The ozonosphere could be regarded as stretching from the mesopause on the lower margins of the ionosphere to the surface of the globe. Within the ozonosphere the partial pressure of ozone is conditioned by numerous processes including diffusion downwards from the ionosphere, transport from areas of local production, destruction by ionisation and via chemical means and just plain mixing of ozone rich with ozone poor air.
Beyond the equatorial latitudes, at lower altitudes and at low sun angles ozone is safe from the pressure of ionisation. EUV is used up in the ionosphere above the mesopause. The ionisation of oxygen demands wave lengths shorter than 240 nm. Ozone, being a large molecule is ionised by UVB. The longer the atmospheric path, the less there will be of these destructive wavelengths because they are used up in the process. Recent work suggests that the complement of ozone in high latitudes is increased via cosmic ray activity. The safest zone for ozone is the winter hemisphere where the atmospheric path is long. Where the atmosphere is in the shadow of the Earth ionisation of ozone is not possible.
On that basis we would expect that ozone partial pressure should increase all the way to the surface of the planet. In practice, erosion from below by NOx prevents the increase in ozone partial pressure at lower elevations. This erosive process gives rise to a higher tropopause in the tropics where atmospheric uplift is most vigorous.Both chemical destruction and transport processes are instrumental in elevating the tropopause in low latitudes.
The polar vortex is another zone of ozone erosion and in this instance from above. This could be the most important source of change in the system. Inside the vortex a variable amount of ozone deficient air is introduced in winter. The feed rate depends upon surface pressure. As surface pressure declines so does the velocity of the zonal wind in high latitudes and the penetration of this mesospheric air.
This chapter looks specifically at aspects of vortex rotation and the mixing processes that are involved in determining ozone partial pressure in the wider ozonosphere.
PROCESSES WITHIN THE ANTARCTIC POLAR VORTEX
At 1 hPa the rotation of the atmosphere is west to east in the same direction as the Earth itself but at a faster rate. Zones of high ozone partial pressure (low surface pressure) form over the warmer waters in the lee of the continents and in particular in the western Pacific Ocean and to the south of Australia. These are zones of enhanced convection where ozone accumulates at the highest elevation. The data below is reported here:
Looking now from the polar perspective we can observe the ingress of ozone rich air into the vortex structure (circled) and using snapshots at six hourly intervals we can see the rate of rotation inside the vortex. Observe the structure that looks like a plant sprouting from soil. Follow the black circle to observe the rotation rate as this structure is carried about within the vortex.
Some features of the circulation worthy of note:
- The vortex at 1 hPa is not uni-cellar in structure but exhibits multiple cells of descent that drag in ozone rich air from the ozone rich periphery.
- The ‘periphery’ at 1 hPa represents an ‘annular’ or ring like structure, albeit quite asymmetrical in its ozone content.
- The diagrams span the time between zero hour on the 13th June to 6 am 15th June with plots at six hourly intervals.It takes 2.5 days for one full rotation to occur within the vortex.
- The zone of high ozone partial pressure outside the vortex does not rotate about the pole in 2.5 days. It is sticky, hanging in the East Indian- West Pacific sector. Here, ozone partial pressure is maintained in spite of the influence of erosive activity emanating from the lower mesosphere and perhaps some ionising radiation impacting from above (but likely very little). This node of enhanced ozone is fed from lower levels per agency of low pressure anticyclones that form near the tropopause, propagate to the surface and lift ozone rich air to the top of the atmosphere. These low pressure cells are ozone collectors. The air circulating within them morphs together to create the vortex upwards of 50 hPa. There is a very wide zone of low surface pressure between the Antarctic continent and the latitude of New Zealand to promote the sticky presence of low pressure cells.
- Ozone is continuously drawn into the multi vortex structure within the generally ozone deficient core of mesospheric air. This has the effect of raising the ozone partial pressure within the core as it descends thereby actively reducing the ozone differential between core and perimeter air. Mini vortex structures of elevated ozone partial pressure persist but only so long as they are supplied from the incomplete annular ring of ozone rich air. When cut off from a source of ozone rich air these mini vortexes lose ozone partial pressure and become invisible until they re-connect with the source of ozone rich air.
- New feeds of ozone rich air are created and drawn into the cone of descending mesospheric air from the ozone rich sector on a continuous basis.
- Only traces of virginal mesospheric air that is relatively deficient in ozone can be seen within the vortex. The rate of mixing ensures that there will be much less difference between the ozone content of the air inside and outside the vortex at the 50 hPa level. Nevertheless there will always be a substantial difference in air temperature across the vortex between internal air of mainly mesospheric origin and stratospheric air outside the vortex warmer in part because it derives from the mid latitudes. As we see below, there is a marked difference in the temperature of the air above the 250 hPa level in winter by comparison with summer. This shows us the extent of the descent of mesospheric air and its involvement in the evolution of the polar arm of the Jet Stream.
IMPLICATIONS FOR SURFACE CLIMATE
- An increase in the intake of mesospheric air will dilute the ozone content of the ozonosphere generally. As the ozone content of the air above the polar cap is diluted the temperature of the air will fall. Large variations in the temperature of polar cap air occur on inter-annual and longer time scales. As the ozone content of the air rises and falls so too does polar cyclone activity and with it there is a change in the distribution of atmospheric mass between high and other latitudes.This is the essence of the most significant modes of climate variability observed on the planet. These modes are well documented as the Arctic and the Antarctic Oscillations.These modes involve a change in the pressure differentials driving the planetary winds and therefore change in the equator to pole temperature gradient.
