Budgets of Finite-Amplitude Local Wave Activity of the Wintertime Northern Hemisphere

Baroclinic wave packets grow and decay in the storm track regions. Metrics with known budget components, such as eddy kinetic energy (EKE) and wave activity, are useful for delineating the driver of Rossby waves into contributions from different physical processes. The finite-amplitude local wave activity (FALWA) based on quasi-geostrophic dynamics proposed by Huang and Nakamura (2016) quantifies both the amplitude and meridional extent of Rossby waves. More importantly, its budget is given by the divergence of generalized Eliassen-Palm flux in the absence of non-conservative forces. With the validity of such formulation beyond small-amplitude wave assumption, a direct comparison between longitudinally localized wave activity tendency and the fluxes is made possible even when waves deform or potential vorticity gradient reverses.

The seasonal averages of FALWA budget terms in boreal winter along the Pacific and Atlantic storm track regions computed with ERA-Interim reanalysis data hint on the major balance between different processes that maintains the wave activities. Even though the waves are found to be fueled by low-level meridional heat flux over both oceans, the counteracting process over the Pacific is a net damping, while that over the Atlantic is via horizontal flux divergence. FALWA can be approximately decomposed into stationary and transient components in a season. The climatology of transient component of FALWA in winter has spatial patterns consistent with that given by small-amplitude local wave activity based on zonally asymmetric basic state proposed by Plumb (1986). A decomposition of wave activity budget in synoptic time-scale over the Pacific and Atlantic regions via cospectral analysis shows that the horizontal FALWA flux convergence and net diabatic heating contribute to FALWA budget comparably. These analyses would promote understanding of the maintenance and variabilities of the storm tracks.