James R. Holton

Stratosphere-troposphere exchange

In the past, studies of stratosphere-troposphere exchange of mass and chemical species have mainly emphasized the synoptic- and small-scale mechanisms of exchange. This review, however, includes also the global-scale aspects of exchange, such as the transport across an isentropic surface (potential temperature about 380 K) that in the tropics lies just above the tropopause, near the 100-hPa pressure level. Such a surface divides the stratosphere into an “overworld” and an extratropical “lowermost stratosphere” that for transport purposes need to be sharply distinguished. This approach places stratosphere-troposphere exchange in the framework of the general circulation and helps to clarify the roles of the different mechanisms involved and the interplay between large and small scales. The role of waves and eddies in the extratropical overworld is emphasized. There, wave-induced forces drive a kind of global-scale extratropical “fluid-dynamical suction pump,” which withdraws air upward and poleward from the tropical lower stratosphere and pushes it poleward and downward into the extratropical troposphere. The resulting global-scale circulation drives the stratosphere away from radiative equilibrium conditions. Wave-induced forces may be considered to exert a nonlocal control, mainly downward in the extratropics but reaching laterally into the tropics, over the transport of mass across lower stratospheric isentropic surfaces. This mass transport is for many purposes a useful measure of global-scale stratosphere-troposphere exchange, especially on seasonal or longer timescales. Because the strongest wave-induced forces occur in the northern hemisphere winter season, the exchange rate is also a maximum at that season. The global exchange rate is not determined by details of near-tropopause phenomena such as penetrative cumulus convection or small-scale mixing associated with upper level fronts and cyclones. These smaller-scale processes must be considered, however, in order to understand the finer details of exchange. Moist convection appears to play an important role in the tropics in accounting for the extreme dehydration of air entering the stratosphere. Stratospheric air finds its way back into the troposphere through a vast variety of irreversible eddy exchange phenomena, including tropopause folding and the formation of so-called tropical upper tropospheric troughs and consequent irreversible exchange. General circulation models are able to simulate the mean global-scale mass exchange and its seasonal cycle but are not able to properly resolve the tropical dehydration process. Two-dimensional (height-latitude) models commonly used for assessment of human impact on the ozone layer include representation of stratosphere-troposphere exchange that is adequate to allow reasonable simulation of photochemical processes occurring in the overworld. However, for assessing changes in the lowermost stratosphere, the strong longitudinal asymmetries in stratosphere-troposphere exchange render current two-dimensional models inadequate. Either current transport parameterizations must be improved, or else, more likely, such changes can be adequately assessed only by three-dimensional models.

The quasi‐biennial oscillation

The quasi-biennial oscillation (QBO) dominates the variability of the equatorial stratosphere (∼16–50 km) and is easily seen as downward propagating easterly and westerly wind regimes, with a variable period averaging approximately 28 months. From a fluid dynamical perspective, the QBO is a fascinating example of a coherent, oscillating mean flow that is driven by propagating waves with periods unrelated to that of the resulting oscillation. Although the QBO is a tropical phenomenon, it affects the stratospheric flow from pole to pole by modulating the effects of extratropical waves. Indeed, study of the QBO is inseparable from the study of atmospheric wave motions that drive it and are modulated by it. The QBO affects variability in the mesosphere near 85 km by selectively filtering waves that propagate upward through the equatorial stratosphere, and may also affect the strength of Atlantic hurricanes. The effects of the QBO are not confined to atmospheric dynamics. Chemical constituents, such as ozone, water vapor, and methane, are affected by circulation changes induced by the QBO. There are also substantial QBO signals in many of the shorter-lived chemical constituents. Through modulation of extratropical wave propagation, the QBO has an effect on the breakdown of the wintertime stratospheric polar vortices and the severity of high-latitude ozone depletion. The polar vortex in the stratosphere affects surface weather patterns, providing a mechanism for the QBO to have an effect at the Earths surface. As more data sources (e.g., wind and temperature measurements from both ground-based systems and satellites) become available, the effects of the QBO can be more precisely assessed. This review covers the current state of knowledge of the tropical QBO, its extratropical dynamical effects, chemical constituent transport, and effects of the QBO in the troposphere (∼0–16 km) and mesosphere (∼50–100 km). It is intended to provide a broad overview of the QBO and its effects to researchers outside the field, as well as a source of information and references for specialists. The history of research on the QBO is discussed only briefly, and the reader is referred to several historical review papers. The basic theory of the QBO is summarized, and tutorial references are provided.

International Geophysics Series

Publisher Summary This chapter lists the names of the editors of the book Geophysical Field Theory and Method, Part B, Electromagnetic Fields I . These editors include Beno Gutenberg, Joseph W. Chamberlain, S. K. Runcorn, C. E. Junge, Robert G. Fleagle, Joost A. Businger. L. Dufour, R. Defay, H. U. Roll, Richard A. Craig, Willis L. Webb, Michele Caputo, S. Matsushita, Wallace H. Campbell, K. V. A. Kondratyev, E. Palmen, C. W. Newton, and many more people.

