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The Brewer–Dobson circulation in the stratosphere and mesosphere – Is there a trend?
Advances in Space Research 38 (2006) 2446–2451 www.elsevier.com/locate/asr
The Brewer–Dobson circulation in the stratosphere and mesosphere – Is there a trend? H.K. Roscoe British Antarctic Survey, NERC, Madingley Rd, Cambridge CB3 0ET, UK Received 14 September 2004; received in revised form 17 February 2006; accepted 17 February 2006
Abstract The Brewer–Dobson circulation brings tropospheric air, accompanied by CFCs and greenhouse gases, into the stratosphere. Many models predict an increased circulation associated with an increase in greenhouse gases, such as that since the 1960s. A recent observation supports this: the rate at which total ozone increases in Antarctica during early winter is consistent with the descent and convergence that are part of the Brewer–Dobson circulation; at 65°S the rate doubled between the 1960s and 1990s. Another recent observation may also support this: the decrease in temperature since 1960 in the Antarctic mid-winter lower stratosphere is much less than the decrease calculated from the greenhouse effect of increased H2O, suggesting less CH4 oxidation in the 1970s; this could be caused by an increase in Brewer–Dobson circulation during the 1970s. An important paradox may be resolved by an increase in Brewer–Dobson circulation: the decrease in tropical cold-point temperature since the 1960s conflicts with the increase in mid-latitude H2O in the lower stratosphere, if it represents an increase at tropical entry of H2O; the conflict could be resolved if dehydration during stratospheric entry is incomplete and the circulation has increased. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Brewer–Dobson circulation; Stratosphere; Mesosphere
1. Introduction Air from the troposphere enters the stratosphere in the tropics, and moves towards the winter pole over a period of years, returning at middle and polar latitudes. This Brewer–Dobson circulation (Brewer, 1949) might be thought to be forced by radiative cooling of the stratosphere in the darkness near the winter pole, which reduces heights at fixed pressures there, creating a poleward gradient in height and so a poleward force. However, this force is turned to the east by Coriollis force, thereby creating the strong polar jet (the stratospheric vortex) in winter and spring. Any poleward circulation would then cease but for wave breaking, which transfers momentum preferentially against the vortex winds (wave drag), thereby allowing a poleward flow. In the stratosphere, planetary-scale E-mail address: [email protected]
Rossby waves break at mid-latitudes in the winter hemisphere in the so-called surf zone. Because of the lower density aloft, gravity waves (buoyancy waves) grow in amplitude as they propagate upwards, breaking throughout the stratosphere and mesosphere. The breaking of these waves in the stratosphere and mesosphere are the major contribution to forcing and control of the Brewer–Dobson circulation (e.g., Shepherd, 2002; Plumb, 2002). This circulation is important to stratospheric chemistry because it carries gases whose products ultimately deplete ozone, such as CFCs and N2O, into the stratosphere. It is important to tropospheric chemistry because it brings ozone down into the troposphere, supplying a substantial proportion of the ozone in the troposphere in the absence of pollution. It is important to climate because it carries greenhouse gases from troposphere to stratosphere, thereby changing the temperature of the stratosphere, troposphere and surface (Forster and Shine, 1997).
0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.02.078
H.K. Roscoe / Advances in Space Research 38 (2006) 2446–2451 450
The subsidence at the winter pole associated with the Brewer–Dobson circulation, plus the poleward flow of the circulation itself, is associated with convergence in the stratosphere at polar latitudes. This convergence causes an increase in total ozone during the winter. Hence the rate of increase of total ozone during the winter is one measure of the strength of the Brewer–Dobson circulation (Fig. 1). In the 1960s, total ozone in Antarctica in winter could only be measured by a Dobson spectrometer observing the Moon, a difficult method that ensured few measurements. Nevertheless, there were sufficient nights of measurement at Faraday in Antarctica (65°S) to allow a robust and significant rate of increase to be deduced from the full winter series (Fig. 2). Subsets of the data gave a very similar rate (Roscoe et al., 2004, and the early winter line in Fig. 2), but with reduced significance.
Lunar Dobson
350
300
250
200
2. Increase in an important indicator of Brewer–Dobson circulation since the 1960s
Faraday,1960 to 1968
400
total ozone (DU)
The strength of the vortex associated with the Brewer– Dobson circulation also affects surface pressure, winds and temperatures in winter (Polvani and Kushner, 2002), probably via downward reflection and refraction of planetary waves. A stronger vortex at the time of a warming is associated with the major modes of variability in surface climate, in the north the Northern Annular Mode and the Arctic Oscillation, in the south the Southern Annular Mode and the Antarctic Oscillation (Baldwin et al., 2003; Gillett and Thompson, 2003).
