The ENSO Driver

This is a story of ozone in the troposphere, ultraviolet radiation and changing surface air pressure relating to a systematic change in cloud cover that cools or warms the tropics, and ultimately the globe. This is a story that lets man off the hook as the perpetrator of ‘anthropogenic climate change’. This is a humanitarian story for the poor people of the world, hungry for cheap energy. Its message is that climate change is natural, normal and nothing to worry about. It is driven by emanations from the sun interacting with the upper atmosphere. Carbon dioxide can cease to be regarded as a pollutant. It never was. We can, in good conscience, celebrate its beneficial role in building plant tissue and greening the world.

This is also a message that will help us to understand changes in the weather and to predict the future of weather over longer periods of time. It is written by a farmer, for farmers.

ENSO refers to El Nino/ Southern Oscillation, a pattern of fluctuation in the degree of warming in the tropical ocean well expressed in the Pacific Ocean. Energy gained at low latitudes is vital to high latitude temperature, especially in winter when the ocean is the main source of enduring warmth. Figure 1 demonstrates the dependency of global temperature on tropical temperature.

Figure 1

Tropical and global temperature change

The atmosphere is generally well mixed. However one gas is actually created in situ and can only exist if there is an ongoing attack on the oxygen molecule by incoming short wave radiation. That gas is ozone. Once created this gas needs a dry and cold environment because it is soluble in water. Ozone is present in higher concentration in the drier parts of the tropical atmosphere away from the inter-tropical convergence. It is transported downwards by the pressure cells that return air to the surface. The concentration of ozone in the air, and the reaction of ozone to independently varying ultraviolet B, jointly influences temperature between 500hPa and the tropopause. That in turn determines upper troposphere cirrus cloud density. Cirrus density determines the amount of light that is turned away from the Earth in relation to that which reaches the surface to be absorbed by the sea. Light penetrates the sea to 200 metres or more and the energy incorporated determines surface temperature right across the globe. 60% of the northern hemisphere is water and 80% of the southern Hemisphere.

The spatial variability in ozone is illustrated in Map 1. The black lines relate to the continental landmasses. Ozone can be seen to peak in the downdraft areas over the oceans in the winter hemisphere pole-wards of 10° of latitude. Inter-annual variability is strong.

Map 1

Source http://www.temis.nl/airpollution/o3tropo.html

Temperature at 200hPa is less volatile seasonally but more volatile on an inter-annual basis than temperature at the surface. It responds to several influences. The first is surface temperature. The second is due to the ozone response to short wave energy from the sun. Thirdly ozone responds strongly to long wave radiation from the Earth offering a destabilizing multiplier effect, especially in the middle of the year when outgoing long wave radiation reaches its annual peak due to the heating of the northern land masses. Because the concentration of ozone increases gradually from 500hPa up to the tropopause at about 100hPa, and because the amount of short wave energy from the sun also increases with elevation, the temperature response increases with altitude as can be seen in figure 2.

Notice the strong shift in 200hPa temperature in 1978, the origin of the great Pacific Climate Shift, more correctly the ‘Great Global Climate Shift’ because the origins of this change lies mainly, as we shall see, in the southern hemisphere and it affected the entire globe. This is the change that has given rise to the global warming scare.

Figure 2

The potential to form cirrus cloud in the sub freezing mid to upper troposphere depends upon the supply of moisture by deep convective cloud over the ITCZ and tropical rain forests. The water vapour content of the air in the mid to upper troposphere is relatively slight; relative humidity is therefore low and varies little across the seasons. In this circumstance temperature is the main driver of air saturation phenomena and therefore cloud cover. The cloud is cirrus consisting of multi-branching ice crystals that are highly reflective. Zerefos et al. 2007 observed that cirrus cloud density over South East Asia is strongly (-0.8 at the 99% confidence level) negatively correlated with temperature at the 200hPa level.

