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HOME > Stratosphere Home > Winter Bulletins > Northern Hemisphere Winter 1999-2000 Summary
Northern Hemisphere Winter Summary

Northern Hemisphere Winter Summary


National Oceanic and Atmospheric Administration

April 2000

National Weather Service

National Centers for Environmental Prediction



  • Angell, J.K. ERL/Air Resources Laboratory
  • Flynn, L. NESDIS/Climate Research and Applications Division
  • Gelman, M.E. NWS/Climate Prediction Center
  • Hofmann, D. ERL/Climate Monitoring and Diagnostic Lab.
  • Long, C.S. NWS/Climate Prediction Center
  • Miller, A.J. NWS/Climate Prediction Center
  • Nagatani, R.M. NWS/Climate Prediction Center
  • Oltmans, S. ERL/Climate Monitoring and Diagnostic Lab.
  • Wang, J. ERL/Air ResourcesLaboratory
  • Zhou, S. Research and Data Systems Corporation

Concerns of possible global ozone depletion (e.g., WMO,1999) have led to major international programs to monitor and explain the observed ozone variations in the stratosphere. In response to these, and other long-term climate concerns, NOAA has established routine monitoring programs using both ground-based and satellite measurement techniques (OFCM, 1988).

Selected indicators of stratospheric climate are presented in each Summary from information contributed by NOAA personnel. A Summary for the Northern Hemisphere is issued each April, and, for the Southern Hemisphere, each December. These Summaries are available on the World-Wide-Web at the site with location products/stratosphere/winter_bulletins

Further information may be obtained from:

Melvyn E. Gelman
NOAA, Climate Prediction Center
5200 Auth Road Camp Springs, MD 20746-4304
Telephone: (301) 763-8071 ext.7558
Fax: (301) 763-8125



For March 2000, over the Arctic, total ozone values were 10 to 20 percent lower than the average since 1979, and were lower than values observed during spring since 1997. Lower stratosphere temperatures observed over the north polar region during December to March were also substantially lower than average. These cold conditions led to widespread chemical destruction of ozone in the lower stratosphere over the Northern Hemisphere polar region. At middle latitudes, total ozone values ranged from near the long-term average to slightly lower than average. Total ozone has generally decreased over the midlatitudes of the Northern Hemisphere at the rate of 2 to 4 percent per decade, from 1979 to the early 1990s, but the downward trend has not continued in recent years. The amounts of chlorine and other ozone destroying chemicals in the stratosphere in recent years have been reported to have reached peak values around 1997-98. Year to year differences in north polar winter-spring stratospheric ozone destruction may be explained as being due to different conditions associated with interannual meteorological variability.


The data available are listed below. This combination of complementary data, from different platforms and sensors, provides a strong capability to monitor global ozone and temperature.

Method of Observation

Parameter Ground-Based Satellite/ Instrument
Total Ozone Dobson     NOAA/ SBUV/2
Nimbus-7/ SBUV
Ozone Profiles Balloon - Ozonesonde NOAA/ SBUV/2
Nimbus-7/ SBUV
Temperature Profiles Balloon - Radiosonde NOAA/ TOVS

We used the total column ozone data from the NASA Nimbus-7 SBUV instrument from 1979 through 1984; NOAA-9 SBUV/2 from January 1985 to December 1988; the NOAA-11 SBUV/2 from January 1989 to December 1993; the NOAA-9 SBUV/2 from January 1994 to December 1996; the NOAA-14 SBUV/2 from January 1997 to June 1999, and the NOAA-11 SBUV/2 since July 1999. Solar Backscatter Ultra-Violet instruments can only produce data for daylight-viewing conditions, so no SBUV/2 data are available at high polar latitudes during winter darkness. Also, NOAA satellites have precessed in their orbit, so varying amounts of data at sub-polar latitudes have been available, depending on the local time of viewing of each satellite during winter months. NOAA-11 total ozone data since July 1999 have not yet been fully validated. From comparisons of coincident data, however, we know that recent NOAA-11 total ozone amounts may be about 2 percent too high. This impacts results determined for the recent period.


