Meridional Transport of Eddy Sensible Heat in Winters Marked by Extremes of the North Atlantic Oscillation, 1948/49–1979/80

Abstract

Composite patterns of eddy sensible heat transport at 700 mb on the Northern Hemisphere are derived for winters [December, January, February (DJF)] marked by extremes of the North Atlantic Oscillation (NAO) occurring in the 1948/49?1979/80 period. Statistically significant heat flux differences between the two NAO modes (GA: Greenland above; GB: Greenland below, normal temperatures relative to northern Europe) occur dominantly in the North Atlantic sector. These are connected to marked reversals in sea level pressure (SLP) and an out-of-phase behavior of the zonal component of the geostrophic wind related to changes in the amplitude of wavenumber 2. Teleconnections of the NAO, in terms of both pressure and eddy heat transport, to the North Pacific and east Asian regions are found to be weak for those extreme events occurring since 1948/49. Partitioning the total meridional heat flux into its transient and quasi-stationary components confirms the dominance of the latter for the hemisphere as a whole, but also shows large differences in the relative contributions for the North Atlantic sector as a function of NAO winter type. There, the poleward transport by the mean eddies relative to the transients is the stronger (weaker) in the GB (GA) winter. Satellite cloud vortex analysis for two winters, each representative of an opposing extreme of the NAO (1974/75, 1976/77), reveals that the composite changes in meridional eddy heat transport for the North Atlantic are matched by concomitant variations in the total and latitude-band occurrences of baroclinic systems.