The United Nations I.P.C.C. maintains that the solar influence on climate over recent decades has been responsible for cooling. This is incorrect. The sun drove almost continuous warming in the tropics until 1998 but particularly between 1978 and 1998 and is responsible for the gradual cooling since that time.There is evidence that a warming event will manifest before the end of March 2009.
The story thus far:
We live in a lopsided world. Forty percent of the northern hemisphere is land but there is little land in the Arctic. Twenty percent of the southern hemisphere is land and the large land mass of Antarctica which doubles in area in winter due to ice accumulation sits right on the pole. A lopsided atmospheric circulation is the result with a strong draft of cold air descending over Antarctica and enhanced downdraft too over the cold southern oceans in all seasons. Cold air tends to be ozone rich and this makes it highly reactive to both short wave radiation from the sun and long wave radiation from the Earth. The reactivity of the upper troposphere is strongest near South America and weakest in the Indian Ocean and the East Pacific. An easterly wind blows between the two. Its strength waxes and wanes according to change in surface pressure in the East.
Here is an example to assist in understanding the dynamics of the atmospheric circulation that drives the easterly winds: In the brick making industry you often find a number of kilns each connected to a central chimney by an underground passage. So the air from a kiln has to go down and along a passage before it goes up through the chimney. If the plant is shut down everything cools down including the chimney. Then a problem arises. When a fire is lit in a kiln, the smoke will not be drawn down into the underground passages. Smoke and flame just comes out of the firebox straight into your face. The solution is to light a fire in the chimney, preferably above the level of the kiln. In that way you start an updraft, drawing the air from the kiln through the underground passages towards the chimney. The same applies to the atmosphere. If energy is applied at the top of the column, the air there will be drawn upwards and the pressure at the surface will drop. What is happening in the South East Pacific is that the heating of the upper atmosphere works to reduce the strength of the established strong circulation. The chimney is over Indonesia were there is never a shortage of warmth. The heating of the upper troposphere in the East Pacific pulls against the natural downdraft and weakens the easterly flow towards Indonesia.
The temperature at the top of the atmospheric column rises in relation to temperature near the surface only if it is heated at the top. Ozone rich air is heated by radiation, first from above (ultraviolet) and then from below (long wave radiation from the Earth). As the temperature rises at the top of the troposphere, cirrus cloud disappears because relative humidity falls away. So, more sunlight reaches the surface and radiation from the surface amplifies the temperature response in the upper troposphere. Around the globe, when 200hPa temperature rises, the amount of cirrus cloud in the upper troposphere falls away. Cirrus is ice cloud with reflective and absorptive properties. The upper troposphere is fed with water vapour via tropical convection and frontal uplift. Relative humidity there tends to be higher than in the middle troposphere.
The warmest waters on the globe are north of the equator (figure 1) and tend to circulate into the relatively smaller ocean areas of the northern hemisphere. Repeated bouts of warming in the tropics has the effect of markedly increasing winter temperatures (5°C) at high latitudes with little change in the topics (about 1°C) where more heat drives evaporation rather than surface temperature increase. The same occurs in the southern hemisphere but the ocean is larger, it is cooled by downdraft in Antarctica and it shares its warmth by taking it over the equator to the northern hemisphere. As the Earth has historically warmed, summer temperatures in Antarctica have actually fallen. This, and warmer winters at high latitudes (in both hemispheres), is strong evidence of a lopsided circulation and a globe warming via an increase in ocean temperature.
Part of the reason for gradual warming over time is the loss of atmospheric humidity (figure 2). In mid year when the land masses of the northern hemisphere warm the atmosphere, progressively year after year, more cloud is eroded because atmospheric humidity has fallen away. This enables more solar energy to reach the ocean, especially in the southern tropics where the cloud area over the ocean is affected by the warming of the atmosphere in mid year. Relative humidity has fallen at both 850hPa (low cloud) and 300hPa (high ice cloud) in both hemispheres. The high humidity at 300hPa in the northern hemisphere by comparison with the southern hemisphere reflects the fact that moisture is carried into the upper atmosphere by convective transport. The zones of greatest convective transport lie north of the equator.
