El Nino and the Southern Oscillation (ENSO) seems to be perceived as a change in the state of the tropical oceans, the focus being on the ENSO 3.4 region in the Pacific. It is thought that change in the Pacific feeds into temperature change elsewhere. The word ‘teleconnections’, is a mantra of climate science. It seems to be shorthand for “we know not how this happens but its regular’. There is also an opinion that ENSO change is temperature neutral on decadal and longer timescales.
I want to tip this perception of ENSO on its head. ENSO is the tropical manifestation of change in sea surface temperature that is most vigorous away from the equator. It is only when we look outside the 10°N to 10°S latitude band that we see the forces that create the phenomenon that we know as the El Nino Southern Oscillation.
A person unfamiliar with the way in which an automobile works might suggest that the turning of the wheels is responsible, via the ‘transmission’, for the up and down motion of the pistons. This is the case while the fuel supply is cut off but not so during acceleration. I assert that ‘wheels moving pistons’ is the mindset in relation to ENSO.
The end point of this essay is a realization that ENSO is not a tropical phenomenon at all. It is a driven by conditions at the poles, particularly Antarctica, and ultimately by the interaction between the mesosphere and the stratosphere.
All data is obtained from the very useful NCEP/NCAR reanalysis that is referenced as: Kalnay, E. and Coauthors, 1996: The NCEP/NCAR Reanalysis 40-year Project. Bull. Amer. Meteor. Soc., 77, 437-471. Thus data is available at http://www.cdc.noaa.gov/cgi-bin/data/timeseries/timeseries1.pl
In this work the average monthly temperature between January 1948 and August 2009 is computed. The difference between that average and the actual figure for each month is then obtained. That difference is referred to as an ‘anomaly’. What is shown here is therefore de-seasonalised data. It is de-seasonalised only in the sense that the method identifies change from the average for the entire period of record for this dataset. The anomaly thus calculated is occasionally presented as a five month running mean centred on the third month and this statistic is shown in the graphs as ‘5MMA’. This average is computed rather than relying on the function in Excel so as to preserve the integrity of the time axis. The figure is centred on the third month.
Sea surface temperature depends upon the stratosphere
Figure 1 shows the five month moving average of 20hPa temperature over Antarctica (in maroon) driving sea surface temperature (henceforth SST) in the Arctic (in blue). Relatively large changes in 20hpa temperature drive relatively large changes in sea surface temperature. Temperature in the stratosphere clearly leads the temperature at the sea surface. The lag is variable but it is of the order of a month or two and sometimes longer. The peak in 20hpa temperature occurs between September and November. I have confidence in asserting a strong causal relationship here. The mechanics of that causal relationship will be discussed later.
Figure 2 shows the five month moving average of 20hpa temperature in the Arctic stratosphere (in red) driving sea surface temperature in the 20-30°S latitude band (in blue). Here, smaller changes in 20hpa temperature drive minor changes in sea surface temperature with a range of about 1°C in the five month moving average (more on a monthly basis). The rhythm is not as regular, well defined or as consistent as in figure 1. There are good reasons for this that will become apparent. Clearly, the upper turning points in stratospheric temperature lead the upper turning points in sea surface temperature. Peaks in 20hpa temperature in the Arctic stratosphere occur between February and March. Occasionally a peak in 20hpa temperature occurs in mid year as in 1994, 1997, 2004 and 2006. This is likely an effect of the southern vortex which varies in strength in mid winter.
The questions that arise include: By what mechanism does the stratosphere drive surface temperature? If the Antarctic stratosphere drives Arctic sea surface temperature why does the Arctic stratosphere not drive SST in the Antarctic? What drives stratospheric temperature over the poles? Does change in temperature at the poles regulate the temperature of the stratosphere at lower latitudes. If so, how does this affect the flux of stratospheric and sea surface temperature at different latitudes?
First, let’s look at the way surface temperature varies at different latitudes. In the graphs that follow the vertical scale is the same in both hemispheres. The Northern hemisphere appears first and the same latitudes of the Southern hemisphere appear immediately below.
Comparing figures 3 and 4:
Marked SST increase occurred early in the period (1994 through to 1997) in both hemispheres. This generated the warmth for the El Nino of 1997-8. The ocean does not care where the warming occurs.
In general, the more sustained and bulky increases in SST over the widest latitude band occurred in the southern hemisphere in mid year.
The higher the latitude, the more extreme is the temperature fluctuation.
Comparing figures 5 and 6:
SST shows much more flux in the mid latitudes of the northern hemisphere than the southern.
Warm anomalies occur in December to January (winter) in the northern hemisphere.
In the southern hemisphere warm anomalies predominantly occur early in the year (summer) but the anomaly peak is later (during winter) at higher southern latitudes. The mid latitudes of the southern hemisphere are a transition point where a weak northern vortex competes with a strong and more persistent southern vortex in regulating stratospheric and sea surface temperature.
Comparing figures 7 and 8:
The most defined and seemingly erratic fluctuations (matching the pattern that prevails at higher latitudes) occur away from the equator at 20-30° latitude (in black)
The fluctuation of SST between equator and 10° south latitude is frequently out of sync with other southern latitudes occasionally peaking prior to year end like the SST of the northern tropics. This is likely due to the mixing of northern and southern waters.
The peaks of SST variation at the equator are much broader, rather amorphous and less well defined than at higher latitudes.
