Distinct roles of cyclones and anticyclones in setting the midwinter minimum of the North Pacific eddy activity. Part II: Eulerian eddy statistics and energetics

Satoru Okajima, Hisashi Nakamura, Yohai Kaspi

The characteristics and dynamics of midlatitude storm-tracks have been long investigated. Nevertheless, our understandings of the storm-tracks, especially the midwinter minimum of the North Pacific storm-track activity, are still limited, partly because Eulerian eddy statistics are incapable of separating cyclonic and anticyclonic contributions. Here we investigate the detailed seasonal evolution of the contributions from cyclonic and anticyclonic vortices, identified with local wind curvature, to the Eulerian eddy statistics, eddy feedback forcing onto the climatological-mean westerly jet, and the energetics associated with the North Pacific storm-track. We demonstrate that the midwinter minimum of the net energy conversion/generation rate normalized by the eddy total energy over the entire North Pacific is more distinct for anticyclonic vortices than for cyclonic vortices, to which the midwinter minimum of the probability of anticyclonic vortices contributes. The main factors include the reduction of the baroclinic energy conversion into midwinter and the reduction of the net energy outflux into spring. To long-term modulations of the MWM of the NP storm-track activity, both cyclonic and anticyclonic vortices contribute constructively. Together with our companion paper, the present study reveals the primal importance of migratory anticyclones for the midwinter minimum of the North Pacific storm-track activity. We posit a viewpoint of considering roles of both migratory cyclones and anticyclones for the storm-track dynamics, the latter of which has been long disregarded unconsciously.