Some weird and wonderful ideas have been put forward to explain ‘sudden stratospheric warmings’ and the relative abundance of ozone in the northern hemisphere by comparison with the southern hemisphere. Consider the following passages:
From the UK Met Office:
Sometimes the usual westerly flow can be disrupted by natural weather patterns or disturbances in the lower part of the atmosphere, such as a large area of high pressure in the northern hemisphere. This causes the Polar Jet to wobble and these wobbles, or waves, break just like waves on the beach. When they break they can be strong enough to weaken or even reverse the westerly winds and swing them to easterlies. As this happens, air in the stratosphere starts to collapse in to the polar cap and compress. As it compresses it warms, hence the stratospheric warming.
And a somewhat more academic approach from: ‘Dynamics, Stratospheric Ozone, and Climate Change’. Theodore G. Shepherd* Department of Physics, University of Toronto 60 St. George St., Toronto ON M5S 1A7 2007
Accessed at: http://www.tandfonline.com/doi/pdf/10.3137/ao.460106
Between early fall and late spring, when stratospheric winds are westerly (reflecting, through thermal-wind balance, the low temperatures found in polar regions due to the lack of solar heating), planetary-scale Rossby waves generated in the troposphere by topography and land-ocean thermal contrasts can propagate up into the stratosphere where they grow in amplitude, break, and dissipate. In this process the waves transfer negative angular momentum from the troposphere to the stratosphere and the resulting negative torque, known as wave drag, drives a poleward circulation in the stratosphere. Mass continuity and the relaxational nature of infrared radiative cooling then leads to upwelling in the tropics and downwelling in the extratropics. The effect on ozone is to bring ozone down from its production region in the tropical upper stratosphere into the extratropical lower stratosphere, where its lifetime is long. Although transport in itself cannot change ozone abundance, the rapid chemical replenishment of ozone in the upper stratosphere together with the poleward transport implies a net increase in extratropical column ozone during the winter-spring season.
We have in this extract references to:
- The cooling of the atmosphere in high latitudes due to lack of solar heating in winter.
- Planetary scale Rossby waves (i.e. the polar front) that is ‘different’ in the northern hemisphere driving the Brewer Dobson pole-ward circulation bringing ozone into the extra-tropics of the northern hemisphere supposedly pumping up the ozone content of the northern hemisphere with less activity in the southern hemisphere where there is accordingly less ozone.
- These waves being more active in winter implies a net increase in extratropical column ozone during the winter-spring season.
Thereby contriving to persuade us that all change begins in the lower troposphere, or in other words ‘it is the tail that wags the dog’.
There is some resemblance to poetry “growing in amplitude, propagating, breaking out and disappating while wave drag, known as negative torque that is associated with mass continuity considerations involving upwelling in low latitudes and downwelling in the extratropics conveys ozone into a sweet spot where its chemical lifetime is long.”
No, its gibberish. The imagination is a wonderful thing but lets start with things that can be observed.
THE WORLD OF ‘CLIMATE SCIENCE’
In winter as surface atmospheric pressure increases a tongue of very cold, ozone deficient mesospheric air penetrates the stratosphere over the poles to about the 300 hPa pressure level. The upper third of the atmosphere is affected by the influx of this air. If one admits this phenomenon and nevertheless wishes to deny any effect on the wider atmosphere one must conceive of an impenetrable wall between the mesospheric air and the wider stratosphere stopping any interaction. That is common. It is embodied in the term ‘Strong vortex’. One must conceive of cyclones of ascending air with a cold core that ‘just happen’, perhaps made up of just ‘air’ in which case it is easy to imagine that the particular sort of air that is uplifted is in inexhaustible supply and will have no consequences for the distribution of ozone, the temperature of the air at elevation or geopotential height that is so carefully monitored here. One can then remain blissfully ignorant of the fact that the massive uplift that occurs via the vortex (not at the equator) proceeds to the top of the atmosphere (not just to the tropopause) at 1 hPa inflating upper air temperature (need another reason for that rather than replacement of cold air with warm air). One must never ask the question what happens to the air so uplifted; is there a balancing descent anywhere else? Does the upper air spin out towards the equator? Is it entrained in the descending air of the mid latitudes? One must never admit that the air so uplifted is ozone rich producing pockets of ozone rich air above sticky low pressure systems that tend to establish over warm waters in the lee of the continents. One must remain blissfully unaware that it is uplift by polar cyclones that is responsible for low surface pressure in high latitudes attracting NOx rich and ozone starved air from tropics between the 300 hPa and the 50 hPa pressure levels giving rise to severe contrasts in atmospheric density that is responsible for the continuous regeneration of these polar cyclones, and in spring at the time of the final warming trapped below 50 hPa to constitute the ‘ozone hole’.