Figure 1 shows monthly average anomalies of zonal mean total ozone, as a function of latitude and time, from January 1979 to March 2000. The percent anomalies are derived relative to each month's 1979-2000 average. Largest anomalies are shown in winter and spring months for the polar region of each hemisphere. In the north polar region, positive anomalies of more than 10 percent prevailed in 1979 and the early 1980s, but mostly negative anomalies predominated in the 1990s. Exceptions to this were in the winter and spring of 1997-98 and 1998-99, when positive zonal mean total ozone anomalies were seen. However, for March 2000, negative anomalies were again observed in high latitudes of the Northern Hemisphere. In middle latitudes, near average conditions or slightly negative anomalies prevailed. The Scientific Assessment of Ozone: 1998 (WMO, 1999) reported that total column ozone decreased at northern midlatitudes (25-60N) between 1979 and 1991, with estimated linear trend downward of 4 percent per decade. However, since the recovery from the 1991 Mt. Pinatubo volcanic eruption, the downward trend of total ozone has not continued. The trend for the middle latitudes, based on the SBUV and SBUV/2 data sets and updated from 1979 through March 2000, is -1.2 percent per decade for 30-40 N, and -2.4 percent per decade for 40-50 N, with a 95 percent confidence estimate of 2 percent. No significant trend has been found over the equatorial region. In the tropics, high anomaly is seen in 2000, part of a quasi-biennial oscillation of total ozone.

The NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) operates a 16-station global Dobson spectrophotometer network for total ozone trend studies. Figure 2 shows the total ozone data for four central U.S. stations from1979 through 1999. The large annual variation is a result of ozone transport processes which cause a winter-spring maximum and summer-fall minimum at northern mid-latitudes. Figure 3 shows the four-station average percent deviation from their long-term monthly means. Large negative anomalies are shown for these stations in the winter and spring of 1998-99, but with near average condition in autumn and winter 1999. These anomalies derived from ground-based measurements are consistent with the anomalies shown in Figure 1, from SBUV/2 satellite ozone measurements. Middle latitude total ozone values in the years since 1993 have not continued to decline as they had declined from 1979 to 1993. The implication of these changes needs to be examined in the context of changes in amounts of ozone depleting gases in the atmosphere and varying meteorological conditions.

The map in Figure 4 shows Northern Hemisphere monthly mean total ozone amounts for March 2000. Low ozone extends from low latitudes, over western Europe, to the Arctic region. High ozone predominates from northern Canada, across Alaska to Siberia. Figure 5 shows the monthly mean total ozone percent difference for March 2000 from the mean for eight March monthly means, 1979-1986 (Nagatani et al., 1988). The 1979 to 1986 base period is chosen because these values are indicative of the early data record. Most notable are the negative anomalies of 10 to 18 percent over the Arctic and northern European regions. Negative anomalies of this magnitude were previously seen three years ago in March 1997, but not in the last two years. Positive anomalies appear throughout sub-tropical latitudes. As mentioned in the previous section, however, total ozone data from recent NOAA-11 are about 2 percent too high. That bias must be taken into account when interpreting results using the preliminary NOAA-14 data.

Figure 6 shows monthly mean temperature anomalies at 50 hPa for three latitude regions, 90N-65N, 65N-25N, and 25N-25S. The temperature anomalies for the winter and spring of 1999-2000 were below average values for north polar latitudes, and near average values for mid-latitudes and the equatorial region. The pattern of zonal mean temperature anomalies closely corresponds to the pattern of zonal mean ozone anomalies at middle and high latitudes.

Extremely low temperatures (lower than -78 C) over the Arctic region in the lower stratosphere are linked to depletion of ozone. Temperatures in the lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Low temperatures contribute to the presence of polar stratospheric clouds (PSCs). PSCs enhance the production and lifetime of reactive chlorine, leading to ozone depletion in the presence of sunlight (WMO, 1999). Daily minimum temperatures over the polar region, 65N to 90N at 50 hPa (approximately 19 km) are shown in Figure 7. During the winter and spring 1999-2000, daily minimum temperatures were much below average values. In January and February, temperatures increased markedly at 10 hPa and above, in association with stratospheric warming events. However the warming affected the lower stratosphere only weakly, so the lower stratosphere polar vortex remained cold, strong, and relatively undisturbed. This is discussed below.

