Impacts of the Planetary-Scale Eddies on the Midwinter Suppression in North Pacific Storm Track Intensity

During boreal winter, synoptic-scale eddy activity is concentrated in two regions, one over the North Pacific Ocean and the other over the North Atlantic Ocean. The observation that the North Pacific storm track weakens during midwinter has posed a long-standing question to the storm track dynamics research community because baroclinity, upon which baroclinic eddies feed on, is maximum during midwinter. The proposed mechanisms to date consider processes that are local to the North Pacific storm track region. In this study, the midwinter minimum question is approached from the perspective of eddy-mean flow interaction which includes both synoptic-scale storm track eddies and planetary-scale eddies. Motivated by previous studies of the dynamics of planetary-scale waves, the authors hypothesize that the midwinter minimum – suppression of synoptic-scale wave activity – occurs when planetary-scale waves are anomalously active because the latter reduces the amount of the zonal available potential energy (ZAPE) available for the growth of synoptic-scale eddies. This hypothesis is tested by analyzing January eddy lifecycle events during years of severe midwinter minima and years of mild midwinter minima.

Supporting the hypothesis, the results of the analysis show that during the severe years, the synoptic-scale (defined here as zonal wavenumbers 5-8) eddy growth is preceded by enhanced planetary-scale (zonal wavenumber 1-3) eddy growth and reduced ZAPE. The opposite behavior holds during the mild years. The planetary-scale wave growth during the severe years is manifested in the form of an anomalously sharpened subtropical jet over the North Pacific, and the associated local Eady growth rate is anomalously large. Again, the opposite occurs during the mild years. This result indicates that to understand the midwinter minimum phenomenon, the global wave-mean flow perspective is more useful than a local baroclinity perspective. Moreover, the enhanced planetary-scale wave activity during the severe years is preceded by enhanced convection over the western warm pool region. These results suggest that the midwinter minimum is caused by heightened warm pool convection during midwinter which, by exciting strong planetary waves, results in a weaker growth of synoptic-scale waves.