There is cold air on one side of the Polar Front and much warmer air on the other side. The green line on the map below traces the front.
Below we see the wind speed response to the differences in the character of the air across the front.
There is a very large difference between the hemispheres in terms of the temperature of the air in the atmospheric column above the winter pole as documented below.
We can notice that:
- The Arctic is warmer at the surface than the Antarctic for most months of the year.
- There is not much difference in the temperature of the air at 300 hPa between the two polar caps in summer but in winter the Antarctic air is colder and the higher the elevation the colder it is.
- The Antarctic air is warmer at the highest elevations in summer due in part to 6% greater solar irradiance in January due in turn to orbital considerations (proximity of the sun in January) despite the fact that the surface is much colder than the Arctic.
- The surface of both polar regions is colder in winter than the air above it. The near surface air is warmer because it has come from lower latitudes and to some extent it may be warmer than the surface due to the presence of ozone.
- The temperature of the air above the 300 hPa pressure level is unrelated to the temperature of the surface, or cooling processes during the polar night or the lack of irradiance during the polar night. Very cold air arrives in winter. It arrives from above. Because its ozone content is light and the flow in Antarctica is enhanced by comparison with the Arctic this mesospheric air dilutes the ozone content of the air in the southern hemisphere and indeed globally. Its flow rate is a major influence on the ozone content of the air in the southern hemisphere on all time scales. There may be other influences, equally important like ozone production at the poles due to ionisation by cosmic rays but the influence of this mesospheric flow differentiates the hemispheres.
If the flow of mesospheric is curtailed, as it is in summer and periodically in winter, especially in the Arctic, the temperature of the polar atmosphere is seen to increase by as much as 40°C and we have a phenomenon called a ‘sudden stratospheric warming’. At this time the tropical stratosphere is seen to cool, reflecting an increase in atmospheric mass in the tropics perhaps due to a shift of cold mesospheric air from the mid latitudes equator-ward.
The shift in the atmosphere from the poles towards the equator can be due to enhanced polar cyclone activity as the ozone content of the air on the outer margins of the polar front increases. Increased polar cyclone activity is associated with a decline in atmospheric pressure over the pole. In play we have the electromagnetic nature of the atmosphere and the possibility that the zonal wind is enhanced due to a change in the solar wind. An enhanced zonal wind indicates increased descent within the vortex. An increase in the ozone content of the air due to cosmic ray activity can not be ruled out as part of the interactive processes involved in promoting shifts in the atmosphere to and from high latitudes.
THE SURFACE PRESSURE DYNAMIC
The Arctic Oscillation index represents the ratio between surface pressure in the mid latitudes and the high Arctic. Because mid latitude surface pressure is relatively invariable while Arctic surface pressure varies a lot the AO is a good proxy for Arctic surface pressure. But one must be aware that the relationship is inverse because mid latitude pressure is the numerator. In the equation that defines the index mid latitude pressure is above the line and Arctic pressure is below the line. Mid latitude/Arctic pressure = AO index. If polar pressure falls the AO index rises. See the relationship depicted below, note that surface pressure in the right hand axis is inverted.
