Active winter in the stratosphere ends

There is evidence for the approaching end of the winter also in the stratosphere:
Large-scale dynamic activity has been increasing since mid December 2009 and seems to collapse during the next days - an indication for the reversal from winter to summer circulation.

Up to 30 years ago, when for the first time the ozone hole appeared, hardly anybody was interested in the altitude range between about 15 to 50 km, called stratosphere. Based on the fact that about 50 % of the atmospheric air masses are situated below a height of 5 km, the focus was concentrated on this region, which influences directly human life through changing weather. However, the ozone maximum in the stratosphere, the so-called ozone layer shielding us from UV radiation, dangerous in large amounts, also directly impacts us.

The occurrence of the ozone layer in general and the ozone hole in particular depend on both, chemical as well as dynamical atmospheric processes, so-called planetary waves. Planetary waves are wave phenomena, that span the whole globe and that are responsible for large-scale energy and momentum transport in the atmosphere. In certain altitude range ozone follows the movement of planetary waves, thus making them visible. When studying the ozone concentration in the atmosphere a picture of the dynamical situation is being produced as well. Such analyses are being generated on a routine basis in the form of the so-called Dynamical Activity Index (DAI), daily updated and made available in the ICSU/ WMO World Data Centre for Remote Sensing of the Atmosphere, WDC-RSAT.

The DAI displays the activity of planetary waves calculated separately for the particular stratospheric hemisphere. It typically increases during winter until it collapses about March (see Figure 1). This is connected with the change of the atmospheric circulation from winter to summer conditions. The continuous increase is usually interrupted once per winter, mostly accompanying a so-called stratospheric warming, one of the most dramatic phenomena of the atmosphere: the very cold winterly stratosphere warms up by some ten degree Kelvin within only a few days, at times there even is a large-scale reversal of the circulation, a short-term change from winter to summer. This phenomenon opposes the development of the ozone hole, because air masses from low latitudes are being mixed with polar air masses, shown in Figure 2. Very distinctly the displacement and the break open of the low-ozone region can be recognised on 26 January 2010 based on the data of the GOME-2 mission on the satellite MetOp, being assimilated into a 4D-Var atmospheric model on a routine basis at DLR and in cooperation with EUMETSAT. Stratospheric warmings are an essential reason for the classical ozone hole having been observed only in the southern hemisphere up to now. There are also indications of a stratospheric warming in the DAI time series of the current winter season at the end of January 2010 (see Figure 3). Following this event the activity of planetary waves is increasing again. It is marked by pulses of various strengths that can be attributed to planetary waves themselves and their interactions. For the next few days the actual rise is expected to end due to the final reversal of the circulation.

 

Figure 1: The DAI of the northern hemisphere based on OMI data (Source: http://avdc.gsfc.nasa.goc/index.php?site=2045907950) is marked by relatively high amplitudes during months December to March.

Figure 2: The region of low ozone, marked bright blue, is not located symmetrically around the North Pole, as would be the case for undisturbed conditions. Instead it is moved towards northern Europe and disintegrated above the northern Atlantic. This is a significant indication for a stratospheric warming.

Figure 3: The DAI time series of the current winter shows a rapid decrease of the
dynamical activity end of January of this year, which hints to a stratospheric warming.

 

For more information on the Dynamic Activity Index (DAI) based on OMI data please visit: http://wdc.dlr.de/data_products/DYNAMICS/dai_latest.php

GOME-2 images and value added products are available at WDC-RSAT: http://wdc.dlr.de/sensors/gome2

For further information and/or questions, please contact wdc@dlr.de.