Theories of the generation mechanisms of large-scale atmospheric disturbances and for determining their predictability in weather and seasonal climate forecasts are reviewed. The roles of dynamical instability in the genesis of storm tracks, blocking, teleconnection patterns, intraseasonal oscillations and convectively coupled equatorial waves are discussed. Regime transitions associated with weather prediction, climate prediction and climate change are examined. The ensemble predictability of strong zonal flow to blocking regime transitions is analysed. The seasonal variability of large-scale instabilities and teleconnection patterns is examined. The causes of the boreal spring predictability barrier associated with seasonal climate prediction of coupled ocean-atmosphere models are examined.

The dynamical reasons for the changing Southern Hemisphere climate during the second half of the twentieth century, including the dramatic reduction in observed winter rainfall in the south west of Western Australia since the mid-1970s, and the recent drought across southern Australia, are examined. The corresponding changes in dynamical modes of variability ranging from storm tracks, blocking modes, northwest cloud-band disturbances, Antarctic low frequency modes, intraseasonal oscillations and African easterly waves are described. The projected changes and trends during the 21st century, in rainfall and baroclinic instability in SRES scenarios, using results from CMIP3 climate change models, are discussed.