Climate dynamics

Abstract:

We present a synthesis of theoretical results concerning the probability distribution of the concentration of a passive tracer subject to both diffusion and to advection by a spatially smooth time-dependent flow. The freely decaying case is contrasted with the equilibrium case. A computationally efficient model of advection-diffusion on a lattice is introduced, and used to test and probe the limits of the theoretical ideas. It is shown that the probability distribution for the freely decaying case has fat tails, which have slower than exponential decay. The additively forced case has a Gaussian core and exponential tails, in full conformance with prior theoretical expectations. An analysis of the magnitude and implications of temporal fluctuations of the conditional diffusion and dissipation is presented, showing the importance of these fluctuations in governing the shape of the tails. Some results concerning the probability distribution of dissipation, and concerning the spatial scaling properties of concentration fluctuation, are also presented. Though the lattice model is applied only to smooth flow in the present work, it is readily applicable to problems involving rough flow, and to chemically reacting tracers. (c) 2000 American Institute of Physics.

Abstract:

The dominant tracer transport processes in the equatorial and subtropical latitudes of the middle and upper stratosphere are investigated. Distributions of water vapor in Northern Hemisphere winter from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite are employed, together with a three-dimensional Stratosphere Mesosphere Model that incorporates a representation of the quasi-biennial oscillation (QBO). The model reproduces the observed tracer distributions, in particular a "staircase" feature that is present in northern winter of 1992 (easterly QBO phase) but not in 1993 (westerly QBO phase). This feature is highly asymmetric about the equator. The model circulation is diagnosed to show that while the induced QBO circulation in the lower stratosphere of the model is relatively symmetric about the equator, in the middle and upper stratosphere it is highly asymmetric and in the correct sense to give rise to the staircase feature. Model experiments are compared in which trajectories are advected by (1) the full three-dimensional circulation and (2) the residual mean circulation only, thereby removing the local effects of isentropic mixing by planetary waves on the trajectory distributions. These confirm the importance of advection by the QBO circulation at equatorial and subtropical latitudes. However, sharpening of the tracer gradients at the subtropical edge of the surf zone by the action of planetary waves is shown to be important in the formation of a subtropical "cliff" between 10 and 20 mbar at 20°-30°N. The model results also suggest that the prominence of the summer subtropical peak in easterly phase years compared with westerly phase years is not entirely due to increased summer upwelling of the large-scale global circulation caused by the stronger planetary wave driving. The depression of the winter half of the equatorial peak by the local asymmetric QBO circulation is also shown to be important. Copyright 2000 by the American Geophysical Union.