It was exceptionally cold in the ozone layer in the past month. During the first half of March there were several days with record-low temperatures in the stratosphere at altitudes of about 20 km, and the cold period continued for the entire month. This led to sustained low ozone values over the polar region.

The area with low ozone over the North Pole was observed by several satellite sensors, e.g. SCIAMACHY on the EnviSat satellite, and the Ozone Monitoring Instrument OMI on EOS-Aura. This is demonstrated by the figure, which shows the monthly-mean ozone amount for March 2011 compared to the months March in the period 1979-2010. The situation this year resembles the years 1996, 1997 and 1990, where similar low ozone levels over the North Pole were observed.

The continuing cold spell in the stratosphere, where the ozone layer is located, was remarkable. In contrast, the previous years 2009 and 2010 showed an early so-called stratospheric warming in January and mid-February. Temperatures in the stratosphere in the first half of March this year were low enough for polar stratospheric clouds to form. In these clouds chlorine compounds are formed from CFCs which break down ozone over the North Pole when sunlight is present. In addition to the cold temperatures, the transport of ozone-rich air from the tropics was reduced this year because of the strong winds around the North Pole at 20km altitude.

The ozone layer over the North Pole nevertheless has a thickness of more than 250 Dobson units (a unit expressing the number of molecules ozone per square meter). This is much more than what is observed for the ozone hole over the South Pole in September-November. 250-300 DU is comparable to the thickness of the ozone layer in the tropics. The relatively large ozone breakdown leads to reduced ozone levels over the Northern Hemisphere this Spring because it will take some time before the lost ozone has been replenished.

The figure above shows the March monthly-mean total ozone values for the period 1979-2011 as it is monitored by MACC partner KNMI. The period 1979-2008 is derived from the multi-sensor reanalysis (MSR) of the ozone layer. The analysis for 2009-2011 is based on an assimilation of SCIAMACHY ozone columns.

The figure to the right shows a time series of the zonal mean total ozone values from the MACC near-real-time monitoring and forecasting system. This figure shows clearly the much lower ozone concentrations at high nothern latitudes this winter compared to the previous winter. Also visible are the ozone hole conditions over the Antarctic during October/November.

MACC partner BIRA-IASB has produced a video showing the development of the low stratospheric ozone concentrations from the MACC global system relative to the normal situation. The animation clearly illustrates the anomalous conditions this year compared to last year.