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

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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 c_R when a blocking occurs. The energy level at c_R exceeds the saturation spectrum E, denoted by E=ac^2, which is derived theoretically from the criterion of Rossby wave breaking ∂q/∂y<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 c_R. The wave cannot break down easily at the wave regime near c_R. The result suggests that an atmospheric blocking occurs when the energy level at c_R exceeds the Rossby wave saturation spectrum of E=ac^2. The blocking structure is characterized by ∂q/∂y<0, which represents the saturation criterion of Rossby waves to derive the spectrum of E=ac^2.
社団法人日本気象学会の論文
2006-04-25