AR4 reported a poleward displacement of Atlantic and southern polar front jet streams from the 1960s to at least the mid-1990s and a poleward shift of the northern hemispheric storm tracks. However, it was also noted that uncertainties are large and that NNR and ERA-40 disagree in important aspects. SREX also reported a poleward shift of NH and SH storm tracks. Studies since AR4 confirm that in the NH, the jet core has been migrating towards the pole since the 1970s, but trends in the jet speed are uncertain. Additional studies assessed here further support the poleward shift of the North Atlantic storm track from the 1950s to the early 2000s.
Subtropical and mid-latitude jet streams are three-dimensional entities that vary meridionally, zonally, and vertically. The position of the mid-latitude jet streams is related to the position of the mid-latitude storm tracks; regions of enhanced synoptic activity due to the passage of cyclones (Section 2.6). Jet stream winds can be determined from radiosonde measurements of GPH using quasi-geostrophic flow assumptions. Using reanalysis data sets (Box 2.3), it is possible to track three-dimensional jet variations by identifying a surface of maximum wind (SMW), although a high vertical resolution is required for identification of jets.
Various new analyses based on NCEP/NCAR and ERA-40 reanalyses as well as MSU/AMSU lower stratospheric temperatures (Section 2.4.4) confirm that the jet streams (mid-latitude and subtropical) have been moving poleward in most regions in the NH over the last three decades (Fu et al., 2006; Hu and Fu, 2007; Strong and Davis, 2007; Archer and Caldeira, 2008a; Fu and Lin, 2011) but no clear trend is found in the SH (Swart and Fyfe, 2012). There is inconsistency with respect to jet speed trends based upon whether one uses an SMW-based or isobaric-based approach (Strong and Davis, 2007, 2008; Archer and Caldeira, 2008b, 2008a) and the choice of analysis periods due to inhomogeneities in reanalyses (Archer and Caldeira, 2008a). In general, jets have become more common (and jet speeds have increased) over the western and central Pacific, eastern Canada, the North Atlantic and Europe (Strong and Davis, 2007; Barton and Ellis, 2009), trends that are concomitant with regional increases in GPH gradients and circumpolar vortex contraction (Frauenfeld and Davis, 2003; Angell, 2006). From a climate dynamics perspective, these trends are driven by regional patterns of tropospheric and lower stratospheric warming or cooling and thus are coupled to large-scale circulation variability.
The North Atlantic storm track is closely associated with the NAO (Schneidereit et al., 2007). Studies based on ERA-40 reanalysis (Schneidereit et al., 2007), SLP measurements from ships (Chang, 2007), sea level time series (Vilibic and Sepic, 2010), and cloud analyses (Bender et al., 2012) support a poleward shift and intensification of the North Atlantic cyclone tracks from the 1950s to the early 2000s (Sorteberg and Walsh, 2008; Cornes and Jones, 2011).