Development and Maintenance Mechanism for the Semiannual Oscillation of the North Pacific Upper-Level Circulation

The north–south semiannual oscillation (SAO) of the North Pacific jet stream, part of the atmospheric SAO in the Northern Hemisphere, can be well depicted by the semiannual component of the monthly-mean eddy streamfunction. Expressed by the semiannual eddy streamfunction budget, the dynamic processes develop and maintain the SAO, including the adjustment between vorticity advection and convergence of vorticity flux of the monthly-mean mode and the convergence of transient vorticity flux. An empirical orthogonal function analysis of these dynamic processes shows an east–west elongated cyclonic (anticyclonic) cell of the semiannual eddy streamfunction anomaly, which appears in January and July (October and April) south of the Siberia–Alaska landmass. The maximum (minimum) adjustment processes by the monthly-mean mode and the maximum (minimum) feedback impact of transient activity on the SAO occur in December and June (September and March), a month ahead of the maximum (minimum) north–south SAO of the North Pacific jet stream. Because vorticity is supplied by the convergence of vorticity flux associated with divergent flow, the SAO for the rotational flow is established by diabatic heat and heat transport through the divergent circulation over the North Pacific Ocean, and by precipitation maintained by convergence of water vapor flux along the oceanic storm track. Additionally, the feedback impact of the modulated transient activity affects the SAO development of the atmospheric rotational and divergent circulations, and the hydrological cycle.