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May
June
July
Fig. 2. Total ozone during winter at Faraday, Antarctica, from Dobson lunar measurements between 1960 and 1968. The long line is the fit to all points, and has slope 9.2 ± 2.8 DU/month (1r). The short line is the fit to points in May and June except the point at 221 DU, and has slope 6.9 ± 5.6 DU/month.
In the 1990s, a new zenith-sky visible spectrometer at Faraday allowed daily measurements at twilight throughout winter, so that a significant rate of increase in early winter only could be deduced (Fig. 3). This is important because the ozone hole now begins in mid-winter at its sunlit edge region (Roscoe et al., 1997 and Fig. 3). The measurements in Figs. 2 and 3 are only a subset of the available measurements. Including other lunar Dobson measurements of reduced frequency in the 1980s and 1990s, the rate of increase is 2.10 ± 0.45 times that of the 1960s (Roscoe et al., 2004) – the rate has approximately doubled.
Fig. 1. The winter cell of the Brewer–Dobson circulation, which brings greenhouse gases and sources of reactive gases (CO2, H2O, CH4, N2O, and CFCs) into the stratosphere.
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Faraday, 1990 to 1995
zenith-sky visible spectrometer
total ozone (DU)
400
350
300
250
200
May
June
July
Fig. 3. Total ozone during winter at Faraday from SAOZ zenith-sky visible measurements, between 1990 and 1995 (exc. 1992 and 1993 because of Pinatubo aerosol). The line is the fit to points in May and June, and has slope 15.7 DU/month, with error ±4.9 DU/month allowing for correlation between adjacent daily values.
3. Increase in Brewer–Dobson circulation in the 1970s to reconcile stratospheric temperatures in Antarctic winter with the global trend in H2O If we consider the local radiative effect of stratospheric trace gases: H2O has a greenhouse effect throughout the stratosphere, including the lower stratosphere, whereas CO2 only has a significant effect in the middle and upper stratosphere (Forster and Shine, 1999); O3 is strongly heated by sunlight, but in the dark its greenhouse effect is small. This means that H2O is uniquely important in the lower stratosphere in Antarctic winter. Hence, the trend in temperature at 100 hPa in Antarctic winter should be an indicator of the global trend in stratospheric H2O (Roscoe et al., 2003), dynamical factors being equal. Fig. 4 shows the changes at Halley in Antarctica in winter, with and without sonde corrections. The main features of the changes are independent of corrections: a cooling of 1.5 to 2 K since 1980, and no overall trend for the full time series. The changes at other sites in the core of the Antarctic -74
temperature (˚C)
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Uncorrected
-78 -80 -82
vortex (where air parcels see little sunlight in winter) have similar characteristics (Roscoe et al., 2003). Stratospheric water vapour has been increasing at about 1%/year at mid-latitudes in the middle stratosphere since the mid-1960s, and in the lower stratosphere since the mid-1950s (Rosenlof et al., 2001). In a model with fixed dynamical heating, Forster and Shine (2002) calculated the changes expected in temperature from a 1%/year increase in stratospheric H2O, listed in Table 1 for Antarctica at 100 hPa. Although the measured and calculated trends agree since 1980, they greatly disagree since 1960. One can speculate about the causes of 1960–2000 disagreement: (a) it could be caused by changes in vortex eccentricity, but this is unlikely as the temperature changes were similar at two other widely separated Antarctic sites (Roscoe et al., 2003); (b) it could be caused by changes to dynamical heating, but this is unlikely as variability in dynamical heating is small at 100 hPa (Shindell et al., 1997); (c) air arriving at the lower stratosphere in Antarctica in winter originates in the middle to upper stratosphere at mid-latitudes, and between a third and half its H2O has come from CH4 oxidation. Hence a change in the rate of CH4 oxidation could cause the disagreement. Item (c) is possible because we have a disagreement between the proxy water vapour trend in Antarctica and the actual water vapour trend at mid-latitudes, and methane oxidation is the only source of water vapour after air enters the stratosphere. A change in rate of methane oxidation can only be caused if temperatures change the reaction rates, if OH amounts change, or if the speed of the Brewer– Dobson circulation changes (a slower circulation gives more time for oxidation in the stratosphere and so increases its apparent rate, as air leaving the stratosphere contains less CH4). Hence if there were a rapid increase in Brewer–Dobson circulation in the early 1970s, it would reduce the rate of oxidation of CH4 for several years before returning to normal, causing a temporary reduction in water trend in Antarctic winter as implied by the observed temperatures. Just such an effect in the upper stratosphere was observed by HALOE (Smith et al., 2000) and modelled (Considine et al., 2001), due to transient changes in circulation
Corrected (minus 4˚C)
-84 -86 1950
1960
1970
1980
1990
2000
Fig. 4. Average temperature in June plus July at 100 hPa at Halley in Antarctica (76°S), smoothed by a 3-year triangular filter, with and without radiation and lag corrections for sonde types. Corrections due to solar heating do not apply in the dark of winter, so only the smaller corrections due to infrared absorption and emission need be considered. Note that the character of the changes in temperature are similar whether or not corrections are applied.