Globe 1 illustrates the difference between filmy and dispersed high up cirrus catching the rays of the sun (red) and dense low altitude cloud that appears white. You are looking at the Pacific with North America top right. Notice the extensive area of cirrus bottom right. Notice the small zone of cirrus off California exhibiting a donut centre and cirrus margins. Also apparent is the river of near surface tropical moisture impinging on British Columbia, Washington and Oregon. This air originated in the hot house environment of south East Asia. It travels laterally bringing torrential rains, a feature of La Nina conditions in the tropics and a relatively stalled high altitude circulation between equator and poles. The tropics are cool. The moisture hangs. As I write Brisbane is experiencing a deluge. In these conditions the warming land masses attract wet maritime air and the little storms can be ferocious.

Globe 1

The sun colours high cirrus cloud red

Zones of appearing and disappearing cirrus cloud that lie over the oceans in the tropics are responsible for tropical warming and cooling events. That potential disappears if the affected zone lies over land. We shall see that the southern hemisphere oceans are the main field of activity.

The Southern Oscillation Index was invented by the English observer Sir Gilbert Walker who discovered that air pressure variation between the Indian Ocean and the eastern Pacific is associated with change in the strength of the trade winds and the power of the Indian monsoon. Because the distribution of warmth in surface waters is closely associated with the strength of the easterlies the SOI is an excellent proxy for tropical warming events as can be seen in figure 3. In figure 3, I show, not equatorial anomalies or ENSO 3.4 anomalies because I consider that the entire latitude band between the equator and 10° north, where the waters are warmest, is the better indicator of the state of the tropics. It is the pattern of change over the globe as a whole that I am interested in.

Figure 3

The SOI is calculated from variation in surface air pressure between Darwin and Tahiti. Both lie in the latitude band 10-20° south. Surface pressure depends heavily upon the tendency for uplift or subsidence in the upper troposphere. That is in turn related to the temperature of the air at 200hPa. Figure 4 shows that when pressure is low in Darwin it is high in Tahiti and vice versa, reflecting the movement of the zone of strong convection from one side of the Pacific to the other. Interestingly, the pressure difference between the two zones has declined over time. However the pressure relationship is tending to recover since the 1997-8 El Nino.

Figure 4

La Nina cooling is a situation of high surface air pressure in the East Pacific as seen in figure 5. El Nino warming is a situation where low surface pressure prevails in the east Pacific. This is associated with uplift due to a warming upper troposphere and cloud free skies.

Figure 5

The inverse relationship between 200hpa temperature and surface pressure is shown in figure 6. Low surface pressure prevailed in the East Pacific for long periods after 1978 apart from the La Nina episodes of 1988 and 2000. However, since 1978 200hPa temperature has gradually tended downwards and surface pressure is recovering.

Figure 6

When 200hPa temperature over Tahiti falls it enhances the rise in surface pressure and strengthens the easterly winds across the tropics. This shifts warm waters westwards inducing an upwelling of cooler waters in the East, symptomatic of a La Nina event. Strong La Nina events occurred in 2000 and in 2008. The evolution and strength of these cooling events is reflected in 200hpa temperatures shown in figure 7. Warming events are characterized by 200hPa temperatures higher than the mean and cooling events are characterized by 200hPa temperature cooler than the mean. Between 1994 and 2000 mean temperatures were above the mean. In recent years 200hPa temperatures have fallen below the mean. This is associated with the fall off in short wave radiation and the solar wind as solar cycle 23 has drawn to a close.

The effect of a fall in 200hPa temperature is to throw up an umbrella of cirrus cloud.

Figure 7

Warming events in the Pacific are associated with warming elsewhere. Where? These dynamics occur wherever there are high pressure cells that carry ozone rich air into the upper troposphere. The map below identifies the areas of sinking air in shades of red according to rates of vertical velocity at 500hPa, about the middle of the troposphere.

Map 2. 500 hPa vertical velocity (Pa/S) in July from ERA-40 reanalysis, 1979-2001 average. Negative (blue) values represent rising air; positive (red) values indicate sinking air. This map is an excellent illustration of the Hadley cell. From Wikipedia. Descending air is red. Main zones marked with a circle.

Map 3 shows that cells of descending air have relatively low levels of cirrus cloud in the period 1984-2000 as would be expected given the relatively high level of 200hPa temperature over that period. This was a period of strongly rising sea surface temperature culminating in the extreme El Nino of 1997-8.

