3.5.7 Summary

Changes from the late 1970s to recent years generally reveal decreases in tropospheric geopotential heights over high latitudes of both hemispheres and increases over the mid-latitudes in DJF. The changes amplify with altitude up to the lower stratosphere, but remain similar in shape to lower atmospheric levels and are associated with the intensification and poleward displacement of corresponding Atlantic and southern polar front jet streams and enhanced storm track activity. Based on a variety of measurements at the surface and in the upper troposphere, it is likely that there has been an increase and a poleward shift in NH winter storm-track activity over the second half of the 20th century, but there are still significant uncertainties in the magnitude of the increase due to time-dependent biases in the reanalyses. Analysed decreases in cyclone numbers over the southern extratropics and increases in mean cyclone radius and depth over much of the SH over the last two decades are subject to even larger uncertainties.

The decrease in long-lasting blocking frequency over the North Atlantic-European sector over recent decades is dynamically consistent with NAO variability (see Section 3.6), but given data limitations, it may be too early to define the nature of any trends in SH blocking occurrence, despite observed trends in the SAM. After the late 1990s in the NH, occurrences of major sudden warmings seem to have increased in the polar stratosphere, associated with the occurrence of more neutral states of the tropospheric and stratospheric vortex. In the SH, there has been a strengthening tropospheric antarctic vortex during summer in association with the ozone hole, which has led to a cooling of the stratospheric polar vortex in late spring and to a two- to three-week delay in vortex breakdown. In September 2002, a major warming was observed for the first and only time in the SH. Analysis of observed wind and SWH support the reanalysis-based evidence for an increase in storm activity in the extratropical NH in recent decades (see also Section 3.6) until the late 1990s. For heat flux, there seem to have been NAO-related variations over the Labrador Sea, which is a key region for deep water formation.