Table 1 Measured changes in temperature at 100 hPa at 76°S in winter (Roscoe et al., 2003), compared to changes calculated at the same location given a 1%/year increase in stratospheric H2O (Forster and Shine, 2002, their Fig. 3)
Measured Calculated
1980 to 2000
1960 to 2000
1.6 ± 0.6 K 1.6 K
0.6 ± 0.5 K 4.2 K
H.K. Roscoe / Advances in Space Research 38 (2006) 2446–2451
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Fig. 5. Locations of tropical radiosonde stations (top) and saturation mixing ratios at the sonde cold-points (below). Open symbols are monthly means, solid symbols are annual means (from Zhou et al., 2001).
following the radiative response to stratospheric aerosol from Mt. Pinatubo in 1991. 4. Increase in Brewer–Dobson circulation since the 1960s to reconcile the trend in H2O with the trend in tropical coldpoint temperature Stratospheric water vapour has been increasing at about 1%/year at mid-latitudes in the middle stratosphere since
the mid-1960s, and in the lower stratosphere since the mid-1950s (Rosenlof et al., 2001). If this represents the increase at the tropical entry point, it disagrees with the decrease observed in tropical cold-point temperatures (Fig. 5), which would naively imply decreasing stratospheric H2O at the entry point. However, dehydration during stratospheric entry is incomplete or inefficient: convective overshoot probably only occupies 0.5% of the tropics at any one time
Fig. 6. Cumulative distribution functions of saturation mixing ratios of coldest points on ECMWF trajectories entering the stratosphere in the tropics (Bonazzola and Haynes, 2004).
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(Gettelman et al., 2002); ultra-thin tropical cirrus is not ubiquitous and typically could only dehydrate by 0.35 ppmv (Luo, 2003); and ice falls slowly during horizontal transport and eventual ascent through the tropical tropopause layer (Gettelman et al., 2002), so that when the air mass is later warmed, most of the ice evaporates into the same air mass and little dehydration has taken place. The inefficiency of the dehydration process is emphasised by new trajectory analyses of air entering the tropical lower stratosphere (Bonazzola and Haynes, 2004; Fueglistaler et al., 2004), where the mean minimum temperatures encountered often have H2O saturation mixing ratios of 1–2 ppmv (Fig. 6), rather than the 3.6 ppmv observed at entry (Dessler, 1998). Given an inefficient dehydration process, if its rate remained constant, an increase in Brewer–Dobson circula-
Table 2 Trends in NO2, measured at Lauder NZ from 1980 to 1998, and calculated with a stacked box model (Fish et al., 2000) Change in NO2 (%/decade) am
pm
Measured (45°S)
+5.0–6.8
+4.1–5.2
Calculated (a) N2O trend (b) Temperature trend (c) O3 trend (d) H2O trend
+2.5 ± 0.3 +1.2 ± 0.4 +0.9 ± 0.3 0.6 ± 0.2
+2.5 ± 0.3 +0.1 +0.0 0.6 ± 0.2
Sum of (a) to (d) (e) 20% decrease in background aerosol
+4.0 ± 0.6 +1.9
+2.0 ± 0.4 +2.4
Sum of (a) to (e)
+5.9 ± 0.6
+4.3 ± 0.4
tion would tend to cause an increase in H2O at entry. An increased circulation should also further cool temperatures at the entry point, as observed. If the tendency to increase H2O at entry because of inefficient dehydration were greater than the tendency to reduce it because of cooling, the increase in H2O would be reconciled with the cooling of the tropical cold-point. 5. Decrease in Brewer–Dobson circulation since 1980 to reconcile the trend in NO2 with the trend in N2O The trend measured in total NO2 at 45°S from 1980 to 1998 was twice the trend in N2O (the source of stratospheric reactive nitrogen). Trends in stratospheric temperature, O3 and H2O could not have caused this, but a decrease in background aerosol surface by 20% per decade could (Table 2). The large sensitivity to stratospheric aerosol arises because of hydrolysis of N2O5 to HNO3 on the aerosol surface, removing NO2 as it reacts to form more N2O5 during subsequent nights. Such a decrease in background aerosol (i.e., between large volcanic eruptions) could easily have occurred between 1980 (just before El Chichon) and 1998 (long after Pinatubo). New measurements in Antarctic summer, where there is no N2O5 and so the sensitivity of NO2 to aerosol is much reduced, show a similar but slightly larger trend to that at 45°S, since 1990 (Fig. 7). This suggests that there has been an increase in the rate of oxidation of N2O in the stratosphere, rather than a reduction in background aerosol. Other factors being equal, this suggests a decrease in Brewer–Dobson circulation since at least 1990. Because of the difference in timing, this is not necessarily in conflict
Fig. 7. Total NO2 at Faraday and Rothera in Antarctica (65 and 67°S). In mid-summer, am and pm values are identical despite the 3-hour time difference, so no N2O5 is present and the sensitivity of NO2 to background aerosol is small. The trend in summer, excluding 1991and 1992 when there is residual sensitivity to volcanic aerosol due to BrNO2 hydrolysis, is 7.6 ± 0.5%/decade, at least as big as at 45°S. These are preliminary values because chemically modified Langley plots have not yet been analysed to correct the drift in zero, which is obvious between the minima of 1990 and 1991, and may persist later.
