Historical trends in the jet streams Cristina Archer

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

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

Jet streams: theory

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

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

Jet stream centroids Wind speed Altitude Latitude

Jet stream wind speed

Jet stream wind speed

Jet stream wind speed NH SHT SHP

Wind speed trends s NH, SHT weakening s SHP strengthening

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

Latitude anomaly trends s All jets are moving poleward

Thermal Wind s Zonal wind speed trends similar in both reanalyses s Zonal thermal wind explains partially NH wind speed trends only

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

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