Numerical Simulation of a Life-Cycle of Atmospheric Blocking and the Analysis of Potential Vorticity Using a Simple Barotropic Model

Abstract

In this study, we conducted a series of numerical experiments to investigate atmospheric blocking, using a simple barotropic model that featured a wavemaker to excite synoptic disturbances. The model has a resolution equivalent to R20 and consists of only five physical processes: a wavemaker as baroclinic instability, topographic forcing, biharmonic diffusion, zonal surface stress, and Ekman pumping.Results of time integrations show that persistent dipole blockings appear one after another in the model, showing a reasonable life-cycle. In the model atmosphere, the synoptic disturbances are amplified exponentially by the wavemaker. The exponential growth soon saturates with nonlinear scattering of energy from synoptic to planetary waves associated with a Rossby wave breaking. The analysis of potential vorticity (PV) indicates that the onset of blocking is brought on by the Rossby wave breaking. The overturning of high and low PVs tends to occur at the topographic stationary ridge.Once a block is formed by the Rossby wave breaking, subsequent Rossby waves are blocked and undergo meridional stretch. The stretched wave then breaks down, depositing fresh low PV at the north and high PV at the south of the blocking system to maintain the block. The result is consistent with the so-called eddy straining mechanism. The result suggests that the exponential growth of synoptic disturbances is essential both for the onset and the maintenance of blocking.

Local Spectral Energetics Analysis of the Formation of Pacific Blocking Anticlones using an Orthonormal Wavelet Expansion.

“…A significant contribution of the upstream synoptic disturbances, as presented in this paper, is in agreement with the findings in the eddy straining mechanism (e.g., Shutts 1983;Chen and Juang 1992;Kimoto et al 1992;Nakamura et al 1997). The present study, based on waveletbased local spectral energetics, has extended the analysis in the previous studies that support the upscale energy cascade mechanism, equivalent to the eddy straining hypothesis (e.g., Saltzman 1959;Hansen and Sutera 1984;Kung and Baker 1986;Tanaka 1991). Figures 14 and 15 in local spectral energetics can be regarded as an extension of Fig.…”

“…The traditional spectral energetics (e.g., Tanaka and Kung 1988;Hansen and Sutera 1984;Kung and Baker 1986) indicated that the eddy kinetic energy of the atmospheric blocking is primarily maintained by the upscale energy cascade from synoptic eddies through the wave-wave interaction, L n , during the formation. In this study, both the scales and longitudinal locations of the energy source region are clarified by applying the local spectral energetics analysis using the orthonormal wavelet expansion.…”

“…In view of the traditional spectral energetics introduced by Saltzman (1957Saltzman ( , 1970, the blocking may be developed and maintained by the nonlinear transfer of kinetic energy through the wavewave interaction from synoptic waves to planetary waves (e.g., Hansen and Sutera 1984;Kung and Baker 1986). Kung et al (1989) and Tanaka (1991) recognized the energy transfer from the zonal baroclinic component to the barotropic component of planetary waves via synoptic-scale conversions using threedimensional normal mode energetics in the observed and simulated atmospheres, respectively. In addition, Tanaka (1998) suggested that the growth of synoptic disturbances played essential roles both in the onset and the maintenance of blocking.…”

“…Kung et al (1989) and Tanaka (1991) recognized the energy transfer from the zonal baroclinic component to the barotropic component of planetary waves via synoptic-scale conversions using threedimensional normal mode energetics in the observed and simulated atmospheres, respectively. In addition, Tanaka (1998) suggested that the growth of synoptic disturbances played essential roles both in the onset and the maintenance of blocking. The results of Tanaka (1998) are consistent with the so-called eddy straining mechanism proposed by Shutts (1983) and supported by Mullen (1987) and Nakamura and Wallace (1990).…”

“…In addition, Tanaka (1998) suggested that the growth of synoptic disturbances played essential roles both in the onset and the maintenance of blocking. The results of Tanaka (1998) are consistent with the so-called eddy straining mechanism proposed by Shutts (1983) and supported by Mullen (1987) and Nakamura and Wallace (1990).…”