1. Historical trends in the jet streams Cristina Archer and Ken Caldeira Department of Global Ecology Carnegie Institution for Science Cristina Archer and Ken Caldeira Department of Global Ecology Carnegie Institution for Science AMS Meeting, New Orleans, 21-24 January 2008

2. Scientific questions IF, HOW, WHY have the jet streams changed in last decades? Methods  Definition of jet streams  Observed trends  Interpretation Methods  Definition of jet streams  Observed trends  Interpretation Tools Reanalyses (ERA-40 and NCEP/NCAR)

3. Jet streams: the theory

4. Jet streams: the reality Koch et al. (2006) Northern Hemisphere Southern Hemisphere

5. Jet streams: the reality Koch et al. (2006) Northern Hemisphere Southern Hemisphere SpiralAnnular

6. Jet stream centroids Wind speed Altitude Latitude

7. Jet stream wind speed

9. NH SHT SHP

10. Wind speed trends  NH, SHT weakening  SHP strengthening  NH, SHT weakening  SHP strengthening

11. Altitude trends  ERA-40: all jets rising significantly  NCEP/NCAR: SHP rising significantly, NH and SHT nearly constant  ERA-40: all jets rising significantly  NCEP/NCAR: SHP rising significantly, NH and SHT nearly constant

12. Latitude anomaly trends  All jets are moving poleward

13. Thermal Wind  Zonal wind speed trends similar in both reanalyses  Zonal thermal wind explains partially NH wind speed trends only  Zonal wind speed trends similar in both reanalyses  Zonal thermal wind explains partially NH wind speed trends only

14. Surface Temperature  Jets rising overall, but more variations in NH  Trends in surface temperature do not explain trends in jet height  Jets rising overall, but more variations in NH  Trends in surface temperature do not explain trends in jet height

15. Conclusions  Centroid approach to define jet streams  Jet streams have risen in altitude and moved poleward in both hemispheres  NH and SHT jets have weakened, whereas the SHP jet has strengthened  Impact of global warming (via surface temperature) mild in NH jet, not evident in SH jets - HIGHLY SIMPLIFIED APPROACH!  Centroid approach to define jet streams  Jet streams have risen in altitude and moved poleward in both hemispheres  NH and SHT jets have weakened, whereas the SHP jet has strengthened  Impact of global warming (via surface temperature) mild in NH jet, not evident in SH jets - HIGHLY SIMPLIFIED APPROACH!