TY - JOUR
T1 - The counter-propagating Rossby-wave perspective on baroclinic instability. Part IV
T2 - Nonlinear life cycles
AU - Methven, J.
AU - Hoskins, B. J.
AU - Heifetz, E.
AU - Bishop, C. H.
PY - 2005/4
Y1 - 2005/4
N2 - Pairs of counter-propagating Rossby waves (CRWs) can be used to describe baroclinic instability in linearized primitive-equation dynamics, employing simple propagation and interaction mechanisms at only two locations in the meridional plane - the CRW 'home-bases'. Here, it is shown how some CRW properties are remarkably robust as a growing baroclinic wave develops nonlinearly. For example, the phase difference between upper-level and lower-level waves in potential-vorticity contours, defined initially at the home-bases of the CRWs, remains almost constant throughout baroclinic wave life cycles, despite the occurrence of frontogenesis and Rossby-wave breaking. As the lower wave saturates nonlinearly the whole baroclinic wave changes phase speed from that of the normal mode to that of the self-induced phase speed of the upper CRW. On zonal jets without surface meridional shear, this must always act to slow the baroclinic wave. The direction of wave breaking when a basic state has surface meridional shear can be anticipated because the displacement structures of CRWs tend to be coherent along surfaces of constant basic-state angular velocity, Ū. This results in up-gradient horizontal momentum fluxes for baroclinically growing disturbances. The momentum flux acts to shift the Jet meridionally in the direction of the increasing surface Ū, so that the upper CRW breaks in the same direction as occurred at low levels.
AB - Pairs of counter-propagating Rossby waves (CRWs) can be used to describe baroclinic instability in linearized primitive-equation dynamics, employing simple propagation and interaction mechanisms at only two locations in the meridional plane - the CRW 'home-bases'. Here, it is shown how some CRW properties are remarkably robust as a growing baroclinic wave develops nonlinearly. For example, the phase difference between upper-level and lower-level waves in potential-vorticity contours, defined initially at the home-bases of the CRWs, remains almost constant throughout baroclinic wave life cycles, despite the occurrence of frontogenesis and Rossby-wave breaking. As the lower wave saturates nonlinearly the whole baroclinic wave changes phase speed from that of the normal mode to that of the self-induced phase speed of the upper CRW. On zonal jets without surface meridional shear, this must always act to slow the baroclinic wave. The direction of wave breaking when a basic state has surface meridional shear can be anticipated because the displacement structures of CRWs tend to be coherent along surfaces of constant basic-state angular velocity, Ū. This results in up-gradient horizontal momentum fluxes for baroclinically growing disturbances. The momentum flux acts to shift the Jet meridionally in the direction of the increasing surface Ū, so that the upper CRW breaks in the same direction as occurred at low levels.
KW - Home-bases
KW - Self-induced phase speed
KW - Wave breaking
KW - Zonal propagation
UR - http://www.scopus.com/inward/record.url?scp=27644539289&partnerID=8YFLogxK
U2 - 10.1256/qj.04.23
DO - 10.1256/qj.04.23
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AN - SCOPUS:27644539289
SN - 0035-9009
VL - 131
SP - 1425
EP - 1440
JO - Quarterly Journal of the Royal Meteorological Society
JF - Quarterly Journal of the Royal Meteorological Society
IS - 608
ER -