Summary UV Bulletin

Summary of the 2007 ozone hole season, issued 2/28/08

Synopsis:
The size of Antarctic ozone hole in the austral fall of 2007 was slightly above the 10-year average, both in depth and overall area. The ozone hole area reached a maximum of approximately 25 million km² in mid-September, according to data of the Ozone Monitoring Instrument (OMI). The minimum total ozone column within the vortex was 107 Dobson Units (DU) and was observed by OMI on 30 September 2007. While the 2007 ozone hole was not as deep and large as the ozone hole of 2006, it lasted exceptionally long. Small areas with ozone columns below 230 DU were still present during the last week of December.

The most important parameters affecting ultraviolet (UV) radiation at the Earth's surface are the Sun's elevation above the horizon, the total column amount of ozone, and cloud cover. UV intensities in Antarctica are particularly large when total ozone is low during November and December, months with large solar elevations. The late break-up of the ozone hole in 2007 led to record-high UV levels during December at the South Pole. Unusually large UV levels were also observed during the second half of December at McMurdo. The effect of clouds is reduced at Antarctic sites compared to mid-latitudes due to moderation of cloud attenuation by highly reflective, snow- and ice-covered ground.

The center of the ozone hole was shifted toward the Atlantic (away from McMurdo) during most of October 2007. As a consequence, McMurdo was outside the perimeter of the ozone hole during much of October, and UV levels were well below average between 19 September and 26 October. During 5 – 16 November, the ozone hole was centered above the South Pole and this led to above-average UV intensities both at McMurdo and at the South Pole. Palmer Station is located 2800 km away from the South Pole and is even more affected by the position of the ozone hole than McMurdo. UV intensities at Palmer varied about the long-term mean during the austral spring of 2007 and exhibited large day-to-day variability. A record-high UV Index of 8.2 was observed on 28 December 2007 when a pocket of low ozone moved over the station. UV levels at Ushuaia also varied about the long-term mean with the exception of two distinct peaks on 17 October and 23 November when the edge of the ozone hole moved over this southern city.

South Pole, Antarctica:
Total ozone at the South Pole remained below 150 DU up 10 October 2007 according to OMI and SUV-100 measurements. Between 11 October and 2 November, total ozone was generally above the long-term mean (calculated from measurements of the years 1991 – 2006) and ranged from 140 to 198 DU. Between 3 – 28 November, total was below the mean; the lowest value during this period was 151 DU and observed on 9 November 2007.

The ozone hole is typically closed by the first week of December and total ozone at the South Pole returns to values above 250 DU thereafter. For the last three weeks of December, the average total ozone column calculated from observations of the years 1991 – 2006 is 280 DU. Measurements from 2007 were substantially below this climatological mean: total ozone ranged between 189 DU and 250 DU for the period 9 – 18 December and between 230 and 250 DU for 24 – 29 December. The minimum of 189 DU was observed on 12 December.

UV intensities in October were somewhat below the long-term mean but increased substantially between 1 and 17 November in response to rising solar elevations and dropping ozone columns. UV levels were significantly above the long-term average between 5 November and 29 December, with the exception of a short period between 28 November and 2 December. The largest increase was observed on 11 December when the UV Index was almost 50% larger than the climatological average.

Figure 1 compares measurements of the UV Index at the South Pole from 2007 with historical measurements of the years 1991 – 2006. Due to the influence of the ozone hole, UV Indices show a large year-to-year variability between October and December (austral spring). The variability for the months January – March (austral summer) is very small. The UV Index in 2007 was below average for most of October and above average for most of November and December. Daily maximum UV Indices observed during the latter two months were between 1.3 and 3.4. Record-high UV Indices were recorded between 9 – 14 and 26 – 29 December. Measurements during these two periods exceeded maximum UV Indices of the last 16 years (gray line in Figure 1). The largest UV Index in 2007 was 3.4 and was observed on 11 December.

