zonal Wavenumber Characteristics of Northern Hemisphere Transient Eddies

Abstract

Eleven winters and twelve summers of twice-daily National Meteorological Center (NMC) analyses of geopotential height and temperature at five levels for the extratropical Northern Hemisphere are partitioned into long-wave fields with zonal wavenumbers m ≤ 5 and short-wave fields with zonal wavenumbers m ≥ 6. The data are also bandpass-filtered to investigate fluctuations of periods 2.5?10 days. From these fields, the standard deviation of 500 mb geopotential height and the poleward geostrophic transient eddy fluxes of sensible heat and zonal momentum are calculated and presented. The results are compared with theoretical and numerical modeling studies of nonlinear, baroclinic instability. Both the observations and modeling studies produce evidence that baroclinic eddies have a definite life cycle and that the strongest baroclinic eddies are of intermediate size (zonal wavenumbers 6?8). Some interesting differences between observations and modeling results also emerge.