The Southern Oscillation Index is a reference point for the strength of the Trade winds. It represents the difference in atmospheric pressure between Tahiti and Darwin. In figure 1 the SOI is the red line with its values on the right axis. A negative SOI reflects slack trade winds and a warming ocean. A positive index relates to a cooling globe. Note that the right axis in figure 1 is inverted.
How is it that change in surface atmospheric pressure is so closely associated with a change in the temperature of the tropical ocean? This is the major unsolved riddle in climate science. If temperature is so obviously associated with pressure on an inter-annual basis why not in the long-term? In this article I show that pressure and temperature are intimately related on all time scales. In other words, ENSO is not an ‘internal oscillation of the climate system‘ that can be considered to be climate neutral. ENSO is climate change in action. You can’t rule it out. You must rule it in. Once you do so, the IPCC assertion that the recent increase in surface temperature is more than likely due to the works of man is not just ‘in doubt’, it is insupportable.
If the IPCC can’t explain ENSO it can not explain climate change. It is not in a position to predict surface temperature. Its efforts to quantify the rise in temperature must be seen to be nothing more than wild imaginings. Its prescriptions for ‘saving the planet’ must be viewed as ridiculous.
Surface pressure data: http://www.longpaddock.qld.gov.au/seasonalclimateoutlook/southernoscillationindex/soidatafiles/index.php. Monthly temperature data: http://www.esrl.noaa.gov/psd/cgi-bin/data/timeseries/timeseries1.pl
Temperature change is linked to change in surface atmospheric pressure
Figure 1 Left axis Temperature in °C. Right axis three month moving average of the monthly southern Oscillation Index
The Southern Oscillation Index leads surface temperature on the upswing and also on the downswing. Some factor associated with change in surface pressure is plainly responsible for temperature change.
How and why does atmospheric pressure change?
The evolution of surface pressure throughout the globe depends upon the activity of the coupled circulation of the stratosphere and the troposphere in Antarctica and in the Arctic. These circulations have become more aggressive over time resulting in a loss of atmospheric mass in high latitudes and gain at low latitudes. The gain at low latitudes reflects the seasonal pattern of increased intensity in the respective polar circulations. The stratosphere and the troposphere couple most intensely in February in the Arctic and in June through to September in the Antarctic. The pattern of enhanced activity at particular times of the year is reflected in the timing of the increase in sea surface pressure in equatorial latitudes, as seen in figure 2.
Figure 2 Gain in average monthly sea level pressure between the decade 1948-1957 and the decade 2001-2010. hPa
The coupled circulation in the southern hemisphere produces a deep zone of low pressure on the margins of Antarctica that encircles the entire globe as is clearly evident in figures 3 and 4. In previous posts I have documented the change in high latitude pressure since 1948 and the associated change in wind strength, sea surface temperature and by inference, since the atmosphere is warmed by the descent of ozone into the troposphere, a change in cloud cover.
Figure 3 Mean sea level pressure January
Figure 4 Mean sea level pressure July
The pressure deficit on margins of Antarctica is deepest in July (winter) as is the pressure gradient between the interior of Antarctica and the southern Ocean .
It is of interest therefore to look at the evolution of the pressure relationship between Tahiti and Darwin (that is the essence of the SOI) over time.
Bear in mind that as atmospheric mass moves from high latitudes to the equator atmospheric pressure increases at Darwin more than it does at Tahiti and the trade winds slacken. The increase in pressure at Darwin is well correlated with the increase in atmospheric pressure in equatorial latitudes globally. The plunge in atmospheric pressure at high latitudes that enables the increase in pressure at the equator is associated with cloud loss and increased sea surface temperature in mid and low latitudes. The most abbreviated explanation of mechanism behind the loss of cloud can be found here: http://wattsupwiththat.com/2011/08/20/the-character-of-climate-change-part-3/
Figure 5 Thirty day moving average of the difference in daily sea level pressure between Tahiti and Darwin hPa.
The excess of pressure in Tahiti with respect to Darwin over the period 1999-2011 is shown in figure 5. The pressure differential plainly evolves over time and an indication of the direction of change is given by the polynomial curve.
Secondly, we can see that the pressure differential exhibits a pattern of seasonal variation. In general the pressure differential is high at the turn of the year and low in mid year.
The pattern of the average daily differential for the entire period for which daily data is available (1992 -2011) is shown in figure 6.
Figure 6 Average daily sea level pressure differential between Tahiti and Darwin over period 1992-2011. hPa
We observe that the pressure differential between Tahiti and Darwin:
• Reflects strong variability even when averaged over a period of twenty years.
