Brewer-Dobson Circulation

The ozone abundances in different regions of the atmosphere are deter-mined by a balance between photochemical processes (production and loss), catalytic destruction and, transport. Ozone is produced in the trop-ical stratosphere but most of ozone is found at higher latitudes away from its production source. Ozone is transported through a slow atmo-spheric circulation in the lower to middle stratosphere that moves the upwelling air parcels from the tropics poleward, and then subsides in ex-tratropics and high latitudes where it builds up. The circulation is a broad hemispheric-scale meridional overturning which is limited to the winter season and historically known as the Brewer-Dobson circulation (BDC).

The main features of the poleward drift of air masses in stratosphere were

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CHAPTER 6. TOTAL OZONE TRENDS AND VARIABILITY DURING 1979 – 2012 FROM MERGED DATASETS OF VARIOUS SATELLITES

1000

1980 1990 2000 2010

-40

La Nina Nino3.4 index

-4

Antarctic AAO

1980 1990 2000 2010

Time [years]

Figure 6.1:Time series of the different explanatory variables used in this study are displayed in the panels: Stratospheric loading of ODS (first panel) in terms of Equivalent Effective Stratospheric Chlorine (EESC) in pptv. The EESC concentration (black curve) peaked in 1997 and started a slow de-cline afterwards. Quasi-Biennial Oscillation (QBO) index at 10 hPa (red) and at 30 hPa (blue) are shown in the second panel. The equatorial zonal winds at both levels are out of phase by about π/2. The third panel dis-plays the 11-year solar cycle as expressed by the solar flux at 10.7 cm (red) and core-to-wing ratio of the MgII line at 280 nm (blue). Time series of mean optical thickness at 550 nm (orange) to account for volcanic aerosol enhancements are presented in the fourth panel; the dominant features are the 1982 eruption of EL Chicohón and the 1991 eruption of Mt. Pinatubo.

Nino 3.4 index describing the state of the El Niño/Southern Oscillation (ENSO) is shown in panel five: El Niño (red) and La Niña (blue). The next two panels show the teleconnection patterns: Arctic Oscillation (AO) index in red and Antarctic Oscillation (AAO) index in blue. In the last panel, extra-tropical eddy heat flux at 100h Pa averaged over midlatitudes (area weight averaged between 45^◦N and 75^◦N) and averaged from Octo-ber to March in the NH (magenta) and from April to SeptemOcto-ber in the SH (green).

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6.1. Main Contributors to Ozone Variability

inferred from Brewer [1949] and Dobson [1956] water vapour and ozone measurements, respectively.

This meridional circulation is driven by the planetary scale atmo-spheric waves (Rossby and gravity waves) which are generated in the troposphere and propagated upwards to the stratosphere [Haynes et al., 1991; Rosenlof and Holton, 1993; Newman et al., 2001; Plumb, 2002]. The waves break and dissipate at a critical level and deposit their momentum, this decelerate the mean zonal air flow [Andrews et al., 1987]. As a result, a poleward motion is established to balance the Coriolis force and pres-sure gradient (geostrophic balance) and due to mass conservation large-scale vertical motions in the tropics and extratropics set in, this induces meridional overturning from equator to pole. In the tropics, the rising air cools down due to adiabatic expansion while in the polar region the subsiding air is heated by adiabatic compression. This leads to reducing (enhancing) of stratospheric temperatures below (above) the local radia-tive equilibrium temperature in the tropics (extratropical) region and as a result diabatic heating warms the tropical upwelling air and diabatic cooling cools the extratropical downwelling branch [Haynes et al., 1991;

Newman et al., 2001]. The diabatic circulation of air (also considered as residual circulation) from the tropical tropopause to the lower polar stratosphere has a mean transport time of five to six years [Waugh and Hall, 2005] and confined in the winter hemisphere [Rosenlof and Holton, 1993; Chipperfield and Jones, 1999] during which the upward propagat-ing waves deposit their easterly momentum (by wave breakpropagat-ing) in the stratospheric mean westerly flow.

The influence of the meridional circulation in a given winter impacts the ozone variability well into spring and summer [Fioletov and Shep-herd, 2003; Weber et al., 2011]. In the polar region, the accumulation of the lower stratospheric ozone is strongly governed by the intensity of diabatic circulation; the stronger the intensity in the wintertime, the stronger is the meridional mixing and the diabatic descent. This increases the stratospheric temperatures which weakens the polar vortex and less ozone is destructed by heterogeneous reactions [Chipperfield and Jones, 1999; Fusco and Salby, 1999; Randel and Stolarski, 2002].

The magnitude of the easterly momentum deposited in the strato-sphere by wave breaking is approximated by the divergence of the Eliassen Palm (EP) flux F, i.e. [Newman et al., 2001]. The net upward flux of wave activity is measured by the vertical component of the EP flux (Fz) which is proportional to the zonal mean poleward eddy heat flux vT [Andrews et al., 1987]. The winter accumulated lower stratosphere eddy heat flux is considered a good measure (proxy) of the inter-annual variability of ozone due to the BDC variations [Fusco and Salby, 1999; Newman et al.,

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CHAPTER 6. TOTAL OZONE TRENDS AND VARIABILITY DURING 1979 – 2012 FROM MERGED DATASETS OF VARIOUS SATELLITES 2001; Randel and Stolarski, 2002; Dhomse et al., 2006; Weber et al., 2011].

The contribution of the large scale stratospheric circulation to ozone fluctuations can also be determined by other dynamical explanatory vari-ables such as the dominant recurrent non-seasonal (with no particular periodicity) sea level pressure variation pattern north/south of 20^◦S/N latitude known as the Arctic Oscillation (AO) (or North Annular Mode (NAM)) and Antarctic Oscillation (AAO) (or Southern Annular Mode (SAM)) indices [Fusco and Salby, 1999; Hartmann et al., 2000; Appen-zeller et al., 2000; Randel and Stolarski, 2002; Kiesewetter et al., 2010;

Steinbrecht et al., 2011]. The QBO phase (see later section) also influneces the wave propagation and relates to variability in the BDC [Baldwin et al., 2001].