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Brewer-Dobson Circulation Bibliography

Posted on: August 8, 2018

bandicam 2018-08-08 09-47-53-597

  1. 1949: Brewer, A. W. “Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere.” Quarterly Journal of the Royal Meteorological Society 75.326 (1949): 351-363. Information is now available regarding the vertical distribution of water vapour and helium in the lower stratosphere over southern England. The helium content of the air is found to be remarkably constant up to 20 km but the water content is found to fall very rapidly just above the tropopause, and in the lowest 1 km of the stratosphere the humidity mixing ratio falls through a ratio of 10—1.The helium distribution is not compatible with the view of a quiescent stratosphere free from turbulence or vertical motions. The water‐vapour distribution is incompatible with a turbulent stratosphere unless some dynamic process maintains the dryness of the stratosphere. In view of the large wind shear which is normally found just above the tropopause it is unlikely that this region is free from turbulence. The observed distributions can be explained by the existence of a circulation in which air enters the stratosphere at the equator, where it is dried by condensation, travels in the stratosphere to temperate and polar regions, and sinks into the troposphere. The sinking, however, will warm the air unless it is being cooled by radiation and the idea of a stratosphere in radiative equilibrium must be abandoned. The cooling rate must lie between about 0.1 and 1.1°C per day but a value near 0.5°C per day seems most probable. At the equator the ascending air must be subject to heating by radiation. The circulation is quite reasonable on energy considerations. It is consistent with the existence of lower temperatures in the equatorial stratosphere than in polar and temperate regions, and if the flow can carry ozone from the equator to the poles then it gives a reasonable explanation of the high ozone values observed at high latitudes. The dynamic consequences of the circulation are not considered. It should however be noted that there is considerable difficulty to account for the smallness of the westerly winds in the stratosphere, as the rotation of the earth should convert the slow poleward movement into strong westerly winds.
  2. 1997: Tuck, A. F., et al. “The Brewer‐Dobson circulation in the light of high altitude in situ aircraft observations.” Quarterly Journal of the Royal Meteorological Society 123.537 (1997): 1-69. Fast response in situ measurements of a suite of chemical species made from the NASA ER2 high altitude aircraft, between 60°N and 70°S at potential temperatures up to 530 K from March to November 1994 at longitudes 115° W to 150°E, are considered for the view they offer of the Brewer‐Dobson circulation in the lower stratosphere and upper troposphere. In the southern hemisphere, where most of the flights occurred, comparisons are made with measurements taken in August/September 1987 at longitudes 120° W to 60° W to examine temporal and longitudinal differences. Interpretations made suggest conceptual modifications to the simple construct of advection in a two dimensional long‐term mean.
  3. 2006: Butchart, N., et al. “Simulations of anthropogenic change in the strength of the Brewer–Dobson circulation.” Climate Dynamics 27.7-8 (2006): 727-741. The effect of climate change on the Brewer–Dobson circulation and, in particular, the large-scale seasonal-mean transport between the troposphere and stratosphere is compared in a number of middle atmosphere general circulation models. All the models reproduce the observed upwelling across the tropical tropopause balanced by downwelling in the extra tropics, though the seasonal cycle in upwelling in some models is more semi-annual than annual. All the models also consistently predict an increase in the mass exchange rate in response to growing greenhouse gas concentrations, irrespective of whether or not the model includes interactive ozone chemistry. The mean trend is 11 kt s−1 year−1 or about 2% per decade but varies considerably between models. In all but one of the models the increase in mass exchange occurs throughout the year though, generally, the trend is larger during the boreal winter. On average, more than 60% of the mean mass fluxes can be explained by the EP-flux divergence using the downward control principle. Trends in the annual mean mass fluxes derived from the EP-flux divergence also explain about 60% of the trend in the troposphere-to-stratosphere mass exchange rate when averaged over all the models. Apart from two models the interannual variability in the downward control derived and actual mass fluxes were generally well correlated, for the annual mean.
  4. 2006: Austin, John, and Feng Li. “On the relationship between the strength of the Brewer‐Dobson circulation and the age of stratospheric air.” Geophysical research letters 33.17 (2006). The strength of the Brewer‐Dobson circulation is computed for multi‐decadal simulations of a coupled chemistry‐climate model covering the period 1960 to 2100. The circulation strength, as computed from the tropical mass upwelling, generally increases throughout the simulations. The model also includes an age of air tracer which generally decreases during the simulations. The two different transport concepts of mass upwelling and reciprocal of the age of air are investigated empirically from the model simulations. The results indicate that the variables are linearly related in the model but with a change of gradient some time near 2005. Possible reasons for the change of gradient are discussed.
