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What is causing the very unusual weather being experienced world-wide in the past couple of years? Eminent meteorologists recently gathered in Reading, England to debate this and suspicion centred on the newly discovered variability in the Jet Streams.

I apologise for the pictures being of Northern Hemisphere processes-I could not locate any for the Southern Hemisphere.

The Polar Jet Streams (those we usually mean when we talk about “Jet Streams”) occur at 7 000-11 000 m or so altitude, are narrow (several 10’s km ‘wide’, 1 km or so ‘thick’), high speed (200 kph or more), west to east flows which meander from side to side, fluctuate in speed, and undulate up and down as well, however, until the past few decades remained within an “envelope” defining their boundaries.

Jet Streams are a consequence of the general atmospheric circulation set up by the atmospheric pressure differentials between the cold Polar higher pressure regions and the warm, lower pressure Equatorial regions and because of the temperature differences. They were predicted from the theoretical fluid mechanics treatment of the atmosphere but were only really discovered during World War II when high flying aircraft encountered these and suddenly found themselves either flying hundreds of kph faster than they expected when going east, or were almost standing still over the ground while flying westwards through the air at normal speeds. The Jet Stream was noted by airline pilots in South Africa years ago and I recall one early SAA Boeing 727 jet flight from Cape Town to Port Elizabeth which picked up the Jet Stream, then called a “following wind”, arriving far ahead of schedule at PE!

The Polar Jet Streams, centred at around 60° north and south latitudes, separate warm, humid, Subtropical air from cold, very dry Polar air and pull these air masses with them as they move. So if the Jet Stream moves polewards, it brings in warmer air and the surface warms up along with rain from the humid air, and if it moves towards the Equator cold, dry air surges in chilling the land beneath. The interface where the cold and warm air interact with one another is unstable and powerful storms may develop here. Indeed in the flat US mid-west, the interaction between warm humid air from the Gulf of Mexico and very cold air from Canada sets the scene for the huge supercell thunderstorms and horrific tornadoes experienced there.

The Subtropical Jet Streams centred on around 30° north and south latitude, are weaker affairs altogether, higher and with much less effect.
Both Northern and Southern Hemisphere Jet Streams are very similar, like mirror images of each other, driven by the same atmospheric energy sources so we can expect that changes in one hemisphere are generally reflected in changes in the other. There are obvious differences such as that the Southern Ocean is unencumbered by land masses unlike the Northern Hemisphere, and there are far fewer observations in the south.
Recently, there have been changes in the motion patterns of the Jet Streams, those vast “rivers of air” whizzing through the stratosphere, but which nonetheless control our weather. For example, our local “Cold Fronts” form under the Jet Stream and then move generally, but not exclusively, towards us from the west. Hence as the Jet Stream wanders so do the fronts and their intensity fluctuates.

Over many years of observations the Jet Streams seemed to follow patterns that were recognisable and used for longer term weather predictions along with other cyclic indicators including the North Atlantic Oscillation, El Niño/La Niña and the Arctic Vortex.

However, the ongoing effects of Global Climate Change have increased air and sea temperatures in Polar regions by as much as 8°C, much faster than the degree or less in Equatorial regions. This has had highly significant effects on the melting of Arctic sea-ice and on the melting of many glaciers especially on Greenland and the Antarctic Peninsula region. The melting of sea-ice shows what is called a “positive climate feedback” in the parlance of climate specialists. Ice and snow are excellent reflectors of light and heat so absorb only 10% or so of incoming sunlight. The sea under the ice is usually much warmer than the air above and melting often occurs from underneath. But as the ice cover diminishes and is replaced by open water, the absorption of heat increases significantly as water reflects much less heat and absorbs much more so the water is warmer and can melt yet more ice and so on. It appears that the Arctic ice melting has altered climate patterns but this has yet to be verified.

The warming climate has unsettled Arctic dwellers since snowfall has been replaced to a large extent by greatly increased rainfall and the hard permafrost on which dwellings were erected is melting and buildings collapsing. Rivers are less iced over and far more rain-derived fresh water is rapidly entering the Arctic Ocean through the many large rivers making it less saline. Arctic ecosystems have clearly been significantly affected.

However, the temperature gradient between Poles and Equator has decreased in recent decades with the reduction in differential heating caused by Global Climate Change and this is believed to have weakened the Jet Streams and caused more fluctuations in their flow regime. This has allowed the alternate intrusion of more cold air then warm air on a larger scale and induced more fluctuation in temperatures and hence rainfall too over a wider area.

The Arctic Vortex, a large, slowly rotating mass of very cold air centred over the North Pole, which acts like a sort of reservoir of frigid air, has weakened, as has the similar, larger Antarctic Vortex This has allowed periodic “leaking” of formerly trapped higher level extremely cold air into more temperate climes where it spins down to lower levels and mostly comes in from an easterly direction, causing spells of exceptionally cold weather. The lack of any land mass between Southern Africa and Antarctica makes it easier for cold air flows to reach us and bring the freezing level right down, although passing over the warmer intervening sea adds heat to the cold air en route. Mountain climbers in South Africa are acutely aware of the frequent and very sudden decreases of temperature with altitude in our mountains at any time of the year often allied with snow and which seriously endanger the unprepared.

We have already noticed the effects of Climate Change induced expansion in the South Indian and South Atlantic High pressure systems bringing us stronger and more frequent South-easters right here, cooling our west coast waters through enhanced upwelling of cold water, and pushing our cold fronts to the south so many now miss us.

Any sailors who brave the hazards of the “Roaring Forties”, and their violent elder sister, the “Furious Fifties”, will discover to their dismay that these already gale-force westerly winds are now even stronger and pushing up still larger ocean swells to pound them. The swells reach and fling their considerable energy at coastlines in their path-Tasmania and New Zealand have already felt the effects.

The saga of Global Climate Change continues to unfold before us right here.

polar jet

jetstream speeds 260 

 air circulation 640

 jet stream winds 480

boeing 564

Fig: SAA Boeing 727 tri-jet, remember these?