Figure 8 compares the average 100 hPa temperature for each March of the last 22 years with the date the stratospheric polar vortex diminished below a specific threshold size. The size of the vortex was defined by the maximum in the gradient of potential vorticity contours at the 450 K isentropic surface, based on the NCEP/NCAR reanalyses. March 2000 was similar to 1998, about mid-way among the years in persistence of the polar vortex and also for average temperatures. Most other years in the 1990s had low temperatures, along with extended persistence of the polar vortex. Figure 9 shows the relationship between the persistence of the polar vortex and the persistence of high latitude total ozone values of less than 300 DU. The persistence of the Arctic polar vortex in the winter-spring of 2000 was similar to 1998, about average for all years. For 1999 the vortex size did not meet the size criteria, so it does not appear in Figure 9 and Figure 8. Meteorological conditions of a weak polar vortex in March 1999 were associated with limited ozone destruction and relatively high total ozone over the Arctic region. During 1996 and 1997, very low total ozone amounts were observed over northern latitudes due in part to an unusually cold, persistent vortex.

Figure 10 shows the average area, during February and March for each year since 1979, of low ozone (lower than 300 DU). For 2000, low total ozone was greater in extent than for the previous 2 years, but near the average for years since 1990.

A time series, from 1979 to February 2000, of normalized height anomalies from 1000 to 30 hPa, for the north polar region, is shown in Figure 11. Positive height anomalies were predominant in the 1980s, while negative anomalies appear in most of the 1990s. The winter and spring height anomalies were positive for the years 1997-98 and 1998-99. But height anomalies for 1999-2000 were negative, consistent with total ozone anomalies for 2000. Note that the height anomaly pattern in Figure 11 is very similar to the downward propagating Arctic Oscillation signature shown by Baldwin and Dunkerton (1999). Negative polar height anomalies are consistent with positive phase of the Arctic oscillation. Negative height anomaly in 2000 is consistent with the observed positive phase of the Arctic Oscillation. Investigations are underway concerning causes and effects of stratospheric variability, as they may be related to tropospheric and surface changes. For example, during the winter-spring of 1998-99, anomalies appear to propagate to the surface, whereas the 1999-2000 anomalies did not indicate a continuous downward propagation.


In the spring of 2000, negative anomalies of total ozone were prevalent in the high latitudes of the Northern Hemisphere. Lower stratosphere temperatures over the Arctic region during the winter and spring were much below average values. Temperatures were sufficiently low for ozone destruction to proceed on polar stratospheric clouds within the polar vortex. The spring conditions in the Arctic region in 2000 are in contrast to conditions in the Arctic region during 1999, when total ozone was above the average. Chlorine and other ozone destroying chemicals in the lower stratosphere reached peak values around 1997-98, and have remained at high levels. As a consequence, lower stratosphere ozone destruction is strong when meteorological conditions of a strong polar vortex and cold polar temperatures prevail. Low total ozone values in the Arctic region in 2000 are attributed to meteorological conditions which were favorable for ozone destruction, along with the continued presence of ozone destroying chemicals in the stratosphere.

Total ozone declined over mid-latitudes of the Northern Hemisphere at the rate of about 2 to 4 percent per decade from 1979 to 1993. In recent years the strong rate of decline of Northern Hemisphere total ozone has not continued, but current stratospheric ozone amounts continue to be below the amounts measured before the early 1980s. A full explanation of ozone and temperature anomalies must include all aspects of ozone photochemistry and meteorological dynamics. Continued monitoring and measurements are essential toward this end.


Baldwin, M.P. and T.J. Dunkerton, 1999: Propagation of the Arctic Oscillation from the stratosphere to the troposphere. J. Geophys. Res., 104, 30937-30946.

Nagatani, R.N., A.J. Miller, K.W. Johnson, and M.E. Gelman, 1988: An eight year climatology of meteorological and SBUV ozone data, NOAA Technical Report NWS 40, 125pp.

OFCM, 1988: National Plan for Stratospheric Monitoring 1988-1997. FCM-P17-1988. Federal Coordinator for Meteorological Services and Supporting Research, U.S. Dept. Commerce, 124pp.

WMO, 1999: Scientific assessment of ozone depletion: 1998. World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 44.




NOAA/ National Weather Service
National Centers for Environmental Prediction
Climate Prediction Center
5200 Auth Road
Camp Springs, Maryland 20746
Climate Prediction Center Web Team
Page last modified: September 11, 2002
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