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Figure 5.13 Average age of air (years) from a model. (Adapted from Waugh et at. 1189].)

5.2.2 Effect of transport on atmospheric constituents

In this section we discuss the effects of the general circulation of the stratosphere on O. and long-lived constituents.

Odd oxygen We can derive time constants with respect to transport for O. using a "replacement time" formalism: x = /„ /(transport rate in VMR per second), where/ttt is the VMR of O,. We further subdivide the total transport lifetime into lifetimes with respect to vertical and horizontal transport (190]:

where v and w are the horizontal and vertical wind velocities. Eddy transport is often parameterized as a diffusion process (see WMO[l5], Chapter 12, for more discussion of this). In Equations (5.2) and (5.3), K1V and K.. represent the diffusion coefficients for horizontal and vertical diffusion, respectively. Note that, in the stratosphere, K:: is generally negligible. Both Equations (5.2) and (5.3) are made up of two terms: the first one (e.g. v df0 Jdy) is the rate of transport due to advection by the Brewer-Dobson circulation, the second one (e.g. 3(K..v dfD /3y)/r)v) is the rate of

transport due to eddy transport. It is important to recognize that the transport time scales defined in Equations (5.2) and (5.3) contain the spatial derivatives of the tracer abundance. As a result, the transport time-scale is a function of the spatial distribution of the tracer, and two tracers that have different spatial gradients will therefore have different transport time-scales.

Figure 5.14 shows profiles of tvci1 and thon/ for Ov at the equator, 30°N, and 60°N (at the equator, Tfcorl, > 1000 days). Also shown are estimates of the lifetime of O, with respect to production (t(m,„ = [Ol]//,0>) and loss (Tk,ss = MLn). Solid lines represent processes that increase O, while dashed lines represent processes that decrease O,. Production always increases Ot, so it is a solid line in all three panels. Loss always decreases O,, so it is a dashed line in all three panels. Transport can either increase or decrease O,, depending on the wind field and the spatial gradients of O,, and therefore appears in Figure 5.14 as both solid and dashed lines.

Lifetime (days)

Lifetime (days)

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