H.K. Roscoe / Advances in Space Research 38 (2006) 2446–2451
with the earlier evidence for an increase in Brewer–Dobson circulation since 1960. It could indeed be that there was a major increase during the 1970s and a small decrease during the 1990s. 6. Causes and implications of an increase in Brewer–Dobson circulation Models predict that increased greenhouse gases cause more planetary waves to propagate polewards, via the upper troposphere to the stratospheric surf zone, thereby increasing the poleward flow of the Brewer–Dobson circulation, e.g., Butchart and Scaife (2001) predicted 15% more mass flow in 2050 with IPCC increases in greenhouse gases. Note that the steady increase in greenhouse gases since the 1960s would argue against the transient increase in circulation and later small decrease, discussed in the previous paragraph. Other possible causes are decreased filtering of gravity waves in the troposphere, due to changes in wind structure because of increased greenhouse gases, and increases in the poleward gradient of geopotential height because of the change to the difference in ozone between tropics and pole that accompanies ozone loss due to CFCs. An increase in Brewer–Dobson circulation would have caused many changes to the amounts and lifetimes of gases that are sources (CH4, N2O, and CFCs) of reactive ozoneremoving gases in the stratosphere. It would also have increased the return of air to the troposphere via tropopause folds, thereby increasing O3 in the free troposphere at mid-latitudes. References Baldwin, M.P., Thompson, D.W.J., Shuckburgh, E.F., Norton, W.A., Gillett, N.P. Weather from the stratosphere? Science 301, 317–318, 2003. Bonazzola, M., Haynes, P.H. A trajectory-based study of the tropical tropopause region. J. Geophys. Res., in press, 2004. Brewer, A.W. Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere. Q. J. R. Meteorol. Soc. 75, 351–363, 1949.
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Butchart, N., Scaife, A.A. Removal of chlorofluorocarbons by increased mass exchange between the stratosphere and troposphere in a changing climate. Nature 410, 799–802, 2001. Considine, D.B., Rosenfield, J.E., Fleming, E.L. An interactive model study of the influence of the Mount Pinatubo aerosol on stratospheric methane and water trends. J. Geophys. Res. 106 (27), 711–727, 2001. Dessler, A.E. A reexamination of the ‘‘stratospheric fountain’’ hypothesis. Geophys. Res. Lett. 25, 4165–4168, 1998. Fish, D.J., Roscoe, H.K., Johnston, P.V. Possible causes of stratospheric NO2 trends observed at Lauder, New Zealand. Geophys. Res. Lett. 27, 3313–3316, 2000. de Forster, P.M., Shine, K.P. Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling. Geophys. Res. Lett. 26, 3309–3312, 1999. Forster, P.M., Shine, K.P. Assessing the climate impact of trends in stratospheric water vapor. Geophys. Res. Lett. 29 (6), 1086, art, 2002. Fueglistaler, S., Wernli, H., Peter, T. Tropical troposphere-to-stratosphere transport inferred from trajectory calculations. J. Geophys. Res. 109, D03108, 2004. Gettelman, A., Randel, W.J., Wu, F., Massie, S.T. Transport of water vapour in the tropical tropopause layer. Geophys. Res. Lett., 29, doi:10.1029/2001GL013818, 2002. Gillett, N.P., Thompson, D.W.J. Simulation of recent southern hemisphere climate change. Science 302, 273–275, 2003. Luo, B.P., et al. (28 authors). Dehydration potential of ultrathin clouds at the tropical tropopause. Geophys. Res. Lett. 30, paper 1557, 2003. Plumb, R.A. Stratospheric transport. J. Met. Soc. Jpn. 80, 793–809, 2002. Roscoe, H.K., Fowler, C.L., Shanklin, J.D., Hill, J.G.T. Evidence from ozone measurements for long-term dynamical changes at the edge of the Antarctic vortex in early winter. Q. J. R. Meteorol. Soc. 130, 1123–1135, 2004. Roscoe, H.K., Colwell, S.R., Shanklin, J.D. Stratospheric temperatures in Antarctic winter: Does the 40-year record confirm mid-latitude trends in stratospheric water vapour? Q. J. R. Meteorol. Soc. 129, 1745–1759, 2003. Rosenlof, K.H. et al. Stratospheric water vapor increases over the past half-century. Geophys. Res. Lett. 28, 1195–1198 (13 authors), 2001. Shepherd, T.G. Issues in stratosphere–troposphere coupling. J. Met. Soc. Jpn. 80, 769–792, 2002. Shindell, D.T., Wong, S., Rind, D. Interannual variability of the Antarctic ozone hole in a GCM. Part 1: the influence of tropospheric wave varaibility. J. Atm. Sci. 54, 2308–2319, 1997. Smith, C.A.R., Toumi, J.D., Haigh Seasonal trends in stratospheric water vapour. Geophys. Res. Lett. 27, 1687–1690, 2000. Zhou, X.