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gfdl's home page > gfdl on-line bibliography > 2003: Journal of the Atmospheric Sciences, 60(8), 993-1023
Bifurcation in eddy life cycles: Implications for storm track variability
| Orlanski, I., 2003: Bifurcation in eddy life cycles: Implications for storm track variability. Journal of the Atmospheric Sciences, 60(8), 993-1023. |
| Abstract: By analyzing a number of very high resolution, nonhydrostatic experiments of baroclinic lifecycles, it was concluded that the intensity of the near-surface baroclinic development influences the upper-level wave to such an extent that it could produce cyclonic or anticyclonic wave breaking. Since the final jet position is equatorward or poleward, the position depends on whether the waves break cyclonically or anticyclonically, respectively. The low-level baroclinicity plays a very important role in the outcome of the wave and feedback to the mean circulation. Using a shallow water model the hypothesis that the intensity of the eddy forcing from the lower layers of the atmosphere can have a profound effect on the disturbances of the upper layers is tested. From these experiments the following is concluded. |
For weak intensities, the strong effective beta
asymmetries due to the earth's sphericity produce anticyclonic wave
breaking and a poleward shift of the zonal jet will occur. For moderate
forcing, anticyclonic wave breaking occurs and consequently, as before,
a poleward shift of the zonal jet will occur. However, there is an
important distinction between weak and moderate forcing. In the latter
case, the eddy anticyclonic centers are very intense. The influence of
the two anticyclones produces a difluence field that will strain the
cyclonic vortex along the SW-NE direction. Consequently, the
meridional vorticity flux
 '
 '
is positive in the north and negative in the south. This process has two
effects: thinning the cyclone and producing positive vorticity fluxes on
the north, negative fluxes on the south and moving the jet poleward. By
increasing the forcing, the cyclone centers become considerably more
intense than the anticyclones (CVC) and they are able to deform and thin
the anticyclones, thus moving the jet equatorward. This transition is
very abrupt; above a threshold amplitude, the life cycle bifurcates to a
cyclonic wave breaking. |
| The implications for storm track variability are quite direct. In normal years, at the entrance of the storm track, intense baroclinicity produces CVCs with a slight shift of the jet equatorward. At the last half of the storm track, due to much weaker baroclinicity, anticyclonic wave breaking occurs (AVCs) displacing the jet poleward. The eddies at the entrance of the storm track develop from the baroclinicity of the subtropical jet. Downstream fluxing and weaker surface baroclinicity make the upper-level waves more aloft and barotropic by the middle of the storm track. These waves normally break anticyclonically, enhancing the subpolar eddy-driven jet. In the warm phase of ENSO, more baroclinicity (and subtropical moisture flux) is present in the eastern Pacific Ocean. This enhanced baroclinicity could support more CVCs in the eastern basin, maintaining the subtropical jet further east. |