A Mechanism for the Midwinter Minimum in North Pacific Storm-Track Intensity From a Global Perspective

The midwinter minimum in North Pacific storm-track intensity is a perplexing phenomenon because the associated local baroclinity in the North Pacific is maximum during midwinter. Here, a new mechanism is proposed wherein the midwinter minimum occurs in part because global planetary-scale waves consume the zonal available potential energy, limiting its availability for storm-track eddy growth. During strong midwinter suppression years, the midwinter minimum is preceded by anomalously large planetary-scale eddy kinetic energy and subsequent reduction in zonal available potential energy and global baroclinity. Consistent with previous studies, this large planetary-scale eddy kinetic energy takes place after enhanced Pacific warm pool convection, which peaks during winter. These results indicate that the midwinter minimum is in part caused by heightened warm pool convection, which, through excitation of planetary-scale waves, leads to a weaker storm-track. This finding also helps explain the existence of the midwinter North Atlantic storm-track minimum.