1. What are the four time-scales on which ozone levels can vary?
2. What are the atmospheric processes that change ozone amounts?
3. What is the primary difference between ozone variability in the upper and lower stratosphere?
4a. How does the variability in solar ultraviolet radiation (such as occurs with the sunspot cycle) affect ozone?
4b. On what two time scales does this occur?
5. What is the difference between the Quasi-Biennial Oscillation (QBO) and El Nino-Southern Oscillation (ENSO)?
6. How does a tropical phenomenon like the QBO affect ozone at all latitudes in the lower stratosphere?
7. Why is it important to know the cyclical variations of ozone?
1. Ozone can vary on short-term, seasonal, interannual, and long-term scales.
2. Atmospheric processes that change ozone amounts are photochemical and dynamical ones. Photochemical processes refer to the production and loss of ozone driven by solar UV photochemistry. Dynamical processes refer to how ozone is transported from one region to another by winds and large-scale circulation patterns in the atmosphere.
3. Ozone variability in the upper stratosphere is driven by photochemical processes, since it is here that there is the necessary ultraviolet radiation that creates and destroys ozone. Ozone variability in the lower stratosphere is driven by dynamical or transport processes, since there is less ultraviolet radiation. Ozone molecules in the lower stratosphere exist for long enough periods to be transported out of a region and into another.
4a. Variations in solar ultraviolet radiation bring about variations in ozone photochemistry through changes in production and loss rates of both oxygen (O2) and ozone (O3) molecules. When the UV output of the Sun increases, as during a sunspot maximum, there is a greater increase in the more energetic wavelengths (200 nm radiation increases 8%) than the less energetic wavelengths (300 nm radiation increases only 0.2%). Radiation with wavelengths shorter than 240 nm drive the photolysis of O2 molecules while radiation with wavelengths between 240 and 320 nm drive the photolysis of O3 molecules. This means that the production rate of ozone via O2 photolysis increases more than the loss rate of ozone via O3 photolysis during pepriods of increased solar UV radiation output. Ozone production increases at a greater rate than ozone loss, hence ozone concentrations increase with increased UV radiation. The result is a direct, inphase variation of upper stratospheric ozone with UV changes from sunspot activity.
4b. Sunspots occur in quasi-regular 11-year cycles. The period when sunspot activity is at its greates is called a solar maximum, while the period when sunspot activity is at its least is called a solar minimum. During solar maxima, sunspots tend to occur in clusters on one side of the Sun, which rotates every 27 days. Ultraviolet radiation is thus modulated by the 11-year sunspot cycle and by the 27-day solar rotational period. If two active sunspot regions occur on opposite sides of the Sun, the 27-day modulation will be replaced by a 13-day modulation.
5. The Quasi-Biennial Oscillation is an oscillation in the average zonal winds in the tropical stratosphere. Roughly every 27-30 months, the tropical stratospheric winds are observed to shift from west to east (westerly), then from east to west (easterly). The QBO is thought to develop as a result of disturbances, or waves, in the tropical troposphere propagating vertically into the lower stratosphere.
Large variations in equatorial Pacific Ocean surface temperatures are known as the El Nino/Southern Oscillation (ENSO). Normally, the waters of the Eastern Pacific Ocean near South America are quite cool as a result of upwelling ocean currents. The tropospheric trade winds normally traverse from east to west in this region. However, in an El Nino period, the trade winds weaken, allowing the warmer waters of the Western Pacific to migrate eastward. This change in sea surface water temperature causes large-scale shifts in the global circulation patterns in the troposphere and lower stratosphere. This in turn effects the transport of ozone in these regions. This oscillation is very irregular, with a period of 4-7 years between episodes. The QBO and ENSO impact the tropical lower stratosphere. Because the QBO is more regular it is easier to account for its impact in trend analysis. However, both phenomena have some impact on ozone values.
6. The QBO not only changes the circulation in the tropics but it also indirectly causes changes at middle and high latitudes. The waves that cause the reversal of the tropical winds do so by exerting a drag on the flow. During one phase of the QBO, this drag induces a circulation through the stratosphere. This circulation enters the lower stratosphere in the tropics and leaves the middle stratosphere in the extratropics. During the other phase of the QBO, the induced circulation reverses itself. Momentum is thus exchanged between the tropics and the extratropics. Air is also transported between the two latitude regions. These air masses have different ozone concentrations. The induced meridional (i.e., in the latitude-altitude plane) circulation moves ozone poor air into the tropics and ozone rich air out of the tropics during one phase of the QBO and then reverses this process during the other. Ozone in the tropics thus decreases and then increases as we go through the QBO cycle. These induced circulations reach into the midlatitudes, increasing or decreasing the ozone concentration there in the opposite phase to what is happening in the tropics. In this way, the QBO affects ozone at all latitudes in the lower stratosphere even though the QBO is a tropical phenomenon.
7. Once we know the cyclical variations in ozone, we can begin the process of removing these signals in order to determine any long-term ozone trend. After developing a model of the long-term ozone time series, including natural variations and a long-term trend, we can then determine if humans activities have affected the amount of total ozone in the atmosphere.