-L., Geller, M.A., Zhang, M. Cooling trend of the tropical cold point tropopause temperature and its implications. J. Geophys. Res. 106, 1511–1522, 2001.
The Brewer–Dobson circulation in the stratosphere and mesosphere – Is there a trend? H.K. Roscoe British Antarctic Survey, NERC, Madingley Rd, Cambridge CB3 0ET, UK Received 14 September 2004; received in revised form 17 February 2006; accepted 17 February 2006
Abstract The Brewer–Dobson circulation brings tropospheric air, accompanied by CFCs and greenhouse gases, into the stratosphere. Many models predict an increased circulation associated with an increase in greenhouse gases, such as that since the 1960s. A recent observation supports this: the rate at which total ozone increases in Antarctica during early winter is consistent with the descent and convergence that are part of the Brewer–Dobson circulation; at 65°S the rate doubled between the 1960s and 1990s. Another recent observation may also support this: the decrease in temperature since 1960 in the Antarctic mid-winter lower stratosphere is much less than the decrease calculated from the greenhouse effect of increased H2O, suggesting less CH4 oxidation in the 1970s; this could be caused by an increase in Brewer–Dobson circulation during the 1970s. An important paradox may be resolved by an increase in Brewer–Dobson circulation: the decrease in tropical cold-point temperature since the 1960s conflicts with the increase in mid-latitude H2O in the lower stratosphere, if it represents an increase at tropical entry of H2O; the conflict could be resolved if dehydration during stratospheric entry is incomplete and the circulation has increased. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Brewer–Dobson circulation; Stratosphere; Mesosphere
1. Introduction Air from the troposphere enters the stratosphere in the tropics, and moves towards the winter pole over a period of years, returning at middle and polar latitudes. This Brewer–Dobson circulation (Brewer, 1949) might be thought to be forced by radiative cooling of the stratosphere in the darkness near the winter pole, which reduces heights at fixed pressures there, creating a poleward gradient in height and so a poleward force. However, this force is turned to the east by Coriollis force, thereby creating the strong polar jet (the stratospheric vortex) in winter and spring. Any poleward circulation would then cease but for wave breaking, which transfers momentum preferentially against the vortex winds (wave drag), thereby allowing a poleward flow. In the stratosphere, planetary-scale E-mail address: [email protected]
Rossby waves break at mid-latitudes in the winter hemisphere in the so-called surf zone. Because of the lower density aloft, gravity waves (buoyancy waves) grow in amplitude as they propagate upwards, breaking throughout the stratosphere and mesosphere. The breaking of these waves in the stratosphere and mesosphere are the major contribution to forcing and control of the Brewer–Dobson circulation (e.g., Shepherd, 2002; Plumb, 2002). This circulation is important to stratospheric chemistry because it carries gases whose products ultimately deplete ozone, such as CFCs and N2O, into the stratosphere. It is important to tropospheric chemistry because it brings ozone down into the troposphere, supplying a substantial proportion of the ozone in the troposphere in the absence of pollution. It is important to climate because it carries greenhouse gases from troposphere to stratosphere, thereby changing the temperature of the stratosphere, troposphere and surface (Forster and Shine, 1997).
