CONTACT: Barbara McGehan FOR IMMEDIATE RELEASE
Dr. David Hofmann 7/2/96
Ozone over the Hawaiian Islands reached record lows during the winter of 1994-95, National Oceanic and Atmospheric Administration scientists reported in the June issue of Geophysical Research Letters. Ozone values this low have not previously occurred over populated areas except on rare occasions near the extreme northern and southern latitudes.
According to the study, total ozone dropped below 200 Dobson units (DU) in December 1994 for the first time since measurements began at the Mauna Loa Observatory over 30 years ago. Dobson units are a measure of the thickness of the ozone overhead. The ozone layer protects humans and other biological systems from harmful ultraviolet (UV) radiation.
Ultraviolet radiation is measured at Mauna Loa by the Smithsonian Environmental Research Center. According to their measurements, as ozone decreased from 260 to 200 DU (a 23 percent decline) between October and December, UV rays from the sun increased, resulting in about a 40 percent increase in the sun- burning portion of the solar spectrum. It has been difficult to verify this expected relation between decreasing ozone and increasing UV over populated regions because of interfering clouds and air pollution. The pristine, cloud-free atmosphere at Mauna Loa, however, is ideal for such measurements.
According to lead author David Hofmann, director of NOAAþs Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., normally winter in Hawaii is the time of year for minimum ozone and maximum UV. "But some winters have lower ozone amounts than others," he says. "For example, there have been 12 winters in the past 31 years with ozone levels that ranged from 5 to 10 percent below the average winter value. The winter of 1994-95, however, was one of the more unusual ones, with ozone levels at about 13 percent below normal."
Comparing the ozone records at Mauna Loa with equatorial stratospheric winds, the researchers found that each low ozone winter corresponded to a year when the wind changed direction from easterly to westerly during the previous summer.
They believe that these low ozone events occur due to vertical motions of the ozone layer in the tropics which vary depending on whether the winds at the equator are westerly or easterly. Low ozone occurs only following a change in wind direction from east to west, which happens on average only every three years or so. Using these results, it will be possible to predict when a low ozone winter in Hawaii will occur simply by observing the pattern of stratospheric winds at the equator during the previous summer. This information could be beneficial to vacationers who would be interested in knowing when high UV winters in Hawaii are likely to occur.
Hofmann and colleagues indicated that while the average difference in ozone between a "low" ozone winter, typically 220 DU, and a "normal ozone winter," typically 240 DU, is only about 8 percent, the amount of UV that causes sunburning would be about 15 percent higher.
They cautioned that while this was true for Mauna Loa at 11,150 feet altitude, the intervening atmosphere would tend to reduce the amount of UV on the beaches; however, the percentage increase in UV between a high and low ozone winter would be similar.
###For more information on the Climate Monitoring and Diagnostics Laboratory, which is part of the Commerce Department's National Oceanic and Atmospheric Administration, please check the CMDL home page at http://www.cmdl.noaa.gov
To receive press releases electronically, send an e-mail to jslaff@rdc.noaa.gov