Dynamics of Two Distinct Subseasonal Growth Mechanisms of the North Pacific Oscillation

This study investigates the growth mechanisms of the North Pacific Oscillation (NPO), one of the primary causes of winter temperature extremes at midlatitudes. We find that the NPO has two distinct origins. One is the Rossby wave propagating across Eurasia that strengthens during cold or warm surges over East Asia. The corresponding vorticity and thermal anomalies grow into the NPO as they move eastward. The other is a local disturbance at the Asian Pacific jet stream exit that amplifies when propagating westward. The dynamical processes behind these two types of NPO growth are investigated by conducting quasigeostrophic geopotential tendency budget analysis, focusing on the relative importance of the vorticity flux and differential heat flux. It is revealed that the contribution of high-frequency eddies is greater in the NPO that grows from a local disturbance. However, NPO growth in both cases is primarily steered by low-frequency vorticity flux, which facilitates eastward or westward propagation through relative or planetary vorticity advections, respectively. Differential heat flux promotes the growth of the NPO before the onset, but dissipates the NPO anomalies afterward. The net effect of the heat flux, however, allows the NPO to amplify and persist through baroclinic instability by constraining the NPO anomalies to have westward vertical tilt. Accordingly, the NPO grows conforming to the dissipative destabilizing mechanism. From the perspective of potential vorticity (PV), the eastward growth of the NPO originates from the downstream advection of PV anomalies accumulated over East Asia, while westward growth is favored by a strong PV gradient near the jet stream. SIGNIFICANCE STATEMENT: This study examines the origins and growth mechanism of the North Pacific Oscillation (NPO), which causes cold or warm spells in the midlatitudes during winter. Two distinct origins of the NPO are found, which are abnormal temperature events over East Asia and local atmospheric disturbances at the Asian Pacific jet stream exit. Evolutions of the NPO are traced by analyzing the roles of the vorticity and heat fluxes in addition to the contributions of synoptic eddies, which turns out to be more significant for the NPO that grows from a local disturbance. Vorticity flux overall dominates the growth of the NPO, but heat flux also helps the NPO amplify owing to background instability.