0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2006.02.078
H.K. Roscoe / Advances in Space Research 38 (2006) 2446–2451 450
The subsidence at the winter pole associated with the Brewer–Dobson circulation, plus the poleward flow of the circulation itself, is associated with convergence in the stratosphere at polar latitudes. This convergence causes an increase in total ozone during the winter. Hence the rate of increase of total ozone during the winter is one measure of the strength of the Brewer–Dobson circulation (Fig. 1). In the 1960s, total ozone in Antarctica in winter could only be measured by a Dobson spectrometer observing the Moon, a difficult method that ensured few measurements. Nevertheless, there were sufficient nights of measurement at Faraday in Antarctica (65°S) to allow a robust and significant rate of increase to be deduced from the full winter series (Fig. 2). Subsets of the data gave a very similar rate (Roscoe et al., 2004, and the early winter line in Fig. 2), but with reduced significance.
Lunar Dobson
350
300
250
200
2. Increase in an important indicator of Brewer–Dobson circulation since the 1960s
Faraday,1960 to 1968
400
total ozone (DU)
The strength of the vortex associated with the Brewer– Dobson circulation also affects surface pressure, winds and temperatures in winter (Polvani and Kushner, 2002), probably via downward reflection and refraction of planetary waves. A stronger vortex at the time of a warming is associated with the major modes of variability in surface climate, in the north the Northern Annular Mode and the Arctic Oscillation, in the south the Southern Annular Mode and the Antarctic Oscillation (Baldwin et al., 2003; Gillett and Thompson, 2003).
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May
June
July
Fig. 2. Total ozone during winter at Faraday, Antarctica, from Dobson lunar measurements between 1960 and 1968. The long line is the fit to all points, and has slope 9.2 ± 2.8 DU/month (1r). The short line is the fit to points in May and June except the point at 221 DU, and has slope 6.9 ± 5.6 DU/month.
In the 1990s, a new zenith-sky visible spectrometer at Faraday allowed daily measurements at twilight throughout winter, so that a significant rate of increase in early winter only could be deduced (Fig. 3). This is important because the ozone hole now begins in mid-winter at its sunlit edge region (Roscoe et al., 1997 and Fig. 3). The measurements in Figs. 2 and 3 are only a subset of the available measurements. Including other lunar Dobson measurements of reduced frequency in the 1980s and 1990s, the rate of increase is 2.10 ± 0.45 times that of the 1960s (Roscoe et al., 2004) – the rate has approximately doubled.
Fig. 1. The winter cell of the Brewer–Dobson circulation, which brings greenhouse gases and sources of reactive gases (CO2, H2O, CH4, N2O, and CFCs) into the stratosphere.
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Faraday, 1990 to 1995
zenith-sky visible spectrometer
total ozone (DU)
400
350
300
250
200
May
June
July
Fig. 3. Total ozone during winter at Faraday from SAOZ zenith-sky visible measurements, between 1990 and 1995 (exc. 1992 and 1993 because of Pinatubo aerosol). The line is the fit to points in May and June, and has slope 15.7 DU/month, with error ±4.9 DU/month allowing for correlation between adjacent daily values.
