The mean meridional mass transport within isentropic layers in the troposphere is characterized by poleward flow in the upper troposphere and equatorward flow near the ground. The equatorward flow at a given latitude is confined to isentropic layers that, at the given latitude, typically intersect the ground, whereas the poleward flow occurs in the interior isentropic layers that, at the given latitude, rarely or never intersect the ground. This distinction between isentropic layers that do not intersect the ground and those that often intersect the ground suggests an analogous division of a general circulation theory into two intertwined parts: one part accounting for the near-surface dynamics and the other part accounting for the dynamics of the interior troposphere. The present paper focuses on the interaction between the mass transport in interior isentropic layers of the extratropics and the mean thermal structure of the troposphere.
In the extratropics, the mean meridional mass transport within interior isentropic layers is, to a good approximation, proportional to the mean meridional eddy flux of isentropic potential vorticity (PV). To examine the dependence of the PV fluxes and the mean thermal structure on external flow parameters, we conduct a series of experiments with an idealized general circulation model (GCM). In the GCM used, the lower boundary has no topography, thus enforcing a zonally symmetric model climate. Effects of processes such as radiation, moist convection, and subgrid-scale turbulence are represented in an idealized fashion as Newtonian relaxation to a prescribed radiative equilibrium state and as quadratic frictional drag. In this GCM, we examine the dependence of the flow on parameters such as the meridional temperature gradient of the radiative equilibrium state, the planet's rotation rate, and the optical depth of the atmosphere.
We find that, over a wide range of flow parameters, the mean thermal structure of the troposphere observes a strong constraint: the lowest isentrope that crosses the tropopause grazes the ground in the tropics. This means that an isentrope either intersects the ground or the tropopause, but there are no mean isentropes that intersect both the ground and the tropopause. Strong eddy fluxes of PV are confined to isentropic layers that, at least occasionally, intersect the ground. Thus, the eddy-induced mass transport at a given extratropical latitude occurs predominantly in isentropic layers that intersect the ground, not at the given latitude but further equatorward.
In isentropic layers that never intersect the ground, there are no sources or sinks of PV. By way of contrast, whenever an isentrope intersects the ground, the near-surface mass flux acts as a source of PV for this isentropic layer. We present an argument that, based on the PV budget of isentropic layers and on a diffusive closure for the eddy PV flux, accounts for the observed constraint on the mean thermal structure of the troposphere.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics