Blocking Formation by an Accumulation of Barotropic Energy Exceeding the Rossby Wave Saturation Level at the Spherical Rhines Scale

[摘要] In this study, energy spectrum of blocking in the Northern Hemisphere is examined in the framework of the 3D normal mode decomposition using the NCEP/NCAR reanalysis for 51 years. Attention is concentrated to the barotropic component of the atmosphere, where the low-frequency variabilities dominate. The horizontal scale of disturbances is measured by the phase speed of a Rossby mode c instead of the horizontal wavenumber.According to the result of the energetics analysis in the phase speed domain, we find an accumulation of energy at the spherical Rhines speed cR when a blocking occurs. The energy level at cR exceeds the saturation spectrum E, denoted by E = ac2, which is derived theoretically from the criterion of Rossby wave breaking δq/δy <: 0, where a represents a constant proportional to mass of the atmosphere for the unit area, and q is the barotropic potential vorticity. The amplified Rossby wave persists longer at the same geographical location because the Rossby wave is stationary for the scale of cR. The wave cannot break down easily at the wave regime near cR. The result suggests that an atmospheric blocking occurs when the energy level at cR exceeds the Rossby wave saturation spectrum of E = ac2. The blocking structure is characterized by δq/δy < 0, which represents the saturation criterion of Rossby waves to derive the spectrum of E = ac2.

[学科分类] 大气科学