Baroclinic annular variability in the Northern Hemisphere

1 Large-scale variability in the Northern Hemisphere (NH) circulation can be 2 viewed in the context of three primary types of structures: 1) teleconnection patterns; 2) 3 a barotropic annular mode; and 3) a baroclinic annular mode. The barotropic annular 4 mode corresponds to the northern annular mode (NAM) and has been examined 5 extensively in previous research. Here we examine the spatial structure and time 6 dependent behavior of the NH baroclinic annular mode (NBAM). 7 The NAM and NBAM play very different roles in driving large-scale variability in 8 the NH circulation. The NAM emerges as the leading principal component (PC) time 9 series of the zonal-mean kinetic energy. It dominates the variance in the wave fluxes of 10 momentum, projects weakly onto the eddy kinetic energy and wave fluxes of heat, and 11 can be modeled as Gaussian red noise with a timescale of ~10 days. In contrast, the 12 NBAM emerges as the leading PC time series of the eddy kinetic energy. It is most 13 clearly identified when the planetary-scale waves are filtered from the data, dominates 14 the variance in the synoptic-scale eddy kinetic energy and wave fluxes of heat, and has a 15 relatively weak signature in the zonal-mean kinetic energy and the wave fluxes of 16 momentum. Interestingly, the NBM is marked by enhanced spectral power on 17 timescales of ~20-25 days. 18 The NBAM is remarkably similar to its Southern Hemisphere counterpart, 19 despite the pronounced interhemispheric differences in orography and land/sea 20 contrasts. The annular scale of the NBAM is consistent with the weak but significant 21 correlations between synoptic-scale eddy activity over the Atlantic and Pacific sectors of 22 the NH. 23