The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits.
This post revises key concepts that relate to the evolution of climate. Good teaching is about saying it again in slightly different ways until it sinks in. This caters for the students who can’t tune in at a particular time and many others whose perceptual frameworks are sort of ‘frozen’. Its also possible that the message can be delivered without the necessary flair.
Knock-knock. New idea. Fundamental to the nature of Earth is the difference between the warmth of low latitudes and the cold of high latitudes. Without the redistribution of energy by wind and water the extent of the habitable latitudes would be tiny. In the tropics there is little variation in the nature of the air from day to day. But in the mid and high latitudes change is the rule. When the wind changes in a systematic fashion to establish new states, we have climate change. The further we depart from the equator, the greater is the change that is experienced.
The air moves from zones of high to zones of low surface pressure. Pre-eminent in terms of low surface pressure is the Antarctic Circumpolar Trough. It is the zone coloured orange in figure 1.
Kalnay et al’s reanalysis of 1996 to be found here. shows the evolution of surface pressure by latitude over time and is presented in a graphical format in figures 2 and 3.
Plainly, the work that is done in redistributing energy across the latitudes is dependent on the evolution of surface pressure in the Antarctic Circumpolar trough and to a lesser extent the latitudes north of 50-60° north.
Figure 4 plots the temperature of the air as it evolved in the year 2015 at 500 hPa at 40-60° of latitude in the northern hemisphere at left and the southern at right. Plainly there is a north-west to south-east orientation in the movement of the air masses as the atmosphere super-rotates about the Earth in the same direction that the Earth rotates, but faster. The speed of rotation increases in the southern hemisphere where the angle of attack is more aligned with the parallels of latitude. The air spirals from north to south at all latitudes.Warmer parcels will have an ascending tendency while colder parcels will be descending.
THE IMPORTANCE OF POLAR CYCLONES
New Concept: It is polar cyclones that are responsible for the intensity and evolution of the circumpolar trough.
A core theme of this work is that Polar Cyclones are energised by warm, low density cores in that space where the troposphere overlaps with the stratosphere. Differences in the ozone content of the air gives rise to differences in air density. A chain of cyclones on the margins of Antarctica give rise to a rapidly circulating polar vortex in the stratosphere. There are no limits to convection in the stratosphere.
In summer the air rises to the limits of the atmosphere directly over the continent of Antarctica but in winter there is descent. A rising cone of air surrounds the zone of descent. This cone is sometimes described as a polar vortex. The cone begins at 300 hPa over the circumpolar trough and widens to take in the mid latitudes at the highest levels.
The upper troposphere/Lower stratosphere in the region of the circumpolar trough is characterised by intense mixing of air from diverse origins, the troposphere, the stratosphere and the mesosphere.
Between October and March the cone of ascending air below 50 hPa tightens like a hangman’s noose bringing air from the troposphere to the pole, creating an ozone hole, the falling away of surface pressure at this time of the year associated with generalised ascent over the Antarctic continent and so excluding the flow of air from the mesosphere that descends throughout winter.
That the circumpolar trough is due to differences in the ozone content of the upper air should be non-controversial.
THE DENSITY OPACITY OF THE GREEN BLOB
The circumpolar trough is an unremarkable aspect of the atmosphere in the view of UNIPCC. The significance of its presence is unappreciated. This is not an unusual state of affairs in the annals of humanity. In fact, ‘Climate Science’ has not leaned a lot about atmospheric dynamics since the time of the pioneer Bjerknes who published a work on the near surface characteristics of polar cyclones in 1922.
It is realised, at least in meteorological circles, that a trigger is required for the formation of low pressure cells of rotating air in the region of the circumpolar trough. That trigger is an upper level trough, a mass of warmer, low density ozone rich air.
In 1922 it was not apparent that the most vigorous winds are located in the overlap between the stratosphere and the troposphere. Neither was it apparent that cold ozone deficient air from both the mesosphere and the tropical troposphere are drawn towards the circumpolar front in the space shared by the upper troposphere and the lower stratosphere.
In fact the concept of a ‘stratosphere’ was pretty new in 1922. In many respects we have not moved on from that position despite the passage of 100 years. Indeed much that was known prior to the 1970’s has since been forgotten in parallel with the increasing concern that man and the environment in which he lives are incompatible entities. Educators went off in socially responsible directions. A fabulous gravy train was created for scientists and space agencies and all those who aspire to gain their daily bread by looking after the environment, painstakingly monitoring the activities of a an every increasing panoply of despoilers, at one end mighty global corporations and at the other the humble cow that provides the milk for your morning cereal irresponsibly farting in its field of green. Such is the work of the modern missionary.
The intensification of polar cyclones in winter, and the consequent lower surface pressure at that time of the year is due to the proliferation of ozone. Gordon Dobson observed in the 1920’s that, in high and mid latitudes low surface pressure identifies areas with high total column ozone. Dobson measured wind velocity and discovered that the strongest winds were not at the surface but in the region of the tropopause. The tropopause is kilometres lower when surface pressure is low than when surface pressure is high. This circumstance may be described as an upper level ‘trough’, a zone of reduced air density that shows up in elevated geopotential height contours. Had Bjerknes apprehended the structure of the upper air we would not now be worrying about carbon dioxide in the atmosphere. We would be aware that the source of long term climate change, the source of decadal variations, the source of inter-annual variations and indeed our daily weather lies in variations in the ozone content of the stratosphere. We would be at peace with the notion that our ‘rather too cool for comfort’ planet gains and loses energy according to change in the extent of its cloud cover.
There is so much to learn.