The graph above effectively records the ever changing surface pressure over the Arctic Ocean. There is a fine balance between surface pressure in the northern hemisphere between the mid latitudes and the Arctic. If Arctic pressure rises cold air flows southwards from the Arctic to the mid latitudes. If it falls, warm mid latitude air floods into the Arctic.
Changes in Arctic surface pressure is closely associated with change in the distribution of the air at all elevations and this is reflected in the temperature of the air. Below, the first of a number of diagrams looks at the response to a change in Arctic surface pressure at 100 hPa.
We see that, at the 100 hPa level, when Arctic surface pressure falls (a taste of summer in winter) warm air invades the polar cap. In each instance, there is an increase in the extent of the warm zone and this is at the expense of the cold zone.
Also we can observe the intensity, duration and latitudinal extent of the zone of very cold air that is present in the Antarctic in winter and also its relative stability and freedom from warming events. The anomaly in the Antarctic is the cooling that can occur over the polar cap that can extend through till November. Shortly we will see that change in the Antarctic in winter can drive change in the Arctic on all time scales and not just during the period of Antarctic winter.
REVERSE RESPONSE IN THE TROPICS
We see above that, at 10 hPa, an increase in the temperature of the air in the tropics is associated with cooling in the Arctic. Similarly, warming in high latitudes is accompanied by cooling over the equator. These phenomena are due to shifts in atmospheric mass signalled by change in the Arctic Oscillation Index. When surface pressure falls in high latitudes the Arctic stratosphere warms and the equatorial stratosphere cools. As suggested above, this is very likely associated with a change in the composition of the air at elevation in the tropics.
RESPONSE TO CHANGE IN SURFACE PRESSURE IN THE ARCTIC AT 50 hPa
At 50 hPa a fall in Arctic surface pressure initiates warming via the replacement of mesospheric air with warmer stratospheric air (shrinkage of the cold zone) but the warming continues and is heavily amplified, and more particularly so as the temperature of the air increases towards its summer maximum. As Arctic surface pressure rises from mid winter on-wards, the flow of mesospheric air is weakened and the temperature of the air over the polar cap warms. In this circumstance even a minor reduction in surface pressure can initiate a major and long sustained warming episode where the temperature of the air rises to the level that it attains in summer or even warmer. The relative enhancement of ozone partial pressure in the air in winter and spring can achieve this feat via excitation by infrared radiation. Apparently the life of ozone is enhanced at low sun angles even at the top of the atmosphere.
The Arctic at the 50 hPa pressure level is susceptible to large variations in the temperature of the air (the ozone content of the air and its distribution) between November and June. When we look at temperature data we need to remember this spread of activity. The greatest variability may be experienced in January and February but there is a long tail of sustained activity.
RESPONSE AT 10 hPa
At 10 hPa we see a similar relationship between Arctic surface pressure and the temperature of the air. Again we see that the temperature of the stratosphere over the Arctic polar cap can be highly variable between November and May and can rise to levels not seen even in the height of summer. This is apparent before and after the period of the equinox when it is known that the solar wind couples most effectively with the atmosphere producing a regular peak in geomagnetic indices at this time of the year.
THE GEOPOTENTIAL HEIGHT RESPONSE AT 50-80° NORTH LATITUDE.TO A CHANGE IN ARCTIC SURFACE PRESSURE
Above, is charted what is described by meteorologists as Wave 1 episodes measured as episodic increases in geopotential height between 50° and 80° of latitude encompassing that part of the northern hemisphere outside the polar cap where the inflated presence of ozone becomes the driver of atmospheric dynamics in the winter season. There is a 1/1 relationship between the ozone content of the air, its temperature and geopotential height. The aberrations in geopotential height seen in this diagram are a response to the increase in the ozone content of the air, and the extent to which ozone rich air is present in the profile. The diagram shows us the depth of the atmospheric column that is affected. Notice the zones between five and fifteen kilometres in elevation that have an apparent life of their own, independent of what is happening at higher altitudes. This is the ‘hot zone’ where marked differences in the density of the air drive the initiation of polar cyclones. Here the geopotential height response to changes in atmospheric pressure aligns well with change in the AO index that is consistent with that seen at the 100 hPa pressure level, representing a direct warming in the zone where polar cyclones originate. This could be a signature of cosmic ray activity.