  5. 2007: Randel, William J., et al. “A large annual cycle in ozone above the tropical tropopause linked to the Brewer–Dobson circulation.” Journal of the Atmospheric Sciences 64.12 (2007): 4479-4488. Near-equatorial ozone observations from balloon and satellite measurements reveal a large annual cycle in ozone above the tropical tropopause. The relative amplitude of the annual cycle is large in a narrow vertical layer between ∼16 and 19 km, with approximately a factor of 2 change in ozone between the minimum (during NH winter) and maximum (during NH summer). The annual cycle in ozone occurs over the same altitude region, and is approximately in phase with the well-known annual variation in tropical temperature. This study shows that the large annual variation in ozone occurs primarily because of variations in vertical transport associated with mean upwelling in the lower stratosphere (the Brewer–Dobson circulation); the maximum relative amplitude peak in the lower stratosphere is collocated with the strongest background vertical gradients in ozone. A similar large seasonal cycle is observed in carbon monoxide (CO) above the tropical tropopause, which is approximately out of phase with ozone (associated with an oppositely signed vertical gradient). The observed ozone and CO variations can be used to constrain estimates of the seasonal cycle in tropical upwelling.
  6. 2008: Garcia, Rolando R., and William J. Randel. “Acceleration of the Brewer–Dobson circulation due to increases in greenhouse gases.” Journal of the Atmospheric Sciences 65.8 (2008): 2731-2739. The acceleration of the Brewer–Dobson circulation under rising concentrations of greenhouse gases is investigated using the Whole Atmosphere Community Climate Model. The circulation strengthens as a result of increased wave driving in the subtropical lower stratosphere, which in turn occurs because of enhanced propagation and dissipation of waves in this region. Enhanced wave propagation is due to changes in tropospheric and lower-stratospheric zonal-mean winds, which become more westerly. Ultimately, these trends follow from changes in the zonal-mean temperature distribution caused by the greenhouse effect. The circulation in the middle and upper stratosphere also accelerates as a result of filtering of parameterized gravity waves by stronger subtropical westerly winds.
  7. 2008: Li, Feng, John Austin, and John Wilson. “The strength of the Brewer–Dobson circulation in a changing climate: Coupled chemistry–climate model simulations.” Journal of Climate 21.1 (2008): 40-57. The strength of the Brewer–Dobson circulation (BDC) in a changing climate is studied using multidecadal simulations covering the 1960–2100 period with a coupled chemistry–climate model, to examine the seasonality of the change of the BDC. The model simulates an intensification of the BDC in both the past (1960–2004) and future (2005–2100) climate, but the seasonal cycle is different. In the past climate simulation, nearly half of the tropical upward mass flux increase occurs in December–February, whereas in the future climate simulation the enhancement of the BDC is uniformly distributed in each of the four seasons. A downward control analysis implies that this different seasonality is caused mainly by the behavior of the Southern Hemisphere planetary wave forcing, which exhibits a very different long-term trend during solstice seasons in the past and future. The Southern Hemisphere summer planetary wave activity is investigated in detail, and its evolution is found to be closely related to ozone depletion and recovery. In the model results for the past, about 60% of the lower-stratospheric mass flux increase is caused by ozone depletion, but because of model ozone trend biases, the atmospheric effect was likely smaller than this. The remaining fraction of the mass flux increase is attributed primarily to greenhouse gas increase. The downward control analysis also reveals that orographic gravity waves contribute significantly to the increase of downward mass flux in the Northern Hemisphere winter lower stratosphere.