Figure 1: Measurements of the UV Index at the South Pole performed between 1991 and 2007. Black dots show the daily maximum UV Indices for the entire period. The thick red line indicates the mean of all measurements. Red shading marks the area between the 10^th and 90^th percentile of all measurements. Overall maximum UV Indices of all years are indicated by a thin gray line. Daily maximum UV Indices measured between September and December 2007 are shown by the blue line. The vertical broken lines indicate the summer solstice (22 December), and the vernal and autumnal equinoxes. All data are from the "Version 2" revision of the NSF UV Monitoring Network.

Due to lower solar elevations at the South Pole, UV levels are generally smaller than at the Antarctic coast. However, human exposure to UV radiation is significant even for UV Indices below 3 due to radiation reflected off of snow.

McMurdo Station, Antarctica:
Total ozone observation performed at McMurdo Station between 19 September and 27 October ranged from 180 to 360 DU. With few exceptions, ozone columns were well above the long-term mean for this period. The large ozone amounts were mostly a consequence of the position of the ozone hole away from McMurdo. Between 2 – 19 November, McMurdo was below the ozone hole with total ozone ranging from 153 to 220 DU. Total ozone between 8 – 27 December ranged between 246 and 300 DU, which is significantly below the long-term average of 299 DU.

UV intensities were well below the climatological mean (calculated from measurements of the years 1989 – 2006) up to 26 October. The daily maximum UV Index during this period was smaller than 2.3 (Figure 2). The situation reversed for the period of 2 - 19 November when UV levels were above average. The most prominent peak in UV occurred between 15 and 17 November when the UV Index was 5.7, which is 50% above the long-term mean. Between 21 - 30 November, UV intensities were again below average. From 9 to 20 December, and also on 23 and 24 December, UV intensities were close to the overall maximum calculated from measurements of the last 18 years. The daily maximum UV Index was between 4.5 and 5.6. The latter value occurred on 19 December and was 25% above the long-term mean.

Figure 2: Measurements of the UV Index at McMurdo performed between 1989 and 2007. Data sets are similar to those of Figure 1.

Palmer Station, Antarctica:
Palmer Station was affected by the ozone hole during most days between mid-August and mid-October. Total ozone during these months was below 230 DU, except for four short periods of 17 – 20 August, 4 – 8 September, 16 – 19 September, and 7 – 8 October. Between 22 October and 7 November, Palmer was outside the perimeter of ozone hole; total ozone was larger than 400 DU between 27 and 30 October. An unusually small ozone column of below 290 DU was observed between 26 and 30 December when the remnants of the (by now dissolved) ozone hole passed over the station.

UV intensities followed the dynamics of ozone variations but were also greatly modified by cloud cover. UV levels generally stayed within the range defined by measurements of the last 17 years. UV levels were particularly low between 22 and 30 October when total ozone was high. Peaks in UV occurred for the periods of 29 September – 4 October (daily maximum UV Index 4.8), 16 October (UV Index 6.7), 9 November (UV Index 6.6), 19 November (UV Index 8.2), and 22 November (UV Index 8.9). UV Indices as high as 14 have been observed historically during the first week of December when ozone-depleted air masses moved over Palmer Station. Measurements in 2007 remained below historic records with the exception of relatively high UV levels on 28 December. The daily maximum UV Index on this day was 8.2, which is about 70% larger than the climatological mean.

Ushuaia, Argentina:
Total ozone at Ushuaia varied from 169 to 402 DU between September and December. Ushuaia became affected by the ozone hole on 14 September (229 DU), 22 – 24 September (169 - 204 DU), 28 September (213 DU), 4 – 5 October (200 – 227 DU), 16 – 17 October (183 – 228 DU), and 23 November (240 DU).

UV intensities varied about the long-term mean calculated from measurements from the last 18 years, with few exceptions. Most notable were spikes in UV on 17 October (daily maximum UV Index 7.1) and 23 November (UV Index 8.8). Both spikes coincided with sharp drops in total ozone. The highest UV intensities were observed on 28 December (UV Index 9.4), a day with scattered clouds and a comparatively low total ozone column of 286 DU. Typical UV Indices for November and December are 5 and 6, respectively, but UV Indices as high as 11 have been observed historically.

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