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Stratospheric Transport Journal of the Meteorological Society of Japan, Vol. 80, No. 4B, pp. 793--809, 2002 793 Stratospheric Transport R. Alan PLUMB Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA (Manuscript received 3 July 2001, in revised form 15 February 2002) Abstract Improvements in our understanding of transport processes in the stratosphere have progressed hand in hand with advances in understanding of stratospheric dynamics and with accumulating remote and in situ observations of the distributions of, and relationships between, stratospheric tracers. It is conve- nient to regard the stratosphere as being separated into four regions: the summer hemisphere, the tropics, the wintertime midlatitude ‘‘surf zone’’, and the winter polar vortex. Stratospheric transport is dominated by mean diabatic advection (upwelling in the tropics, downwelling in the surf zone and the vortex) and, especially, by rapid isentropic stirring within the surf zone. These characteristics determine the global-scale distributions of tracers, and their mutual relationships. Despite our much-improved understanding of these processes, many chemical transport models still appear to exhibit significant shortcomings in simulating stratospheric transport, as is evidenced by their tendency to underestimate the age of stratospheric air. 1. Introduction have opposite large-scale gradients (HF has a stratospheric source and tropospheric sink, It has long been recognized that strato- CH a tropospheric source and stratospheric spheric trace gases with sufficiently weak 4 sink); nevertheless, the shapes of their iso- chemical sources and sinks have similar global pleths are very similar, with isopleths bulging distributions, in the sense that their isopleths upward in the tropics and poleward/downward (surfaces of constant mixing ratio) have similar slopes in the extratropics. Especially in the spatial shapes. This was evident from some winter or spring hemisphere, there are strong of the earliest global observations of CH and 4 horizontal gradients in the subtropics, rela- N O from the SAMS instrument on the Nimbus 2 tively flat isopleths in middle latitudes, and 6 satellite (Jones and Pyle 1984) and has been strong gradients again near 60 latitude in the confirmed (albeit with some reservations, as we winter hemisphere. The similarity of structure shall discuss below) in subsequent observations between two chemically unrelated tracers is and modeling studies of many such tracers. strong evidence that these characteristics are Examples of these characteristic shapes are determined by transport, and thus by strato- shown in Fig. 1 for monthly mean distributions spheric (and mesospheric) dynamics. around the equinoxes. The two tracers shown Comprehensive perspectives of the global- scale dynamics of the stratospheric circulation Corresponding author: R. Alan Plumb, Depart- have been given elsewhere (e.g., Holton et al. ment of Earth, Atmospheric and Planetary 1995, to which the reader is referred for details Sciences Massachusetts Institute of Technology, and for original references). For present pur- Cambridge, MA 02139, USA. E-mail: [email protected] poses, we will just summarize the main points ( 2002, Meteorological Society of Japan that are relevant to trace gas transport. 794 Journal of the Meteorological Society of Japan Vol. 80, No. 4B Fig. 2. Schematic of the residual mean meridional circulation in the atmo- sphere. The heavy ellipse denotes the thermally-driven Hadley circulation of the troposphere. The shaded regions (labelled ‘‘S’’, ‘‘P’’, and ‘‘G’’) denote re- gions of breaking waves (synoptic- and planetary-scale waves, and gravity waves, respectively), responsible for driving branches of the stratospheric and mesospheric circulation. Fig. 1. Monthly mean meridional distri- butions of (left two panels) HF (parts per billion by volume) and of (right two served distributions of temperature and radia- panels) CH4 (parts per million by vol- ume) during March (top) and Septem- tively active constituents (Rosenlof 1995; ber (bottom), from measurements by Eluszkiewicz et al. 1996), provided one inter- the HALOE instrument (Russell et al. prets ‘‘mean circulation’’ as ‘‘residual mean cir- 1993a) on the Upper Atmosphere culation’’, rather than the straightforward Research Satellite. Calculation of the Eulerian mean (it is the residual mean that is monthly mean distributions for these relevant to tracer transport (e.g., Andrews et al. data is described in Randel et al. (1998). 