In 1984 Van Loon and Shea observed that: Nearly all of the Southern Hemisphere responds to the Southern Oscillation, but the response is largest in the Australia-South Pacific sector. Large anomalies of sea level pressure form well ahead of any on the Northern Hemisphere, and this observation together with the conspicuous anomalies in the region of Australia and the South Pacific suggest that the origin of the Southern Oscillation must be sought in this region.
This work tracks the way the sun and the Earth interact to bring about gradual but reversible temperature change in the tropics and ultimately, the globe. The focus is on an area well south and east of the ENSO 3.4 region at the Equator. Later articles will examine other areas of interest. However, I suspect that few will exhibit the atmospheric dynamics that drive the Southern Oscillation (and ENSO 3.4 temperature) as vigorously as the area described here.
Almost 100 years ago the southern oscillation was identified by Gilbert Walker as a factor in the variation in strength of the Indian Monsoon. His Southern Oscillation Index (SOI) compares atmospheric pressure in the Pacific at Tahiti (17º 52S, 149º 56W) with atmospheric pressure in Darwin (12.28° S latitude and 130.50° E). The oscillation involves a shift in the zone of maximum convection between Indonesia (La Nina) and the mid Pacific (El Nino). This follows a weakening of (higher) atmospheric pressure in the east that forces the trade winds. Associated change in cloud cover determine sea surface temperature across the Pacific and around the globe.
An El Nino event involves a strong increase in sea surface temperature in the entire tropics. This drives an increase in global temperature. El Nino events vary in frequency and intensity within and between solar cycles, and also according to 100 year and longer cycles in solar activity.
This presentation follows a simple format. A question is posed. Data is presented and comment follows thereafter.
All data in this study is drawn from: Kalnay, E. and Coauthors, 1996: The NCEP/NCAR Reanalysis 40-year Project. Bull. Amer. Meteor. Soc., 77, 437-471.at http://www.cdc.noaa.gov/cgi-bin/Timeseries/timeseries1.pl
Where are the areas of descending high pressure air that are associated with low 200hPa temperature ?
Due to the influence of the Antarctic land mass in a hemisphere dominated by ocean, the Southern Hemisphere is well provided with dense, cold air. The effect is to depress 200hPa temperature south of 35° south latitude by comparison with the northern hemisphere.
At what longitude is found the coldest 200hPa air?
The South East Pacific has the coldest upper troposphere. This is close to 220°-240° East (120°-140°West). The area is east of Tahiti (150°East) by a full 30° of longitude. At 20° to 30° south latitude the southern hemisphere is generally colder at 200hPa than is the northern hemisphere.
At 30°- 40° south latitude (the tracking latitude for subtropical high pressure cells) the coldest 200hPa air is located between 220° and 260° east while a similar zone is located near the Greenwich meridian in the South Atlantic.
What is the relationship between upper atmosphere temperature and pressure at the surface?
The strength of high pressure cells varies inversely with temperature in the upper troposphere. A cold upper troposphere accentuates the downdraft and a warm upper troposphere weakens the downdraft and lowers surface air pressure. This is the message of figure 4.
What then drives the temperature of the upper air ?
Figure 5 plots the seasonal movement of temperature at each level between the surface and 10hPa (45km) in the upper stratosphere. Almost 80% of the atmosphere lies below 100hPa. This is the ‘tropopause’ where temperature ceases to decline with altitude and begins to rise due to the heating influence of short wave radiation from the sun and long wave radiation from the Earth. The excitable receptors are oxygen, nitrogen and ozone.
The dynamic controlling atmospheric temperature in the upper troposphere is not complex. It can be explained in this way: There is a seasonal change in atmospheric temperature and cloud cover driven by the unequal distribution of the land between the hemispheres. Some 40% of the Northern Hemisphere is land by comparison with just 20% of the Southern Hemisphere. Land can not absorb solar energy like the ocean. A rise in global atmospheric temperature (and a fall in cloud cover) happens in mid year. Paradoxically, there is a strong loss of cloud cover in the southern tropics. Please inspect the maps below.
Map 1 Cloud cover DJF and JJA.
The loss of cloud over the ocean in the southern hemisphere occurs because while the atmosphere is warmed by northern influences the southern ocean cools in mid year yielding less evaporation, while in the northern tropics sea surface temperature is increasing, and with it evaporation.
The ocean is transparent to solar radiation and is the Earths energy storage vault. The loss of cloud cover in the southern tropics results in a strong addition to the Earth’s energy budget in mid year. This slows the fall in temperature of the southern oceans in winter.
