The atmosphere is continuously in motion. These motions occur over different time scales, ranging from a single day to several decades. It is convenient to separate atmospheric variations into the following four time-scale categories: short-term, seasonal, year-to-year, and long-term. We give examples of variations with these four time scales in the next four subsections.
The study of atmospheric motions makes up a branch of atmospheric science called dynamics, which is discussed in detail in Chapters 2 and 6. Ozone in the lower atmosphere (i.e. the troposphere and lower stratosphere, see Chapter 2 for definitions) has a long lifetime, on the order of months to years. (The "lifetime" of an ozone molecule refers to the period between the creation and destruction of this ozone molecule, as explained in Chapter 3.) This is because the photochemical processes that create and destroy ozone occur very slowly in the lower atmosphere. This, in turn, is because the ultraviolet radiation that drives ozone photochemistry is mostly screened out from the lower atmosphere by the ozone higher in the atmosphere. The primary source of variability in the lower atmosphere is transport processes. As a result, ozone in the lower atmosphere acts as a tracer of atmospheric motions. The specific stratospheric dynamics that drive ozone transport -- i.e., the circulation pattern of ozone -- are detailed in Chapter 6.
Short-term variability refers to day-to-day and week-to-week variations. For example, the effects of the passage of a weather system are classified as short-term variability. A global map of ozone for a given day looks very much like a weather map with high and low ozone amounts corresponding to weather systems, though in the reverse (anticorrelated) sense to high and low pressure systems. The map for the next day will show movement of both these weather systems and the ozone amounts.
The short-term variability occurs on top of a general pattern that repeats every year. This repeating variation is called an annual cycle or a seasonal cycle. This is very much analogous to the annual cycle of temperature near the surface. We know that winter will be cold and summer will be warm. We also know that some winter days are colder than others just as some summer days are warmer than others. Ozone amounts vary in a similar manner. At northern mid latitudes, ozone amounts will be larger in winter and early spring and smaller in summer and fall.
The shape and amplitude of the annual distribution will not be precisely the same from year to year. This year-to-year variability will be referred to as interannual variability. Again we can make an analogy to temperatures. We know that some winters are exceedingly cold on average while others are relatively warm. Likewise, some winters have large amounts of ozone while others have small amounts. Interannual variability can be somewhat regular, like the Quasi Biennial Oscillation (QBO) described in Chapters 3 and 9; or it can be irregular, varying with no particular evident pattern.
Long-term variability is a catch-all term for variations with time scales typically on the order of decades. Cyclic variability with a time scale longer than our available measurement record make up one class of long-term variability. One cause of long-term variability is a trend. For instance, long-term variability in the ozone loss process can result in a trend in ozone amounts. Such long-term variability can have other causes, such as the gradual build-up (i.e. a trend) in the amount of ozone-destroying chlorine in the stratosphere from chlorofluorocarbons (or CFCs; see Chapter 11).
The amount of ozone at any location depends on variations on all of these time scales. The ozone distribution varies by latitude, with different seasonal cycles at different locations. Such variations arise because of the overall ozone circulation pattern known as the Brewer-Dobson circulation, which is discussed in detail in Chapter 6. Here we will explore variations for a typical northern mid-latitude location, Washington, D.C.
Figure 8.01 shows the measurements of the total column amount of ozone (per square centimeter) at Washington, D.C. (latitude=39°N) from January 1991 through December 1992.
The jagged black curve consists of daily ozone measurements for 1991-92. The heavy blue line gives an estimate of the mean annual cycle from an average of 15 years of data from 1979 through 1993. The gray-shaded area shows the range of daily measurements over this 15-year time period. Note that the daily measurements of total column ozone vary about the mean annual cycle. These variations are greater in winter than in summer. The mean annual cycle increases during winter, from about November to April, and this increase occurs sporadically. The cycle decreases during summer in a more regular manner.
The bottom panel of Figure 8.01 is an expansion of the top panel showing the total ozone measurements for the month of January 1992 only. The daily ozone measurements are shown in black, and the average for the month is the heavy red line. The average ozone measured for January 1982 (ten years earlier) is shown by the pink line. Note that the typical daily variations are 10-20 Dobson Units or about 5%. The largest changes up or down from one day to the next are about 50 DU or 15%. Of particular note is that the January 1992 average is 25 DU less than the January 1982 average. This will be shown in Chapter 9 to be due to a combination of interannual variability and long-term variability (which, in turn, may represent a long-term downward trend in total ozone).