The lagged response in GPH may be due to ozone creation facilitated by cosmic ray activity that is facilitated as the temperature of the air increases. The penetration of cosmic rays is temperature dependent. It is due to this dependence on temperature that the muon count at the surface provides a direct proxy for the incidence of ‘sudden stratospheric warmings’.
The lagged response is likely also due to the effect of declines in surface pressure on the mesospheric flow, that takes time to re-establish after disturbance, perhaps due to ozone creation via cosmic ray activity.
It is apparent that the increases in geopotential height follow the episodes where Arctic surface pressure falls away. This is also in part a natural consequence of the impact of polar cyclone activity in lowering surface pressure across high latitudes that curtails the intake of mesospheric air and in consequence advances the ozone content of the air outside the region of the polar cap.The impact is seen outside the polar cap region as a result of the change in rate of flow of mesospheric air. The so-called vortex is profoundly leaky. Its not a barrier but an interaction zone. At its base, in the Antarctic in particular, is a chain of intense polar cyclones acting like a collection of turbocharged cake-mixers arrayed outside the perimeter of the polar cap, the mixers swinging equator-wards into the mid latitudes as if they were suspended from an annular shaped support structure high in the atmosphere and free to walk along it while swinging longitudinally towards the mid latitudes where they lose momentum and torque but are active in introducing very cold air into the subtropics and promoting frontal rainfall as they do so.
TWO VERY DIFFERENT HEMISPHERES
In the broadest context the difference in the ozone content of the northern and the southern hemisphere is a response to the distribution of land and sea. The large land masses of the northern hemisphere promote the formation of competing high pressure cells in winter robbing the Arctic of the opportunity to develop a strong descending anti-cyclonic circulation over the pole. The distribution of ozone rich centres of uplift in the northern hemisphere is discontinuous because of the interruption of the sea by the expansive continents. At best, in early winter, the Arctic exhibits a more limited and confined descent of mesospheric air in the upper stratosphere, that can be centred on the East Asian high, Scandinavia or Greenland instead of the Arctic and much subject to shrinkage and displacement. This escapes detection by those whose habits of mind are hypothetical, abstract and mathematical rather than observational. As the Eurasian continent warms after mid winter, the centre for the development of high surface pressure shifts to the Arctic. The inherent changeability that is built in when there are competing centres of activity and a limited stretch of ocean to support the development of sticky areas of uplift creates a situation that is inherently unstable. The establishment of a strong core of descending air from the mesosphere requires stability. The Antarctic with its central core of land surmounted by a massive mound of ice in turn surrounded by an almost uninterrupted stretch of relatively warm water in high latitudes provides the required stability.
The weakness in the flow of mesospheric air in the northern hemisphere yields an ozone partial pressure that is much greater in the northern than the southern hemisphere. The dilution effect of mesospheric air is smaller. As a corollary, the Arctic, lacking the annular ring of polar cyclones that we see on the margins of Antarctica is not capable of generating the massive shifts in atmospheric mass due to polar cyclone activity that are achieved in the southern hemisphere. As a driver of climate change the Arctic is potent in the short term. In the long term it is a client state of the Antarctic.
THE ARCTIC IS A CLIENT STATE OF THE ANTARCTIC
Above we have the Arctic Oscillation and the Antarctic Oscillation indices.Notice that when the Antarctic is most heavily active between June and December a rise in the AAO is associated with a fall in the AO and vice versa. In other words, high surface pressure in the Arctic is associated with low surface pressure in the Antarctic and vice versa. In the remainder of the year the two move together indicating that the same external circumstances (not atmospherically bound ‘planetary waves’ originating in the troposphere) drive the flux in surface pressure at the poles and with it weather and climate on all time scales. It is necessary to remember that the flux in surface pressure across the globe is the result of interactive activity in the high latitudes of both hemispheres.