The East to West wind and the effect of seasonal changes in pressure
The strongest downdraft of ozone rich air occurs over the cold waters west of the Andes in South America, particularly south of 30° S latitude. This creates a high pressure zone in the east of the Pacific causing easterly winds to blow towards the area of low pressure located in South East Asia sometimes called ‘The Maritime Continent’. Here the tropical waters are warmest, yielding evaporation and causing convectional uplift (the chimney). The strength of the easterlies depends upon the pressure difference between these two zones. When 200hPa temperature rises in the East, surface pressure falls there, the easterlies slacken, and an El Nino heating event occurs. That heating is due to cloud loss in the upper troposphere as a consequence of direct solar warming of the upper troposphere.
Let’s have a close look at the seasonal change in pressure relations across the Pacific.
The S.O.I. charts the difference between atmospheric pressure in Tahiti and Darwin. It is apparent in figure 3 that pressure rises in winter in both locations and falls in summer. The difference between them is slightly greater in summer than in winter.
But, pressures to the east of Tahiti are actually much higher and actually fall away strongly in winter.
Looking at the chimney end of this seesaw, we must take into account the reality that Darwin is not the actual centre of the uplift zone. In summer the chimney is the South Pacific Convergence Zone located over the Solomon Islands. In southern winter the atmospheric flow is to Indonesia and thence to the Asian land mass as part of the Asian monsoon. The true differential between the east and the west of the Pacific is therefore represented by the changing length of the vertical grey arrows in figure 3. This differential is greatest in February- March-April in the height and towards the end of the southern summer.
So the natural state of affairs is for a La Nina pattern of strong easterlies to set in for the southern summer. With a cooling global atmosphere in southern summer (land masses of the northern hemisphere are cold and so is the atmosphere) cloud cover recovers and peaks in southern summer. The recovery in cloud tends to keep the southern hemisphere cooler in summer than it otherwise would be. This is fortunate. The Earth is closer to the sun in January and irradiance is almost 7% stronger than it is in July. Without the recovery of cloud the earth would warm strongly. This is what happens in an El Nino event. As a sidelight one notes that the orbital factor changes slowly over time and is not of importance in determining the southern oscillation. However, it does affect global temperature in the long term (thousands of years).
It is apparent from this discussion that the primary dynamic driving global temperature is cloud cover as it is determined by the temperature of the atmosphere, in turn determined by the distribution of land and sea. The variable influence is the suns effect on the temperature of the upper troposphere via the impact of short wave radiation on ozone. It is not really important which way the surface winds blow or how the zone of convection moves to the central Pacific or what happens to the warm pool or whether more or less cold water wells up from the deep. What is important is the change in cloud cover that accompanies the change in wind strength and direction. We know that the tropics warm during El Nino events and common sense tells us that this warmth is conveyed to higher latitudes by ocean currents. We have the evidence of a 5° temperature gain in winter at high latitudes between the decade starting in 1948 and the decade starting in 1998. That tells us that the high latitude ocean now has much more stored warmth. In quiet airs, winter temperature at high latitudes relate to sea surface temperature while summer temperature relates to cloud cover. In unquiet airs the temperature becomes that of the location from whence the wind blows. Regardless of the multiple influences on air temperature the mean has risen strongly in winter at high latitudes.
If the change in the pressure differential is critical to the flow of wind across the Pacific (and the more important change in cloud cover) what months exhibit the greatest variability in pressure and upper atmosphere temperature?
Figure 4 shows greatest variability in atmospheric pressure in the south east Pacific in July, August and September. Peak variation in 200hPa temperature occurs between June and October.
What are the sources of this variability?
- Variation in atmospheric humidity from year to year will affect the rate of cloud loss in the warm period for the global atmosphere between June and September. After a tropical cooling event the extent of cloud loss in mid year is greater because a cooling cycle precipitates atmospheric moisture. Thus, there will be a strong change in outgoing long wave radiation in mid year from year to year that will affect 200hPa temperature and change surface pressure.
