An Assessment of Year to Year Atmospheric Conditions.
The charts shown here are the records of the situation at 00UTC on 7th May each year. There is no particular significance to that date other than that it is approximately ‘half-way up the ladder’ from the winter to the summer peak positions.
Assessing the progress of the atmospheric structure at the same point each year allows us to consider whether any significant changes are occurring year to year and to assess whether there are any long-term changes evident. The position at around this point each year should be the point at which the shift from late winter to genuine spring should be well achieved.
The principle variation in the atmospheric structure occurs with what is, effectively, the Hadley cell; its position varies seasonally under the influence of the sun, and its size and shape most significantly under the influence of short term solar energy impacts. From a northern hemisphere viewpoint (exactly the same in the southern hemisphere but reversed) the southern limit is delineated by the ITCZ, the northern limit by the northern edge of the sub-tropical high pressure zones. This northern limit is most easily visualised as a ‘crease’ in the atmosphere along which the jet streams form and which is seen to be the ‘steering level’ along which surface low pressure weather systems form and travel.
In addition to the obvious seasonal shift of the cell, variations in size and form occurring under the influence of solar energy impacts will alter the latitude and profile of the ‘crease’ and its influence on surface patterns will be seen to vary; add to that the observed ‘swelling’ of the upper atmosphere above and beyond the lower level structure and it may be seen that the line followed by the ‘steering level’ can be displaced substantially. By monitoring the position of this line as it moves seasonally and under the influence of incoming energy bursts we can assess any changes that may be taking place over the longer term.
We must also take into account the influence of both ‘Russell – McPherron’ effects and ‘Spörer’s law’ both of which influence the level of impact that any solar activity may have on the terrestrial environment.
As always, it must be stressed that sunspots are not the only solar influence on the earthly environment; the presence of a sunspot does not imply an earthly impact nor does the absence of spots imply the absence of such an impact. The sunspot cycle should be viewed simply as a general indicator of overall solar activity.
The key indicator in atmospheric charts is, as always, the mean or average position – the latitude – that has been reached by the 550mb isobar in its seasonal progress, together with the volume of cold air – in simple terms the amount of blue – remaining towards the polar regions compared to the amount of red remaining to the south . Any short term alteration in the size and shape of the Hadley Cell will alter both the position of that line and the gradients in that region. As with any natural phenomenon, nothing is easy or straightforward; localised ‘wiggles’ will occur as a result of thermal variations in that region and sea/land differential heating so a degree of interpretation is needed in smoothing or averaging out the position of the isobars shown.
If we therefore assess these factors in the chart for, say, the year 2000; compare that with the year 1900, then examine the International Sunspot Chart for the same period, we are able to assess whether there is indeed any cross correlation. By maintaining an annual check on the situation at this point we are able to monitor any ongoing developments.
Sunspot Number April 2017 AP index April 2017 TCI index 2010-20
Comparing the sunspot number and the AP index for April 2017, we find that while sunspots continue their decline, the AP index is showing a contrary upsurge which has been maintained, basically, throughout 2015/16 and into early 2017. The upper atmosphere ‘Thermosphere Climate Index’ has followed the overall tendency of the sunspot number, modulated by the ‘Ap’ value.
We can then relate this to the deep atmosphere charts to examine whether there has been any identifiable reversal of the southward tendencies of the critical points evident between 2000 and around 2014.
If we make the direct comparison between 2014, when the Ap index was significantly lower than 2017 we find also that the critical points in the upper atmosphere were also significantly further south in 2014 than in 2017, while in 2015, when the Ap had been high early in the year but dipped sharply lower, we find high levels of heat to the south while the 550mb zone is more heavily looped.
The principal charts in this discussion are the ‘Springtime’ charts of 7th May, however it is also relevant to look at the ‘Autumnal’ charts to see whether any comparable variance is evident there. The group below are the charts for 00z 07 November for the years shown.
2003 2010 2019
In these, the steady southward movement of the ‘Steering Level ‘ is clearly seen.
Trying to look towards future probabilities, the sunspot number will certainly continue to decline inexorably, while the solar wind/Ap values are also likely to decline towards or beyond levels associated with the winters of circa 2010.
The unusual level of cold over UK and Western Europe for this time in May is clearly shown 07/05/2019
Assessing the trends between 2008 and 2015 would give the impression of a gradual overall reduction in atmospheric energy levels. The latitude achieved by the ‘Steering Level’ in its northward movement being significantly higher in ’08/’10 than in ’14/’15.
Charts from the years 2000 and 1990 would seem to confirm and even reinforce this observation. The 552mb Isobar having achieved a position north of Iceland in 1990, but barely achieved northern France in ’14/’15.
When we seek to go beyond this point, we start to run in to problems common in the field, the paucity of available data. Accurate charts are available only going back as far as 1985, beyond that point data simply did not exist in sufficient quantity or with sufficient accuracy to enable charts to be plotted. We can, however, run such data as we do have through our analytical computers and endeavour to re-analyse using modern methods to obtain charts of how the atmosphere should have looked at the time. Whilst such charts are useful and as accurate and valid as they can be made and are based upon actual observed data from the time, it must always be remembered that they are re-creations, not actual real-time plots. They do, however, give us, as closely as possible, a valid image of conditions as far back as 1870 and it is, indeed, dramatically evident that the atmosphere was significantly cooler in the late 1800’s than it was 100 years later.