The increase in atmospheric temperature consequent upon the warming of the northern land masses gives rise to a strong peak in outgoing long wave radiation in mid year. Earth radiation has a median wave length of 10 micrometres (millionths of a metre). Ozone absorbs at 9.6 micrometres producing a strong temperature response wherever it is present. It is a greenhouse gas par-excellence.
Returning to figure 5, it is apparent that upwards of 250hPa there is strong warming in mid year (in direct contrast to cooling at the surface). At 200hPa an August temperature peak is thereby produced, completely inverting the seasonal temperature regime that is observed at the surface. At 100hPa (the tropopause) and 50hPa the mid year peak is actually strongest, diminishing with elevation thereafter. At 10hPa where the dominant influence on atmospheric temperature is simply short wave radiation from the sun, yielding a peak in January, (when irradiance is 7% greater than in July, due to orbital considerations), a small upwards inflection of temperature is still present to remind us of the ozone induced maximum above and below the tropopause.
Yes, above and below the tropopause. This is the critical observation that will revolutionize the science of meteorology in about thirty years when the incumbent perpetrators of the myth of anthropogenic global warming are all safely retired.
At 30-40°south and 220-240°east, temperature in the upper troposphere and the stratosphere is plainly driven by radiation directly heating the atmosphere. That radiation is from two sources, the Sun and the Earth. Ultra-violet B from the sun is absorbed by ozone as is the Earths outgoing long wave radiation. These twin sources complement each other. When ultraviolet radiation from the sun heats the upper troposphere reducing upper atmosphere cloud, long wave radiation from the Earth amplifies the temperature gain.
Why is this important?
When temperatures rise at 200hPa, the high pressure cells are weakened and the easterly winds at the surface falter. In mid year, every year, surface pressure weakens in the south east Pacific because of warming in the upper troposphere. If this warming persists into spring and summer an El Nino warming event occurs and the southern hemisphere soaks up energy.
The essence of an El Nino warming event is a loss of cloud cover as the upper atmosphere warms. The same influence that heats the upper atmosphere near South America bathes all the seas in sunlight on a daily basis, running the ozone gauntlet. Ozone flames up on contact, instantly imparting energy to the air in proportion to its concentration and the strength of the spectrum of wave lengths that excite it.
The phenomenon whereby temperature in the upper troposphere is influenced by radiation from the Sun, and the Earth, is not unique to the south east Pacific. However, it is in this region that it finds its greatest expression. The extent of the phenomenon can be read in the coldness of the upper air, the degree of upwards inflection of the 200hPa temperature curve in mid winter and the extent of the swing in 200hPa temperature from year to year .
Where does the warming begin?
It begins in the South East Pacific where the waters of the Humboldt current are amongst the coldest on the planet. There is no better place to check this out than at: http://www.met.igp.gob.pe/cpntc/sst/ANOMALIAS/archivo.html where a little warming was apparent on the 17th December. This might be a little late in the year to redeem the Southern Hemisphere from a very cool Winter and Spring. But, what can you expect when the sun is as quiescent as it is at this moment.
So, what’s new?
It is an article of faith in meteorology that the troposphere is heated by the surface of the Earth. While there is more than a germ of truth in this proposition, it is not the complete story, especially in relation to the ozone rich high pressure zones of the south east Pacific. In fact radiation from the sun and the Earth has sufficient influence on upper atmosphere temperature in the south east Pacific to set up the Southern Oscillation. That oscillation moves in phase with change in the Earths energy budget because both respond to the same stimulus.
As I noted in an earlier post, there is no evidence that the presence of carbon dioxide in the troposphere changes its basic nature as a ‘turning sphere’, efficiently moving heat from the surface and transporting it upwards to a point where heat can be freely radiated to space. That point is where convection ceases. It is described as the ‘tropopause’. The temperature at the tropopause varies between minus 60°C and minus 85°C. It’s only 10km up.
The temperature fluctuation at the tropopause on a seasonal and inter-annual basis far exceeds that at the surface. That fluctuation is due to radiative influences. Radiation from the Earth varies little from year to year and yet the temperature at the tropopause swings wildly. Lets hope this particular penny drops soon.
van Loon H. and D.J. Shea The Southern Oscillation. Part VI: Anomalies of Sea Level Pressure on the Southern Hemisphere and of Pacific Sea Surface Temperature during the Development of a Warm Event AMS Journals Online Monthly Weather Review Volume 115, Issue 2 (February 1987)
To be continued.
My next post will look at the way the pressure differential between east and west changes over the year, and identify the months where the variability in air pressure is most extreme. This leads into a discussion of how the intensity of short wave radiation that reaches into the troposphere might change over time. We measure it in space, we measure it at the surface but we dont measure it at 200hPa.