Two distinct interannual modes of the north pacific oscillation: role of ocean-atmosphere coupling and transient eddies

The interannual variability of the North Pacific Oscillation (NPO) in boreal winter displayed a significant interdecadal shift, characterized by a local north-south seesaw pattern before the mid-1980s but a global-scale wave-train pattern afterwards, and the dynamic nature associated with these two distinct interannual modes remains unexplored. An analysis with the geopotential tendency equation shows the distinct roles of ocean-atmospheric coupling and transient eddies in modulating the two modes. The local north-south seesaw mode before the mid-1980s was dominated by high-frequency eddies, vorticity advection, and diabatic heating. The high-frequency eddies were conducive to moisture convergence in the low-level cyclonic mean flow and further led to intensification of precipitation process. The anomalous surface heat flux forced by the cyclonic mean flow strengthened the meridional temperature gradient and thus atmospheric baroclinicity in the subtropics, which in turn induced more active high-frequency eddies and maintained the NPO structure. After the mid-1980s, the local diabatic heating response and high-frequency eddy activities associated with the global-scale wave-train mode became weaker whereas the upstream anomalies were significant. Low-frequency eddies provided most of momentum flux through zonal-eddy coupling and primarily contributed to the zonal distribution of the NPO-related circulation anomalies across Eurasia and the Arctic.