The equatorial fluctuation is atypical of other latitudes and is not a fair indication of the degree of warming of the sea globally.
Looking at the SST data as a whole
The most extreme variations in SST are at higher latitudes.
In mid latitudes the northern hemisphere shows much heavier fluctuations than the southern. The effect of the Arctic vortex on SST appears to be much diminished beyond about 30S latitude but these high latitude southern waters vary in temperature more than anywhere else on the globe. The variation here is in the middle of winter.
The seas in the two hemispheres experience peak warming activity at different times but these times are fairly consistent from year to year.
The only way in which the sea can warm simultaneously at all latitudes within a hemisphere is via a loss of cloud cover.
There is a unifying force dictating the pattern of sea surface temperature increase and this is the force that regulates stratospheric temperature.
What are these anomalies telling us about natural climate cycles?
The figure below shows the march of raw sea surface temperature in near equatorial latitudes. The strongest peak is in February-April while a secondary peak shows up in September to November in northern waters. A February-April peak is in the middle of the time of peak anomalies in SST in the Southern Hemisphere. The anomaly will be earlier if driven by the northern stratosphere and later if it is driven by the Antarctic stratosphere.
The secondary peak that shows up in September-November in the northern tropics is driven in part by the global reduction in cloud cover in northern summer and secondary effects from the state of the Antarctic (winter) vortex that shows strongly positive anomalies between September and December in the entire period after 1978.
The movement of southern water temperature prior to the strong El Nino of 1997-1998 shows how an expansive southern ocean can warm equatorial waters while the contribution from the restricted volume of northern hemisphere waters is relatively minor.
Fig 9 Sea surface temperature in close equatorial waters. Raw data.
There is no other force than the sun that will warm the oceans in such an expansive fashion on this relatively rhythmic schedule. There is no chaos in this system. There is order and regulation. The largest sea surface temperature responses are at higher latitudes and the smallest responses are at low latitudes. This mirrors the pattern of temperature variation in the stratosphere.
What we witness in the period 1994-2009 is a simple amplification of the normal pattern of seasonal warming due to an amplification of stratospheric temperature.
Does change in temperature at the poles affect the stratosphere at lower latitudes and how does this affect the flux of temperature at different latitudes?
It is at the highest latitudes that 20hpa temperature fluctuates to the greatest extent. Figure 10 shows the primacy of stratospheric temperature change at the poles in relation to that at the equator. Vertical blue lines show precedence for polar temperature fluctuations. In the mixing process between poles and equator, ozone content and temperature fluctuations are damped. A further, little understood process tends to produce the quasi-biennial oscillation in ozone content, temperature and stratospheric wind that is apparent at 10°N to 10°S. In low latitudes the influx of moisture from overshooting convection erodes ozone because ozone is soluble in water. A further factor influencing conditions in the tropical stratosphere is the enigma of a rise in surface pressure at the equator during SST warming events accompanied by falling temperature at the highest levels of the equatorial stratosphere and a fall in surface pressure at the poles, the subject of my next post.
The unifying force that controls sea surface temperature is the changing concentration of ozone in the upper atmosphere. Change in ozone is a polar phenomenon because that is where the exchange with the mesosphere predominantly occurs.
Adding the anomaly of 20hPa temperature for the north and south accounts for a push pull relationship between the vortexes. Notice the decline in the summed anomalies from 1960 to 1976, a period when the seas cooled.
The question remains: Why does SST follow the temperature of the stratosphere.
This is a subject for speculation. My guess is that ice cloud in the atmosphere above 200hpa (where there is sufficient ozone and the rise and fall in temperature is consequently several times that at the surface), simply varies with temperature. As the upper air warms, relative humidity falls, less water is condensed as ice and consequently more sunlight gets through the atmosphere to reach the surface of the absorbing sea. It is the winter hemisphere vortex that determines the flux in ozone in the global stratosphere. It is in the summer hemisphere that the sun is shining on high latitudes.The relative weakness of the northern vortex vis a vis the southern means that it’s influence is weak beyond 30°S latitude. Between 30S latitude and Antarctica the southern vortex determines the issue.
It will not be easy to verify this hypothesis because the change in ice cloud density in the atmosphere above 200hpa is light and the change tiny.
Why bother? Why is it important to know how it works?
The Antarctic vortex exhibits the greatest change over the period of record. The origin of the climate shift of 1978 is apparent in figure 11. The greater change is at 10hpa indicating the influence of the mesosphere. The warming of the sea between 1978 and 2003 is wholly explicable. The sea warmed because the Antarctic stratosphere warmed. The documented cooling of the sea after 2003 has occurred because the Antarctic stratosphere began to cool about 2003.
In my next post I will look at inverse relationships between atmospheric pressure at the poles and the equator confirming that change in vortex strength lies behind the change in ozone and air temperature above the poles rather than the prevailing idea of ‘planetary waves’ generated by change in SST at the equator and the increase in convection that results from that. The latter is another instance where the wheels are seen to be causing the pistons to move up and down.
The warming and cooling of the globe is due to influences that have been in operation long before the industrial revolution and the burning of fossil fuels. The atmosphere is not capable of retaining warmth like a greenhouse. It is a very efficient vent for surface warmth. It should be compared to a collection of chimneys. Those who disagree with this assessment need to have a closer look at how the atmosphere functions.