Identifying the leading patterns of extratropical climate variability

The leading patterns of large-scale climate variability are examined in the context of the energetics of the extratropical circulation. It is argued that variability in the extratropical flow can be viewed in the context of two types of structures: 1) those that explain large fractions of the variance in the zonal-mean kinetic energy and the eddy fluxes of momentum; and 2) those that explain large fractions of the variance in the eddy-kinetic energy and the eddy fluxes of heat. The two types of structures play very different roles in cycling energy in the middle latitude atmosphere.

Variability in the zonal-mean kinetic energy and the eddy fluxes of momentum is dominated by the annular modes. But variability in the extratropical eddy kinetic energy and the eddy fluxes of heat is dominated by a very different structure that emerges as the leading principal component (PC) time series of the eddy-kinetic field. The annular modes have been extensively examined in the literature. The leading PC of eddy-kinetic energy has received much less attention in previous work, but arguably plays a similarly important role in extratropical climate variability. Implications for the interpretation of large-scale climate variability are discussed.