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A Theory for the Lower-Tropospheric Structure of the Moist Isentropic Circulation

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  • 1 Department of Physics, University of Toronto, Toronto, Ontario, Canada
  • 2 Department of Earth and Environmental Sciences, and Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York
  • 3 Courant Institute of Mathematical Sciences, New York University, New York, New York
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Abstract

A theoretical model describing the structure of the dry and moist isentropic circulations in the lower troposphere is derived. It decomposes the meridional flow in the troposphere into three contributions: a dry equatorward flow, a cold moist equatorward flow, and a warm moist poleward flow in the mixed layer. The model is based on observations of the meridional mass fluxes joint distribution in potential temperature and equivalent potential temperature. It updates an existing model of the dry circulation by emphasizing the role of moisture in the mixed layer. The model is used to derive an expression for the ratio of moist to dry circulation strengths and this expression is used to assess the influence of surface thermodynamics on the circulations. It predicts that the moist circulation should be between 1.5 and 2 times as strong as the dry circulation and that this relative strength should not increase indefinitely with increasing surface temperature variability. The model also yields an expression for the ratio of total meridional heat fluxes to meridional sensible heat fluxes. This expression indicates that while an increase in the total heat fluxes occurs when surface temperature variability increases (via an increase in latent heat flux), it cannot increase indefinitely. The results suggest that changes in surface thermodynamic conditions must be constrained to constrain changes in the meridional overturning circulation associated with a warming climate.

Corresponding author address: Frédéric Laliberté, Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7, Canada. E-mail: frederic.laliberte@utoronto.ca

Abstract

A theoretical model describing the structure of the dry and moist isentropic circulations in the lower troposphere is derived. It decomposes the meridional flow in the troposphere into three contributions: a dry equatorward flow, a cold moist equatorward flow, and a warm moist poleward flow in the mixed layer. The model is based on observations of the meridional mass fluxes joint distribution in potential temperature and equivalent potential temperature. It updates an existing model of the dry circulation by emphasizing the role of moisture in the mixed layer. The model is used to derive an expression for the ratio of moist to dry circulation strengths and this expression is used to assess the influence of surface thermodynamics on the circulations. It predicts that the moist circulation should be between 1.5 and 2 times as strong as the dry circulation and that this relative strength should not increase indefinitely with increasing surface temperature variability. The model also yields an expression for the ratio of total meridional heat fluxes to meridional sensible heat fluxes. This expression indicates that while an increase in the total heat fluxes occurs when surface temperature variability increases (via an increase in latent heat flux), it cannot increase indefinitely. The results suggest that changes in surface thermodynamic conditions must be constrained to constrain changes in the meridional overturning circulation associated with a warming climate.

Corresponding author address: Frédéric Laliberté, Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7, Canada. E-mail: frederic.laliberte@utoronto.ca
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