  8. 2009: McLandress, Charles, and Theodore G. Shepherd. “Simulated anthropogenic changes in the Brewer–Dobson circulation, including its extension to high latitudes.” Journal of Climate22.6 (2009): 1516-1540. Recent studies using comprehensive middle atmosphere models predict a strengthening of the Brewer–Dobson circulation in response to climate change. To gain confidence in the realism of this result it is important to quantify and understand the contributions from the different components of stratospheric wave drag that cause this increase. Such an analysis is performed here using three 150-yr transient simulations from the Canadian Middle Atmosphere Model (CMAM), a Chemistry–Climate Model that simulates climate change and ozone depletion and recovery. Resolved wave drag and parameterized orographic gravity wave drag account for 60% and 40%, respectively, of the long-term trend in annual mean net upward mass flux at 70 hPa, with planetary waves accounting for 60% of the resolved wave drag trend. Synoptic wave drag has the strongest impact in northern winter, where it accounts for nearly as much of the upward mass flux trend as planetary wave drag. Owing to differences in the latitudinal structure of the wave drag changes, the relative contribution of resolved and parameterized wave drag to the tropical upward mass flux trend over any particular latitude range is highly sensitive to the range of latitudes considered. An examination of the spatial structure of the climate change response reveals no straightforward connection between the low-latitude and high-latitude changes: while the model results show an increase in Arctic downwelling in winter, they also show a decrease in Antarctic downwelling in spring. Both changes are attributed to changes in the flux of stationary planetary wave activity into the stratosphere.
  9. 2011: Shepherd, Theodore G., and Charles McLandress. “A robust mechanism for strengthening of the Brewer–Dobson circulation in response to climate change: Critical-layer control of subtropical wave breaking.” Journal of the Atmospheric Sciences 68.4 (2011): 784-797. Climate models consistently predict a strengthened Brewer–Dobson circulation in response to greenhouse gas (GHG)-induced climate change. Although the predicted circulation changes are clearly the result of changes in stratospheric wave drag, the mechanism behind the wave-drag changes remains unclear. Here, simulations from a chemistry–climate model are analyzed to show that the changes in resolved wave drag are largely explainable in terms of a simple and robust dynamical mechanism, namely changes in the location of critical layers within the subtropical lower stratosphere, which are known from observations to control the spatial distribution of Rossby wave breaking. In particular, the strengthening of the upper flanks of the subtropical jets that is robustly expected from GHG-induced tropospheric warming pushes the critical layers (and the associated regions of wave drag) upward, allowing more wave activity to penetrate into the subtropical lower stratosphere. Because the subtropics represent the critical region for wave driving of the Brewer–Dobson circulation, the circulation is thereby strengthened. Transient planetary-scale waves and synoptic-scale waves generated by baroclinic instability are both found to play a crucial role in this process. Changes in stationary planetary wave drag are not so important because they largely occur away from subtropical latitudes.
  10. 2011: Weber, Mark, et al. “The Brewer-Dobson circulation and total ozone from seasonal to decadal time scales.” Atmospheric Chemistry and Physics 11.21 (2011): 11221-11235. The effect of the winter Brewer-Dobson circulation (BDC) on the seasonal and decadal evolution of total ozone in both hemispheres is investigated using satellite total ozone data from the merged GOME/SCIAMACHY/GOME-2 (GSG) data set (1995–2010) and outputs from two chemistry-climate models (CCM), the FUB-EMAC and DLR-E39C-A models. Combining data from both hemispheres a linear relationship between the winter average extratropical 100 hPa eddy heat flux and the ozone ratio with respect to fall ozone levels exists and is statistically significant for tropical as well as polar ozone. The high correlation at high latitudes persists well into the summer months until the onset of the next winter season. The anti-correlation of the cumulative eddy heat flux with tropical ozone ratios, however, breaks down in spring as the polar vortex erodes and changes to a weak positive correlation similar to that observed at high latitudes. The inter-annual variability and decadal evolution of ozone in each hemisphere in winter, spring, and summer are therefore driven by the cumulative effect of the previous winter’s meridional circulation. This compact linear relationship is also found in both CCMs used in this study indicating that current models realistically describe the variability in stratospheric circulation and its effect on total ozone. Both models show a positive trend in the winter mean eddy heat flux (and winter BDC strength) in both hemispheres until year 2050, however the inter-annual variability (peak-to-peak) is two to three times larger than the mean change between 1960 and 2050. It is, nevertheless, possible to detect a shift in this compact linear relationship related to past and future changes in the stratospheric halogen load. Using the SBUV/TOMS/OMI (MOD V8) merged data set (1980–2010), it can be shown that from the decade 1990–1999 to 2000–2010 this linear relationship remained unchanged (before and after the turnaround in the stratospheric halogen load), while a shift is evident between 1980–1989 (upward trend in stratospheric halogen) and the 1990s, which is a clear sign that an onset of recovery is detectable despite the large variability in polar ozone. Because of the large variability from year to year in the BDC circulation substantial polar ozone depletion may still occur in coming decades in selected winters with weak BDC and very low polar stratospheric temperatures.

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