Notice the difference in the AAO between winter 2015 and winter 2016. This represents much reduced atmospheric pressure over Antarctica in the latter year. Very large variations in surface pressure from year to year are the norm. But change can also be unidirectional over long periods of time. The decline in surface pressure in the Antarctic in the last seven decades is mapped below.
The change in sea level pressure over time is documented below in a more useful fashion, according to the month of the year.
These changes reflect the joint impact of both polar circulations on Antarctic surface pressure. Notable is the ongoing decline in Antarctic surface pressure in May against a background of low inter annual variability and the recovery of surface pressure in Antarctica from August through to March. High variability in the months of July through to November is driven from the Antarctic and from December through to March by Arctic processes.
THINKING: EXTERNAL VERSUS INTERNAL MODES OF CAUSATION
Re-visit the description of the origin of sudden stratospheric warming and the abundance of ozone in the northern hemisphere that is provided at the head of this chapter.
There is no process that is inherent in the dynamics of the near surface atmosphere that we designate ‘troposphere’ that can pull atmospheric mass from both poles simultaneously. There is no internal force that can drive down atmospheric pressure in the region of Antarctica over seventy years. That mechanism has to be externally initiated and continually forced.
The troposphere delivers weather only by virtue of its ability to generate polar cyclones in high latitudes, the most variable circumstances in the global pattern of daily weather. It is the ozone content of the upper troposphere from about the 300 hPa pressure level that is responsible for the generation of polar cyclones that are described as ‘cold core’ due to the character of the air within them at the surface. There is no warm land mass to initiate these cyclones. There is no warm moist atmosphere. But, from 300 hPa through to 50 hPa there are large differences in the ozone content of the air between the pole and the equator. Air that has a very different composition, in terms of its ozone and moisture content merges at the polar front. Differences in the nature of the air that merges at the polar front (simply a chain of polar cyclones) drives polar cyclones, the jet stream and surface wind patterns via the impact of polar cyclones on surface pressure across the globe.
The ozone content of the air varies in a systematic fashion over time and very strongly from year to year. The following diagrams for the same time and day for three consecutive years serve to illustrate the differences between the hemispheres as they manifest at the 1 hPa pressure level. Ozone, because it reduces air density gathers in sticky zones of low surface pressure of its own creation and is carried by convection to the top of the atmosphere where it persists apparently immune to ionisation by Ultra Violet B. Perhaps the rate of supply from below is sufficient to maintain the anomaly? If it is, it can come only from Cosmic Ray activity.
Notice the wandering nature of the vortex of low ozone content air that is frequently located outside the polar circle in the Arctic. Its plainly vagrant, of no fixed abode, with no visible means of support. But even in the Antarctic where the intake of mesospheric air is strong, mixing processes within the vortex, normally located over the polar cap, are apparently very strong.
Plainly in evidence is the consistent vigour of the uplift in the Southern hemisphere by comparison with the Northern hemisphere. Variations in vortex activity drive changes in the ozone content of the upper air in the Southern hemisphere. The bleed of ozone rich air towards the inside of the vortex is plainly in evidence in the vigorous southern vortex, much less so in the northern. Accordingly the contrasts between air that has little ozone and air that has a lot is much richer in the southern hemisphere. The data above can be accessed at the following address. Unfortunately it begins only in 2014.
This data is of enormous importance to our rapidly developing understanding of atmospheric processes.
The primary mode of ozone control is via the intake of mesospheric air in winter. Ozone abundance is well correlated with surface pressure that in turn determines the direction and intensity of the planetary winds that determine the equator to pole temperature gradient, a first order influence on surface temperature. The abundance of ozone is also related to geopotential height variations that are well correlated with surface temperature variations that appears to be due to change in cloud albedo.
A secondary mode of ozone control appears to be linked to cosmic ray activity. This, and a well documented correlation between the zonal wind and geomagnetic activity affecting the mesospheric flow, are capable of driving change on all time scales.