• Is greatest between late December and the end of February (strong Trade winds)
• Is least between April and September (weak Trade winds).
• Shows a pattern of enhancement in February- March and also in September- October that plainly relates to the pattern of pressure increase in near equatorial latitudes evident in figure 2. The shift in the atmosphere away from Antarctica tends to enhance the pressure differential driving the trade winds all year, but in particular in September and October. So far as the Arctic is concerned the pressure loss is centered on February and March.
Why do the trades tend to fail in mid year?
Figure 7 Sea level pressure hPa. Seasonal pattern in Tahiti and Darwin.
The erosion of the pressure differential in southern winter relates to the establishment of a high pressure zone over the Australian continent. Compare figures 3 and 4 noting the difference in atmospheric pressure over Australia in summer and winter.
Evolution of the pressure differential (and the trade winds) between solar minimum and maximum in cycles 23 and 24
The low point between solar cycle is frequently marked by La Nina cooling. As geomagnetic activity picks up the first and usually the largest El Nino of the solar cycle occurs and lasts till solar maximum that is frequently associated with La Nina cooling.
In figures 8-11 the evolution of the pressure differential between 1997 and 2000 (Cylce 23) is compared with its evolution between the years 2009-2011 (Cycle 24). For reference the average annual cycle in the pressure differential (figure 6) is represented by the black line.
Figure 8 Daily pressure differential. Tahiti less Darwin. hPa
The first and largest El Nino of solar cycle 23 began in early 1997. The first El Nino in Cycle 24 started in late 2009. The pattern of the differential is shown in figure 8. Plainly, the collapse in the pressure differential was more severe in 1997 than it was in 2009.
The collapse in the differential persisted till March in 2010 and May in 1998. A strong recovery followed.
Figure 10 Daily pressure differential. Tahiti less Darwin. hPa
In 1999 and 2011 we see a strong pressure differential (La Nina) in the early part of the year, and in the case of 1999 this enhanced differential persisted through to the end of the year. The differential in early 2011 was much stronger than it had been in 1999.
It is noticeable that week to week variability is enhanced in 2011. I suggest that this relates to increased plasma density in an atmosphere due to reduced ionizing short wave radiation in solar cycle 24 by comparison with 23. Under these circumstances El Nino and La Nina produce a relatively ‘wild ride’.
We note the extension of La Nina into a second year.
Figure 11 Daily pressure differential. Tahiti less Darwin. hPa
2000 was a La Nina year coinciding with solar maximum. A coincidence of La Nina with solar maximum is more usual than not. On that basis one expects the current La Nina to continue into 2012. However, given the relative deficiency in short wave ionizing radiation in cycle 24 with respect to cycle 23 this time around could be different. The lack of a well-defined peak in cycle 24 will make a difference. If the cycle goes in fits and starts, so too will the ENSO experience.
Is the climate swinging towards El Nino as it warms?
It is a favorite meme of those who suggest that the globe is warming ‘due to change in trace gas composition’ that the climate is likely to become El Nino dominant. Does recent history support this assertion? Is a warming globe associated with increased incidence of El Nino?
Figure 12 Average daily pressure differential Tahiti less Darwin hPa
In the six year period 1992-1997 the average daily pressure differential reveals an El Nino bias in relation to average for the entire period 1992-2011. In this period the globe warmed, but the degree of warming was subdued by the eruption of Pinatub0 in 1991.
Figure 12 Average daily pressure differential Tahiti less Darwin hPa
A cooling bias is evident over the last seven years from 2005 through to 2011.
Figure 13 Average daily pressure differential. Tahiti less Darwin. hPa
Plainly there has been a progression away from an El Nino towards a La Nina state over the twenty years since 1992. In the period to 1998 the globe plainly warmed. In the period since 1998 warming seems to have ceased. There has been a suggestion that some heat that ‘should be there’ has gone missing. Can this be read as an admission that warming has either slowed or has actually ceased?
ENSO is not climate neutral. ENSO is the reality of climate change in action. The progression towards cooling that is evident in the increasing pressure differential between Tahiti and Darwin shows no sign of abating. The ENSO state changes not only on an inter-annual time scale but on very much longer time scales. ENSO is plainly not ‘climate neutral’.
If we look back at figure 1 we will see that the Southern Oscillation Index leads the change in tropical sea surface temperature on the upswing and the downswing. The SOI is more positive (cooling) in 2011 than it has been at any time over the last sixty years.
Until the IPCC can properly account for ENSO cycles they can not ascribe climate change to ‘change in trace gas composition due to the works of man’. We see an excellent correlation between surface pressure and surface temperature and no correlation at all between trace gas concentration and surface temperature.
Where is Science?