The Influence of the Equatorial Quasi-Biennial Oscillation on the Global Circulation at 50 mb

Abstract Monthly mean Northern Hemisphere 50 mb geopotential heights for a 16-year period (1962-77) are composited with respect to the phase of the equatorial quasi-biennial oscillation (QBO). The observed zonal mean geopotential height at high latitudes is significantly lower during the westerly phase of the equatorial QBO than during the easterly phase in all months composited. For this 16-year sample we find that in early winter (November-December) the amplitude of planetary wavenumber 1 is nearly 40% stronger during the easterly phase of the equatorial QBO. In late winter (January-March) the amplitude of planetary wavenumber 2, on the other hand, is nearly 60% stronger during the westerly phase of the equatorial QBO. Data from an additional 6-year sample show a similar wavenumber 1 signal during the November-December period. However, an additional 4-year sample does not support our conclusions concerning wavenumber 2 during the January-March period. Composites based on zonal wind data from a longitudi...

The influence of gravity wave breaking on the general circulation of the middle atmosphere

Abstract The zonal mean solstice circulation of the global middle atmosphere is simulated using a semi-spectral numerical model. Radiative heating and cooling is computed by the algorithm of Wehrbein and Leovy. Mechanical dissipation is represented by the gravity wave breaking parameterization of Lindzen. An inertial adjustment parameterization is used to prevent the development of inertially unstable meridional shears near the equator. It is shown that gravity wave drag and diffusion in the mesosphere can account for the observed large departure from radiative equilibrium in both summer and winter. Experiments incorporating a forced stationary wavenumber 1 disturbance indicate that planetary wave EP flux convergences although they may modify the mean flow profile significantly, cannot provide the major source of mechanical dissipation in the winter mesosphere. A simulated sudden warming is accompanied by an equally strong mesospheric cooling. This cooling is caused primarily by the relaxation of the pola...

The Role of Gravity Wave Induced Drag and Diffusion in the Momentum Budget of the Mesosphere

Abstract A slight modification of the parameterization suggested by Lindzen (1981) for the zonal drag and eddy diffusion effects generated by breaking internal gravity waves in the mesosphere is tested using a severely truncated midlatitude β-plane channel model. It is found that realistic mean zonal flow profiles with zonal wind reversals above the mesopause can be simulated for both winter and summer radiative heating conditions provided that a gravity-wave spectrum is assumed which includes both stationary waves and waves of relatively large phase speeds (∼ ±20 m s−1). These results contrast greatly with the unrealistic mean wind profiles produced when Rayleigh friction is used to parameterize the effects of small scale motions on the mean flow.

A Theory of the Quasi-Biennial Oscillation

Abstract A theory is presented which indicates that the quasi-biennial oscillation of the zonal wind in the tropical stratosphere is a result of the interaction of long-period, vertically propagating gravity waves with the zonal wind. We discuss the theoretical basis and observational evidence for the existence of long-period gravity waves near the equator, and the mechanism of their interaction with the zonal wind, and present some simple numerical results which show how the absorption of the momentum of these waves by the mean flow leads to a downward propagating zonal wind profile. It is shown that the interaction of these gravity waves with the observed semiannual zonal wind oscillation above 40 km will produce a downward propagating quasi-biennial oscillation. We present the results of several numerical experiments with a model of the tropical stratosphere which includes the gravity wave interaction mechanism. The quasi-biennial oscillation is simulated quite successfully. Finally, we discuss possibl...

Horizontal transport and the dehydration of the stratosphere

The coldest tropopause temperatures occur over the equatorial West Pacific during Northern Hemisphere winter. Horizontal transport through this “cold trap” region causes air parcels that reach the tropopause at other longitudes to be dehydrated to the very low saturation mixing ratios characteristic of the cold trap, and hence can explain why observed tropical stratospheric water vapor mixing ratios are often lower than the saturation mixing ratio at the mean tropopause temperature. Horizontal transport of water vapor can also explain how a persistent layer of subvisible cirrus can exist near the tropopause in the cold trap even though observations suggest that there is diabatic cooling and subsidence, rather than diabatic heating and rising through the tropopause in this region. Thus, horizontal transport in the tropical transition layer in the vicinity of the tropopause plays an important role in the water balance of the stratosphere.

Seasonal Variation of Mass Transport Across the Tropopause

The annual cycle of the net mass transport across the extratropical tropopause is examined. Contributions from both the global-scale meridional circulation and the mass variation of the lowermost stratosphere are included. For the northern hemisphere the mass of the lowermost stratosphere has a distinct annual cycle, whereas for the southern hemisphere, the corresponding variation is weak. The net mass transport across the tropopause in the northern hemisphere has a maximum in late spring and a distinct minimum in autumn. This variation and its magnitude compare well with older estimates based on representative 90Sr mixing ratios. For the southern hemisphere the seasonal cycle of the net mass transport is weaker and follows roughly the annual variation of the net mass flux across a nearby isentropic surface.

Numerical simulations of convectively generated stratospheric gravity waves

Abstract The excitation and vertical propagation of gravity waves is simulated in a two-dimensional model of a mesoscale convective storm. It is shown that in a simulated squall line the gravity waves that are preferentially excited are those propagating opposite to the direction of motion of the storm. Solutions for cases with differing stratospheric mean zonal flow profiles are compared. It turns out that, in the absence of storm-relative mean winds in the stratosphere, the primary mode of excitation of gravity waves is by mechanical forcing owing to oscillatory updrafts. The stratospheric response consists of waves whose periods match the primary periods of the forcing. Owing to the tendency of the oscillating updrafts to propagate toward the rear of the storm, gravity wave propagation is limited primarily to the rearward direction, and there is a net downward momentum transport. When storm-relative mean winds are included in the model the waves excited by the oscillating updrafts are weaker, but a new...