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Ozone Change at the South Pole: A Forty-Year Record of ObservationsOzone Change at the South Pole: A Forty-Year Record of ObservationsSamuel Oltmans, Bryan Johnson, Robert Evans, Joyce Harris, David Hofmann

NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado 80305Dorothy Quincy, Mark Clark, Brian Vasel

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado, 80309

Introduction

Observations of total column ozone began at South Pole in 1961and have been made continuously since. During winter months Dobson observations can be made using the moon as a light source but with less precision than direct sun observations. Ozonesonde observations of the ozone vertical profile were done sporadically from 1967-1971 and regularly since 1986. Surface ozone observations began in 1975 and continue to the present.

September 2001

References:Hofmann, D.J., S.J. Oltmans, J.M. Harris, B.J. Johnson, and J.A. Lathrop, Ten years of ozonesonde measurements at the south pole: Implications for recovery of springtime Antarctic ozone, J. Geophys. Res., 102, 8931-8943, 1997.Harris, J.H., S.J. Oltmans, P.P. Tans, R.D. Evans, and D.L. Quincy, A new method for describing long- term changes in total ozone, Geophys. Res. Lett., submitted, 2001.

The extent of the depletion at each 2 km level (figure 9) shows that in the heart of the depletion layer (16-20 km) the loss rate is nearly one Dobson unit per day. In the 2 km layers just above and below this the loss is slightly less at 0.6-0.7 DU/day. While ozone loss rates increased from the mid 1980s when measurements began into the early 1990s (Hofmann et al.. 1997), they have generally remained near the same level over the most recent 5 years. This reflects the nearly complete ozone loss that has been achieved in the 14-20 km layer since 1993 (figure 7) that limits any additional loss. During all of the years since observations began in 1986 temperatures (figure 10) have dipped well below threshold temperatures required to form polar stratospheric clouds (PSC). The increasing September ozone loss rate during the early years of the measurements thus likely reflects the increasing burden of ozone destroying halogens in the stratosphere. Near the top of the ozone hole (22-24 km) the losses may be sensitive to the temperature. In both 1999 and 2000 ozone loss is near the maximum amount for this layer (figure 9). Temperatures in this layer were also at or below the lowest values in the record in September and early October.

Development of the Annual “Ozone Hole”During the austral spring column ozone amounts started to dip consistently well below 200 DU beginning in 1985. When ozonesonde observations were begun in 1986 a characteristic signature could be seen in the profile with ozone “missing” in the 15-20 km region in comparison with profiles obtained late in the previous winter. This region also had ozone amounts markedly lower than was seen in profiles obtained during spring back in the late 1960s. The rapid loss of ozone each spring can be seen in the series of profile obtained for 2000 (figure 5). The development of the depletion in relation to the seasonal pattern can be seen in the abrupt appearance of the low mixing ratios between 14-22 km in the seasonal cross-section (figure 6).

This notch in the ozone profile and its development over the years can be seen in the profiles obtained for minimum column ozone each year since 1986 (figure 7). Beginning in 1989 a region of near zero ozone amount began to appear above 15km. Since 1993 this has encompassed much of the region from 15-20 km.

Conclusions

Beginning in the 1970s ozone began to decline over the South Pole based on the total column ozone record. Recent years have shown nearly complete ozone loss in the 15-20 km layer in late September and early October. Although at some coastal Antarctic sites ozone loss in 2000 was quite dramatic early in the season, at South Pole ozone losses were typical of recent years. The vortex weakened and moved away from South Pole much earlier than any year since 1988, however.

Total Column Ozone Changes

Moon observations are infrequent due to adverse winter weather conditions and limited observing periods and optical measurements using the Dobson spectrophotometer cannot be made at all during twilight periods. To better define the year-round column ozone measurements integrated column ozone amounts from the ozonesondes are used to supplement the Dobson data during the dark and twilight months (figure 1). This new data set has been analyzed using a recently developed technique (Harris et al., 2001) for describing long-term column ozone changes.

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Figure 2: Ozone residuals (+) after removing known variations and fitting a tendency curve (solid line) found from filtering the residuals.

Figure 1: Monthly mean () and model (solid line) total ozoneAt South Pole that accounts for the seasonal, quasi-biennial, solar and detrended temperature dependencies in the data.

Figure 3: Instantaneous growth rate of total column ozone (solid curve) found by differentiating the tendency curve shown in figure 2. The dashed curves show the 95% confidence limits found using a bootstrap method.

Figure 4: Mean column ozone for the October 15-31 period of each year. Mid-October is the earliest in the austral springwhen direct sun Dobson observations can be made.

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The decline in ozone at South Pole began in the 1970s (figure 2) and has persisted through most of therecord since then. Growth rates were mostly negative (figure 3) throughout this period with the largest declines in the 1970s and mid-1980s. The overall growth rate (decline) was –8.0 ± 1.1 %/decade. Figure 7: The partial pressure of ozone with altitude for the profile obtained when the column ozone amount was a minimum.

Figure 5: The series of profiles showing the development of the “ozone hole” in 2000. Figure 6: Time-height cross-section of ozone mixing ratio in 2000.

The progression of the ozone loss in both the total column and the 12-20 km layer is marked by the sharp decline beginning early in September (figure 8). This is in sharp contrast to the profiles obtainedin the late 1960s and early 1970s (black dots in figure 8). Beginning in 1993 the 12-20 km layer has approached amounts near zero. There are large year to year differences in the recovery of the annualdepletion with recovery coming early in November as in 2000 or delayed as late as early December asin 1999. Because the date of the annual recovery has shifted to a later date beginning in the 1990sthere is some suggestion that the ozone loss has stabilized the vortex by maintaining the colderstratospheric temperatures later into the year. However, in 2000 though the loss was similar to that inthe 1990s, and actually appeared earlier than recent years, the recovery occurred early in November.

Figure 8: The total and 12-20 km column ozone obtained from individual ozonesonde measurements for various years. The black dots are for soundings obtained during the period 1967-1971.

Figure 11: a) South Pole surface ozone monthly mean mixing ratios (box) with the modeled fit (solid dots and dashed line) and smoothed curve fit (solid line); and b) instantaneous growth rate found by differentiating the solid curve in a).

Surface ozone mixing ratios at the South Pole decreased throughout the 1980s and early 1990s. However, during the late 1990s surface ozone amounts have recovered significantly to near levels seen before the decline began. The earlier decline appeared to parallel the ozone declines in the low stratosphere but the recovery in surface ozone does not reflect changes in the stratosphere.

Figure 9: Ozone column amount in 2 km layers with 1999 and 2000 values shown separately. Figure 10: 2 km layer average temperatures.JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

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