3. Increase in Brewer–Dobson circulation in the 1970s to reconcile stratospheric temperatures in Antarctic winter with the global trend in H2O If we consider the local radiative effect of stratospheric trace gases: H2O has a greenhouse effect throughout the stratosphere, including the lower stratosphere, whereas CO2 only has a significant effect in the middle and upper stratosphere (Forster and Shine, 1999); O3 is strongly heated by sunlight, but in the dark its greenhouse effect is small. This means that H2O is uniquely important in the lower stratosphere in Antarctic winter. Hence, the trend in temperature at 100 hPa in Antarctic winter should be an indicator of the global trend in stratospheric H2O (Roscoe et al., 2003), dynamical factors being equal. Fig. 4 shows the changes at Halley in Antarctica in winter, with and without sonde corrections. The main features of the changes are independent of corrections: a cooling of 1.5 to 2 K since 1980, and no overall trend for the full time series. The changes at other sites in the core of the Antarctic -74
temperature (˚C)
-76
Uncorrected
-78 -80 -82
vortex (where air parcels see little sunlight in winter) have similar characteristics (Roscoe et al., 2003). Stratospheric water vapour has been increasing at about 1%/year at mid-latitudes in the middle stratosphere since the mid-1960s, and in the lower stratosphere since the mid-1950s (Rosenlof et al., 2001). In a model with fixed dynamical heating, Forster and Shine (2002) calculated the changes expected in temperature from a 1%/year increase in stratospheric H2O, listed in Table 1 for Antarctica at 100 hPa. Although the measured and calculated trends agree since 1980, they greatly disagree since 1960. One can speculate about the causes of 1960–2000 disagreement: (a) it could be caused by changes in vortex eccentricity, but this is unlikely as the temperature changes were similar at two other widely separated Antarctic sites (Roscoe et al., 2003); (b) it could be caused by changes to dynamical heating, but this is unlikely as variability in dynamical heating is small at 100 hPa (Shindell et al., 1997); (c) air arriving at the lower stratosphere in Antarctica in winter originates in the middle to upper stratosphere at mid-latitudes, and between a third and half its H2O has come from CH4 oxidation. Hence a change in the rate of CH4 oxidation could cause the disagreement. Item (c) is possible because we have a disagreement between the proxy water vapour trend in Antarctica and the actual water vapour trend at mid-latitudes, and methane oxidation is the only source of water vapour after air enters the stratosphere. A change in rate of methane oxidation can only be caused if temperatures change the reaction rates, if OH amounts change, or if the speed of the Brewer– Dobson circulation changes (a slower circulation gives more time for oxidation in the stratosphere and so increases its apparent rate, as air leaving the stratosphere contains less CH4). Hence if there were a rapid increase in Brewer–Dobson circulation in the early 1970s, it would reduce the rate of oxidation of CH4 for several years before returning to normal, causing a temporary reduction in water trend in Antarctic winter as implied by the observed temperatures. Just such an effect in the upper stratosphere was observed by HALOE (Smith et al., 2000) and modelled (Considine et al., 2001), due to transient changes in circulation
Corrected (minus 4˚C)
-84 -86 1950
1960
1970
1980
1990
2000
Fig. 4. Average temperature in June plus July at 100 hPa at Halley in Antarctica (76°S), smoothed by a 3-year triangular filter, with and without radiation and lag corrections for sonde types. Corrections due to solar heating do not apply in the dark of winter, so only the smaller corrections due to infrared absorption and emission need be considered. Note that the character of the changes in temperature are similar whether or not corrections are applied.
Table 1 Measured changes in temperature at 100 hPa at 76°S in winter (Roscoe et al., 2003), compared to changes calculated at the same location given a 1%/year increase in stratospheric H2O (Forster and Shine, 2002, their Fig. 3)
Measured Calculated
1980 to 2000
1960 to 2000
1.6 ± 0.6 K 1.6 K
0.6 ± 0.5 K 4.2 K
H.K. Roscoe / Advances in Space Research 38 (2006) 2446–2451
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Fig. 5. Locations of tropical radiosonde stations (top) and saturation mixing ratios at the sonde cold-points (below). Open symbols are monthly means, solid symbols are annual means (from Zhou et al., 2001).
following the radiative response to stratospheric aerosol from Mt. Pinatubo in 1991. 4. Increase in Brewer–Dobson circulation since the 1960s to reconcile the trend in H2O with the trend in tropical coldpoint temperature Stratospheric water vapour has been increasing at about 1%/year at mid-latitudes in the middle stratosphere since
the mid-1960s, and in the lower stratosphere since the mid-1950s (Rosenlof et al., 2001). If this represents the increase at the tropical entry point, it disagrees with the decrease observed in tropical cold-point temperatures (Fig. 5), which would naively imply decreasing stratospheric H2O at the entry point. However, dehydration during stratospheric entry is incomplete or inefficient: convective overshoot probably only occupies 0.5% of the tropics at any one time
Fig. 6. Cumulative distribution functions of saturation mixing ratios of coldest points on ECMWF trajectories entering the stratosphere in the tropics (Bonazzola and Haynes, 2004).