- The variability in 200hPa temperature in March/April and September/ October relates to solar influences. It is well known that geomagnetic activity peaks in March and September when the solar wind couples most effectively with the Earth’s magnetosphere. Ozone maxima occur in the upper atmosphere at 80-90 km over the equator at the equinoxes when mesospheric winds carrying water vapour from the summer to the winter pole are at their weakest. The temperature of the atmosphere at 1hPa (45km) peaks at the equinoxes, testimony to the strength of short wave radiation at this time. The temperature peaks at 1hPa are subject to a biennial fluctuation in intensity, a feature they share with the Southern Oscillation. All this is strong evidence of a solar connection.
There is further evidence of solar influence in the variability of monthly temperatures in the troposphere at 10-20° south latitude where irradiance is stronger than at 30° to 40°S. Please bear in mid that figure 5 (below) relates to a specific latitude band around the entire globe.
Figure 5 shows a fillip in variability in April at 300hPa that increases in amplitude through to 150hPa. A small peak in variability shows at 100hPa in March. But the greatest variability is clearly seen in September. At 300hPa the September variability is disguised by the tendency of 300hPa temperature to fall in October. Upwards of 250hPa the variability in September increases strongly as does the ozone content of the troposphere. At 10° to 20° south latitude the weight of variation in mid year points to a northern hemisphere influence while the skew towards greatest variability in September points to a solar influence aligned with the change in geomagnetic activity.
Can the Southern Oscillation be predicted?
On the first of January 2009 NOAA in their weekly summary announced that based on recent trends in the observations and some model forecasts La Nina conditions are likely through early 2009. The atmospheric pressure data suggests otherwise.
Figure 6 shows that tropical sea surface temperature (20°north latitude to 20°south latitude around the globe) responds to variations in sea surface pressure in the south east Pacific at latitude 30°-40°south and 240°-60°East. Since sea surface temperature rises as surface atmospheric pressure falls, I have inverted the pressure curve so that pressure varies with temperature and the relationship is easier to perceive. Sometimes there is no lag between these series but on most occasions there is a lag of six to 12 months.
A move towards El Nino warming should follow the drop in surface pressure in 2008. The extent of the rise in pressure in 2007 and the strength of the associated La Nina precipitation event suggest that the atmosphere will react strongly to the increase in 200hPa temperature. Any increase in upper troposphere temperature will swiftly remove cloud. The evolution of 200hPa temperature in January and February 2009 will be of great interest.
The ultraviolet index is up year on year to January 2nd 2009. Notice the extreme values in South America and the general increase across the southern tropics. What is happening between the sun and the Earth to produce this?
What is the prognosis for tropical sea surface temperature for the next decade?
Figure 7 shows a fall in sea surface pressure in the south east Pacific between solar cycles 18 and 21. During the entirety of solar cycle 21, sea surface pressure was severely depressed. The Earth system was shocked by a severe warming event with a half degree step change in sea surface temperature at 10° to 20° north latitude. Cycles 22 and 23 also produced long periods of depressed surface pressure representing sustained warming events. However, the trend since the start of cycle 22 is for sea surface pressure to rise, culminating in record peak pressure in 2007. A small solar cycle 24 should continue this trend and the Earth will continue to cool. Surface temperature data can not be trusted. But a cooling trend is apparent in some datasets after 1978 and in satellite records of the temperature of the troposphere.
Figure 8 shows that the southern hemisphere was set up for a state of persistent warming after the climate shift of 1978. However, temperature at 200hpa, 100hpa and in the lower stratosphere has been in decline since 1981, a mark of diminishing short wave radiation intensity in the lower stratosphere and upper troposphere.
The Southern Oscillation is a signal in the atmosphere that tells us whether the Earth is about to cool or warm. It is driven by the sun via its influence on the thermal state of the upper troposphere. This drives changes in surface wind. However, the most important change is in cloud cover. This is the thing that accounts for the change in sea surface temperature in the tropics during warming events, a change that feeds through to high latitudes. The ocean is the energy flywheel for the Earth and it changes slowly. The real test of the Earths energy status is water temperature, not air temperature.
To be continued
In my next exercise in ‘historical climatology’ I want to explore changes in atmospheric humidity and cloud cover.
We have an excellent historical record and I want to thank the NOAA Earth System Research Laboratory and acknowledge the following authors:
Kalnay, E. and Coauthors, 1996: The NCEP/NCAR Reanalysis 40-year Project. Bull. Amer. Meteor. Soc., 77, 437-471.
Their data is freely available via the web at: