the algorithm, and the matplotlib (Hunter 2007), pandas (McKinney 2010), and seaborn (Waskom et\u00A0al. 2018) Python libraries used in plotting and analysis of the results. The code for the tropopause gradient method has been made open-source, Kelleher and Maher (), available at https://github.com/mkstratos/stj_pv and the documentation can be found at https://stj-pv.readthedocs.io . The Natural Environment Research Council and Met Office fund PM as part of the ParaCon project (Grant NE/N013123/1), MK and PS as part of the EuroClim project (Grant NE/M006123/1). We especially want to thank Gloria Manney for their kind and thorough review of the manuscript and our two anonymous reviewers who together helped to greatly improve our study. PM would like to thank Hanh Nguyen and Chris Lucus from the Bureau of Meteorology for interesting discussions on this work and tropical expansion more generally. ERA-Interim courtesy of ECMWF, CFSR/CFSv2 and JRA-55 were obtained from UCAR RDA, courtesy of NCEP and JMA respectively, and MERRA-2 courtesy of NASA. This research used resources of the Compute and Data Environment for Science (CADES) at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. The authors would also like to acknowledge the numpy (Oliphant ), scipy (Jones et al. ), and xarray (Hoyer and Hamman ) Python libraries used in the algorithm, and the matplotlib (Hunter ), pandas (McKinney ), and seaborn (Waskom et al. ) Python libraries used in plotting and analysis of the results.
Keywords
- Subtropical jet stream
- Tropical expansion
- Tropopause gradient method