A Model Study of Zonal Forcing in the Equatorial Stratosphere by Convectively Induced Gravity Waves

A two-dimensional cloud-resolving model is used to examine the possible role of gravity waves generated by a simulated tropical squall line in forcing the quasi-biennial oscillation (QBO) of the zonal winds in the equatorial stratosphere. A simulation with constant background stratospheric winds is compared to simulations with background winds characteristic of the westerly and easterly QBO phases, respectively. In all three cases a broad spectrum of both eastward and westward propagating gravity waves is excited. In the constant background wind case the vertical momentum flux is nearly constant with height in the stratosphere, after correction for waves leaving the model domain. In the easterly and westerly shear cases, however, westward and eastward propagating waves, respectively, are strongly damped as they approach their critical levels, owing to the strongly scale-dependent vertical diffusion in the model. The profiles of zonal forcing induced by this wave damping are similar to profiles given by critical level absorption, but displaced slightly downward. The magnitude of the zonal forcing is of order 5 m s21 day21. It is estimated that if 2% of the area of the Tropics were occupied by storms of similar magnitude, mesoscale gravity waves could provide nearly 1/4 of the zonal forcing required for the QBO.

[1]  D. Durran.  Do Breaking Mountain Waves Deceierate the Local Mean Flow , 1995 .

[2]  T. Tsuda,et al.  Radiosonde observations of equatorial atmosphere dynamics over Indonesia: 2. Characteristics of gravity waves , 1994 .

[3]  Chidong Zhang.  On the Annual Cycle in Highest, Coldest Clouds in the Tropics , 1993 .

[4]  R. Lindzen.  Equatorial Planetary Waves in Shear: Part II , 1971 .

[5]  J. Holton,et al.  A Theory of the Quasi-Biennial Oscillation , 1968 .

[6]  Kaoru Sato,et al.  Short-period disturbances in the equatorial lower stratosphere , 1994 .

[7]  M. Joan Alexander,et al.  Gravity wave momentum flux in the lower stratosphere over convection , 1995 .

[8]  R. A. Plumb,et al.  A model of the quasi-biennial oscillation on an equatorial beta-plane , 1982 .

[9]  T. Dunkerton.  The role of gravity waves in the quasi‐biennial oscillation , 1997 .

[10]  Murry L. Salby,et al.  Equatorial wave activity derived from fluctuations in observed convection , 1994 .

[11]  R. Fovell,et al.  Numerical simulations of convectively generated stratospheric gravity waves , 1992 .

[12]  S. W. Bowen,et al.  Gravity waves generated by a tropical cyclone during the STEP tropical field program: A case study , 1993 .

[13]  K. Rosenlof,et al.  Nonstationary gravity wave forcing of the stratospheric zonal mean wind , 1996 .

[14]  Hans A. Panofsky,et al.  Introduction to dynamic meteorology , 1956 .

[15]  J. Holton,et al.  Stratosphere‐troposphere exchange , 1995 .

[16]  R. Vincent,et al.  Gravity wave activity in the lower atmosphere: Seasonal and latitudinal variations , 1995 .

[17]  J. Holton.  Waves in the Equatorial Stratosphere Generated by Tropospheric Heat Sources , 1972 .

[18]  Mitchell W. Moncrieff,et al.  A Numerical Investigation of the Organization and Interaction of the Convective and Stratiform Regions of Tropical Squall Lines , 1988 .

[19]  R. Lindzen,et al.  Wave Structure of the Tropical Stratosphere over the Marshall Islands Area during 1 April–1 July 1958 , 1975 .

[20]  D. Durran,et al.  A Compressible Model for the Simulation of Moist Mountain Waves , 1983 .

[21]  Brian E. Mapes,et al.  Multiscale variability of deep convection in relation to large-scale circulation in TOGA COARE , 1996 .

[22]  D. Karoly,et al.  Gravity wave activity associated with tropical convection detected in TOGA COARE sounding data , 1996 .

[23]  Jean-Luc Redelsperger,et al.  A three-dimensional simulation of a tropical squall line: convective organization and thermodynamic vertical transport , 1988 .

[24]  J. Holton,et al.  The Gravity Wave Response above Deep Convection in a Squall Line Simulation. , 1995 .

[25]  J. Pyle,et al.  A two-dimensional model of the quasi-biennial oscillation of ozone , 1989 .

[26]  T. Dunkerton,et al.  Climatology of the Equatorial Lower Stratosphere , 1985 .

[27]  J. Holton,et al.  An Updated Theory for the Quasi-Biennial Cycle of the Tropical Stratosphere , 1972 .

[28]  R. A. Plumb.  The Interaction of Two Internal Waves with the Mean Flow: Implications for the Theory of the Quasi-Biennial Oscillation , 1977 .

[29]  B. A. Boville,et al.  A three-dimensional simulation of the equatorial quasi-biennial oscillation , 1992 .

[30]  T. Dunkerton.  Nonlinear Propagation of Zonal Winds in an Atmosphere with Newtonian Cooling and Equatorial Wavedriving , 1991 .

[31]  R. Vincent,et al.  High‐resolution radiosonde data offer new prospects for research , 1995 .

[32]  John M. Wallace,et al.  Observational Evidence of Kelvin Waves in the Tropical Stratosphere , 1968 .