The ozone hole continues to develop. Ozone values in the northern Antarctic Peninsula had reached close to normal, but are now falling. Ozone values in the Weddell Sea area have declined in the past fortnight and are now around 35% below normal. A stratospheric warming occurred over the Weddell Sea area as the centre of the polar vortex displaced towards the Pacific.

a) Ozone. Routine ozone measurements from Halley station started towards the end of August. Prior to that, the sun was not high enough to permit measurements, however a few observations using moonlight were made. These suggest that total ozone values fell from an estimated 300 DU in early July to around 200 DU in mid August. Low accuracy measurements on the zenith sky have now commenced and these suggest that values then rose to around 240 DU and have since fallen to 190 DU in the past 10 days. Ozone values are comparable to those of the past few years, but well below the long-term mean, with depletion at around 35%.

Halley preliminary mean daily total ozone, (DU)
Dobson No 103: Instrument constants revised 1997 May 2.
(0 indicates no data)

1997 August 1 - 1997 September 4

b) Radiosonde data. Stratospheric temperatures at 100 hPa, near the peak of the ozone layer, can be used to reflect changes in ozone amount. Stratospheric clouds, which are crucial to the mechanism of ozone depletion, are likely to be present when the 100 hPa temperature is below -80 deg C and may persist whilst the temperature is below -75 deg C. Data from Halley show that the mean 100 hPa temperature increased substantially towards the end of August, from close to the long term norm of -83 deg C, to -78 deg C, near the upper limit of the long term range. Stratospheric clouds have been observed from the station.

a) Ozone. Ozone measurements from Rothera are made using a SAOZ (Systeme d'Automatique Observations Zenithales) spectrometer. This is a research instrument, but the daily ozone values are available. These show that the mean ozone values rose from around 230 DU at the beginning of August to 290 DU mid month and have since slowly declined to around 270 DU. Day to day variation is around 40 DU.

Rothera preliminary mean daily total ozone, (DU)
(0 indicates no data or data not available)

1997 August 1 - 1997 September 1

b) Stratospheric clouds. Nacreous or mother-of-pearl clouds are regularly seen from stations along the Antarctic Peninsula between early May and October, with a peak in July. There is some evidence that their frequency of occurrence has increased since the mid 1950s. This year they have been observed from Rothera on June 8, 10, 13, 15, July 4, 17, 21, 23, and August 4.

Routine ozone measurements commence at Vernadsky in early August, but some observations, of lower accuracy, are made in July. Mean total ozone values fell from an estimated 350 DU in early July to 240 DU in early August, then rose to over 300 DU by the end of the month (close to the long term mean). Day to day variation is around 30 DU. Particularly low values were recorded on August 2^nd, when the station was affected by one of the ozone reduction events associated with the forming ozone hole.

Vernadsky preliminary mean daily total ozone (DU).
Dobson No 31: Instrument constants revised 1997 August 15

1997 August 1 - 1997 August 30

TOVS satellite images from the US NCEP/NWS/NOAA Climate Prediction Center and EP/TOMS images from the US NASA/GSFC show the developing ozone hole. Intermittent ozone reduction events occurred during late July and early August. The ozone "hole" is beginning to grow and deepen and has become relatively elongate. There are now several separate areas within the vortex showing significant ozone depletion. The normal circumpolar high ozone belt is present. UK Met Office analyses also show that the polar vortex has become highly elongated, stretching from roughly 65-deg south, 160-deg east to 65-deg south, 40-deg west. The analyses and the radiosonde profiles from Halley and Neumayer show a strong warming event over the Weddell Sea area at the end of August as the centre of the vortex displaced towards the Pacific.

Note that all ozone values in this bulletin are preliminary and are subject to revision from time to time when the instrument constants are re-evaluated. All Dobson ozone data is reduced to the Bass-Paur scale as recommended by the WMO. If you use or pass on this data please make acknowledgement to J D Shanklin, British Antarctic Survey.