1987)). A schematic of the residual circulation of the atmosphere (up to the mesopause) is depicted in Fig. 2. In the tropical troposphere, 2. Overview of stratospheric circulation the well-known Hadley circulation can be and transport understood, at least in its simplest form, as a 2.1 The stratospheric circulation nonlinear circulation driven by latitudinal gra- The meridional circulation of the strato- dients in thermal forcing (Held and Hou 1980). sphere, known as the ‘‘Brewer-Dobson’’ circula- Despite the temptation to interpret the appar- tion, after the pioneering deductions of Brewer ently thermally-direct circulation of the strato- (1949) and Dobson (1956) from observations sphere in the same way, it is clear that other of stratospheric water vapor and ozone, respec- processes must be involved, since air following tively, comprises a two-cell structure in the the circulation must lose angular momentum lower stratosphere, with upwelling in the trop- as it moves poleward. The direct, large-scale, ics and subsidence in middle and high lat- effects of friction being utterly negligible in the itudes, and a single cell from the tropics into stratosphere, such loss of angular momentum the winter hemisphere at higher altitudes. can only be ascribed to the impact of waves: the These characteristics have been confirmed from presence of wave drag is thus crucial to the modern radiation calculations based on ob- stratospheric circulation. August 2002 R.A. PLUMB 795 In contrast to the much more quiescent sum- yond the surf zone, deep into the tropics and mer hemisphere, the wintertime stratosphere even across the equator: issues involved in ex- is dominated by large-amplitude, planetary- plaining this behavior are discussed in Plumb scale Rossby waves propagating upward from and Eluszkiewicz (1999). At least a component the troposphere. Intermittently, these waves of the tropical circulation may be a nonlinear, break, stirring air more or less isentropically thermally driven circulation, analogous to the across large distances of the winter strato- tropospheric Hadley cell (Dunkerton 1989; Se- sphere within a region that has become known meniuk and Shepherd 2001a,b). The compo- as the ‘‘surf zone’’ (McIntyre and Palmer 1983), nent driven by wave drag may also extend into bounded by sharp gradients of potential vor- the tropics, either via a similar nonlinear ticity (PV) and of tracers in the winter sub- mechanism or through the effects of other wave tropics and at the edge of the polar vortex. motions within the tropics (Plumb and Elusz- Apart from the direct effects of this stirring on kiewicz 1999; Scott 2002). At the vortex edge, tracers, it has an indirect effect through the theoretical arguments lead us to expect little meridional circulation induced by the stirring penetration of the steady circulation into the of PV. For adiabatic, inviscid flow, and assum- vortex, even under nonadiabatic conditions ing steady state (i.e., solstice conditions), the (Bu¨ hler and Haynes 1999; Sobel and Plumb zonal mean momentum budget can be written 1999), despite the observational evidence there (e.g., Andrews et al. 1987) of strong descent of tracer isopleths (such as in à Fig. 1). ÀvÃPà ¼ v^P^ ; ð1Þ In the lower stratosphere, the effects of syn- where P is Ertel PV, v is northward velocity, optic-scale tropospheric disturbances, present the notation và denotes the mass-weighted throughout the year, are probably responsible zonal mean of v along isentropic surfaces, and for driving the strong two-cell circulation there v^ ¼ v À và the departure from that mean. Eq. and in the upper troposphere, through the (1) simply states the need for zero net flux of same Rossby wave pumping mechanism (the PV in a conservative steady mean state: mean local dynamics of these waves, away from their and eddy fluxes must cancel. Therefore, a non- baroclinic sources, being just Rossby wave zero eddy flux of PV—which will be present dynamics). Near the surface, these same bar- whenever waves are breaking, unless there is oclinic disturbances drive an equatorward re- no background PV gradient—requires a non- turn flow (Held and Schneider 2000; Koh 2001) zero mean circulation in steady state. In fact, in in the form of cold air outbreaks. the usual case of northward mean PV gradient The mesosphere is dominated by a strong à and downgradient eddy flux, v^P^ < 0, and (1) global circulation (with và of several msÀ1) from implies a poleward circulation (since the sign summer pole to winter pole, which manifests of Pà has the same sign as latitude, except per- itself in the dramatic reversal of pole-to-pole haps very close to the equator). Thus, the wave temperature gradient in the upper mesosphere. drag drives the flow poleward, through a mech- This circulation is believed to be driven pri- anism we will here refer to as the ‘‘Rossby wave marily by upward propagating inertia-gravity pump’’1, providing an explanation for the pole- waves. Unlike Rossby waves, inertia-gravity ward
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