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(Gettelman et al., 2002); ultra-thin tropical cirrus is not ubiquitous and typically could only dehydrate by 0.35 ppmv (Luo, 2003); and ice falls slowly during horizontal transport and eventual ascent through the tropical tropopause layer (Gettelman et al., 2002), so that when the air mass is later warmed, most of the ice evaporates into the same air mass and little dehydration has taken place. The inefficiency of the dehydration process is emphasised by new trajectory analyses of air entering the tropical lower stratosphere (Bonazzola and Haynes, 2004; Fueglistaler et al., 2004), where the mean minimum temperatures encountered often have H2O saturation mixing ratios of 1–2 ppmv (Fig. 6), rather than the 3.6 ppmv observed at entry (Dessler, 1998). Given an inefficient dehydration process, if its rate remained constant, an increase in Brewer–Dobson circula-
Table 2 Trends in NO2, measured at Lauder NZ from 1980 to 1998, and calculated with a stacked box model (Fish et al., 2000) Change in NO2 (%/decade) am
pm
Measured (45°S)
+5.0–6.8
+4.1–5.2
Calculated (a) N2O trend (b) Temperature trend (c) O3 trend (d) H2O trend
+2.5 ± 0.3 +1.2 ± 0.4 +0.9 ± 0.3 0.6 ± 0.2
+2.5 ± 0.3 +0.1 +0.0 0.6 ± 0.2
Sum of (a) to (d) (e) 20% decrease in background aerosol
+4.0 ± 0.6 +1.9
+2.0 ± 0.4 +2.4
Sum of (a) to (e)
+5.9 ± 0.6
+4.3 ± 0.4
tion would tend to cause an increase in H2O at entry. An increased circulation should also further cool temperatures at the entry point, as observed. If the tendency to increase H2O at entry because of inefficient dehydration were greater than the tendency to reduce it because of cooling, the increase in H2O would be reconciled with the cooling of the tropical cold-point. 5. Decrease in Brewer–Dobson circulation since 1980 to reconcile the trend in NO2 with the trend in N2O The trend measured in total NO2 at 45°S from 1980 to 1998 was twice the trend in N2O (the source of stratospheric reactive nitrogen). Trends in stratospheric temperature, O3 and H2O could not have caused this, but a decrease in background aerosol surface by 20% per decade could (Table 2). The large sensitivity to stratospheric aerosol arises because of hydrolysis of N2O5 to HNO3 on the aerosol surface, removing NO2 as it reacts to form more N2O5 during subsequent nights. Such a decrease in background aerosol (i.e., between large volcanic eruptions) could easily have occurred between 1980 (just before El Chichon) and 1998 (long after Pinatubo). New measurements in Antarctic summer, where there is no N2O5 and so the sensitivity of NO2 to aerosol is much reduced, show a similar but slightly larger trend to that at 45°S, since 1990 (Fig. 7). This suggests that there has been an increase in the rate of oxidation of N2O in the stratosphere, rather than a reduction in background aerosol. Other factors being equal, this suggests a decrease in Brewer–Dobson circulation since at least 1990. Because of the difference in timing, this is not necessarily in conflict
Fig. 7. Total NO2 at Faraday and Rothera in Antarctica (65 and 67°S). In mid-summer, am and pm values are identical despite the 3-hour time difference, so no N2O5 is present and the sensitivity of NO2 to background aerosol is small. The trend in summer, excluding 1991and 1992 when there is residual sensitivity to volcanic aerosol due to BrNO2 hydrolysis, is 7.6 ± 0.5%/decade, at least as big as at 45°S. These are preliminary values because chemically modified Langley plots have not yet been analysed to correct the drift in zero, which is obvious between the minima of 1990 and 1991, and may persist later.
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with the earlier evidence for an increase in Brewer–Dobson circulation since 1960. It could indeed be that there was a major increase during the 1970s and a small decrease during the 1990s. 6. Causes and implications of an increase in Brewer–Dobson circulation Models predict that increased greenhouse gases cause more planetary waves to propagate polewards, via the upper troposphere to the stratospheric surf zone, thereby increasing the poleward flow of the Brewer–Dobson circulation, e.g., Butchart and Scaife (2001) predicted 15% more mass flow in 2050 with IPCC increases in greenhouse gases. Note that the steady increase in greenhouse gases since the 1960s would argue against the transient increase in circulation and later small decrease, discussed in the previous paragraph. Other possible causes are decreased filtering of gravity waves in the troposphere, due to changes in wind structure because of increased greenhouse gases, and increases in the poleward gradient of geopotential height because of the change to the difference in ozone between tropics and pole that accompanies ozone loss due to CFCs. An increase in Brewer–Dobson circulation would have caused many changes to the amounts and lifetimes of gases that are sources (CH4, N2O, and CFCs) of reactive ozoneremoving gases in the stratosphere. It would also have increased the return of air to the troposphere via tropopause folds, thereby increasing O3 in the free troposphere at mid-latitudes. References Baldwin, M.P., Thompson, D.W.J., Shuckburgh, E.F., Norton, W.A., Gillett, N.P. Weather from the stratosphere? Science 301, 317–318, 2003. Bonazzola, M., Haynes, P.H. A trajectory-based study of the tropical tropopause region. J. Geophys. Res., in press, 2004. Brewer, A.W. Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere. Q. J. R. Meteorol. Soc. 75, 351–363, 1949.
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