- The area to the east of the Antarctic peninsula tends to be ozone deficient and therefore the natural home for a high pressure cell of descending air. Another natural home for a zone of high surface pressure lies to the west of Chile where the ocean is very cool. A third is the Australian continent in winter. The strength of the pressure differential across the Pacific Ocean that drives the trade winds will depend on surface pressure in the broad ozone deficient zone to the west of Chile. This is part of the ENSO dynamic in the southern hemisphere because it determines the pressure differential that drives the trade winds across the Pacific. This differential changes on decadal and longer time scales. There is a similar dynamic driving change in the planetary winds in the North Atlantic and North Pacific.
- The strength of the west wind drift that is driven by the westerly winds in the Southern Ocean and the temperature of the waters streaming northwards on the western margins of South America depends upon the pressure differential between the mid latitudes and the margins of Antarctica. That depends in turn on the ozone content of the air in high latitudes that is responsible for the strength of polar cyclone activity. Polar cyclone activity determines the balance of surface pressure between mid and high latitudes.
THE CIRCULATION IN THE LOWER STRATOSPHERE
Data here. http://www.esrl.noaa.gov/psd/map/time_plot/
In the hovmoller diagram above we see a depiction of air temperature at 250 hPa. The diagram covers the year 2014 for the latitude band 30-40° south. A northwest to southeast pattern manifests strongly in winter. This is produced when cold ozone deficient air from the equator is drawn pole-wards. That air comes from under the high tropopause that prevails in near equatorial latitudes and it is ozone deficient, NOx rich and very cold, as cold in fact as the air that descends from the mesosphere over the pole. It must enter the circulation in the mid latitudes obliquely rather than directly because it must push into and under warmer ozone rich air present at the same elevation due to the low tropopause that prevails in high latitudes. The high latitude circulation is driven by polar cyclones on the margins of Antarctica. Here the air ascends and rotates faster as it ascends. The speed of the circulation depends in part on the strength of the zonal wind that is dependent on electromagnetic influences. It depends also on polar surface pressure that conditions the intake of mesospheric air. The polar cyclones are formed in the region between the low tropopause (8 km) that prevails in high latitudes and 100 hPa (18 km). In this zone there are marked differences in the density of the air according to its origin. These density differences are material to the development of polar cyclones that propagate downwards to the surface and send ozone rich air to the top of the atmosphere where it accumulates at 1 hPa and spreads out towards low latitudes, This ozone rich air is entrained in the descending vortex as described above.
The polar circulation ascends to the top of the atmosphere. The tropical circulation is limited to a high tropopause. What goes up must come down and the dominant zone of descent from the stratosphere is the high pressure cells of the mid latitudes. A smaller zone of descent is via the inside of the polar vortex.
Source of map below here
Notice that in this description of the way in which the wind blows I do not refer to a ‘coriolis force’. There is no such force. This is a meteorologist’s rule of thumb. Nor do I refer to ‘tropopause folding’ or ‘surf zones’ The circulation of the atmosphere is set in high latitudes where its rate of rotation is fastest and it is a product of circumstances that manifest most strongly in winter. Its engine is located between the 300 hPa and the 50 hPa pressure levels. That engine is the difference in air density across the vortex.
Now let us look at this circulation in terms of the distribution of NOx and ozone near the tropopause.
We are looking at a polar stereo-graphic view of the southern hemisphere with Antarctica central. The light grey line overlaid on the diagram at left traces the feathery edge of air with an appreciable NOx content. That line is duplicated, rendered in black and overlaid on the ozone diagram at right. It is apparent that the distribution of ozone south of about 30° south latitude is entirely the product of the distribution of NOx. NOx catalytically destroys ozone. NOx is not apparent in the yellow areas but these are interaction zones where NOx has already done some work in reducing the ozone content of the air.
Let us now examine the circulation at 50 hPa and 100 hPa by tracking the passage of NOx rich cold air of tropical origin into the ozone rich warmer, less dense air at high latitudes. Let us remember that surface pressure is determined by the ozone content of the air. Surface pressure is much lower on the margins of Antarctica. That requires that cold, dense ozone deficient air must flow from the low and mid latitudes to high latitudes where the air is ascending to the top of the atmosphere as in a chimney. The return flow is from the top of the atmosphere. We should be able to track the ingress of NOx rich air anywhere between the 300 hPa and 50 hPa pressure levels. Data is available for the 50 hPa and 100 hPa pressure levels here and is reproduced below.
It is apparent that the air from mid and low latitudes is drawn into the circulation on the margins of Antarctica and progressively loses its separate identity in the process. At the 100 hPa level, the level of the tropical tropopause, that the great contrasts in atmospheric temperature and density are to be found. This is approximately the level where polar cyclones are formed and jet streams generated. According to the contrasts in the ozone partial pressure, temperature and air density polar cyclones wax and wane in activity, shifting atmospheric mass to and from high latitudes.
Here we are looking at the origin of the inter-annual modes of natural climate variation. But it is more than that. We are looking at the engine that drives weather on all time scales. The beating heart of this engine is ozone. The distribution of ozone is not the product of the system. The system is the product of the distribution of ozone.