A quick note on the implications of these dynamics for the northern Hemisphere follows:

In periods of strong tropical warming, northern hemisphere summer temperature is inhibited by cloud persistence associated with high atmospheric moisture levels in turn associated with strong evaporation rates in the tropics. An additional influence on northern hemisphere cloud dynamics is the fact that warmest sea surface temperatures are located between the equator and 10°N lat. There is a strong flow of energy towards the polar regions via ocean currents. In times of warming, winter temperatures tend to be warm at high latitudes and Arctic ice tends to disappear. The frost free period increases greatly, benefiting agriculture in high latitudes.

Map 3

How does the location of these high pressure cells of low loud density compare with the distribution of moisture in the lower atmosphere? Map 3 shows that these high pressure cells are located in the rain shadow zones of the major continents where precipitable moisture is naturally low.

Map 4

How does the distribution of high pressure cells across the globe compare with the pattern of sea surface temperature during a La Nina event like that of 2007-8. This is shown in Map 5

Map 5

It is apparent that these high pressure cells exhibit low sea surface temperatures during a La Nina event.

What does cirrus cloud look like from space? Lets look at globe 2. The reader can find the animation from which this still was drawn at http://www.intelliweather.com/imagesuite_specialty.htm Other views of the world show similar concentrations of cirrus in the high pressure zones of all the oceans.

Alternatively find these images on Anthony Watts blog at ‘Watts up with that’ at http://wattsupwiththat.com/

Globe 2

The nature of the warming of the tropics during El Nino events

1. Since the extent of the cloud reduction depends upon the distribution of high pressure cells and the southern hemisphere is best endowed in this respect, the Southern Ocean is likely to show warming first. However, the entire southern hemisphere is affected by the presence of Antarctica where very cold temperature is associated with strong downdraft in both summer and winter. This establishes a very strong thermal gradient between the Equator and the South Pole. A vigorous return of very cold waters in the East Pacific tends to mask warming activity when the cirrus disappears.

2. The warming activity is determined by shifting and unstable pressure systems of variable size and differing characteristics and this is likely to give rise to variability in the timing and rate of warming in different parts of the tropical oceans.

3. The location of the ITCZ north of the equator, the draw of the northern continents in summer and the distribution of the land masses tends to ensure that the northern hemisphere is a strong beneficiary of tropical warming events. The reverse of course also applies. The shape of the ocean basin determines the ratio between the warmed zone and that to be warmed.

4. The change in temperature along the equator, lets say ENSO 3-4, is not a good indicator of the strength or timing of warming events because it is not directly linked to the force responsible for the change in pressure, wind and cloud cover that causes the warming event.

5. Warming in the tropics during an El Nino event is due to several forces namely (a) Effect on the distribution of the warm pool waters as the wind that concentrates the pool in the west and drives the ocean circulation loses intensity. (b) Change in upwelling of cool waters in the East Pacific as the wind changes. (c) Change in level of insolation of ocean waters over extensive areas of the globe that results in small change in surface temperature but large change in the level of energy stored near the surface.

Conclusion

1. ENSO is not energy balance neutral and neither are the oceanic oscillations that depend upon ENSO. ENSO is not a coupled ocean/ atmosphere phenomenon that owes its character to simple oscillatory behaviour like that of a pendulum. Whether the world warms or cools depends upon change in cloud cover.

2. The sun is responsible for recent warming of the globe. This warming depends upon the flux of cirrus cloud, in turn dependent upon the impact of ultra violet radiation on ozone.

3. ENSO, the periodic warming of the tropics is driven by the sun. It is not possible to remove ENSO from the temperature record. It is the essence of climate change, led by what happens in the tropics where the influence of the sun is greatest.

Reference:

Zerefos, C.S., K. Eleftheratos, P. Zanis, D.S. Balis, and G. Tselioudis, 2007: Search for man-made cirrus contrails over Southeast Asia. Terr. Atmos. Ocean. Sci., 18, 459-474, doi:10.3319/TAO.2007.18.3.459(EA).

Data: Kalnay, E. and Coauthors, 1996: The NCEP/NCAR Reanalysis 40-year Project. Bull. Amer. Meteor. Soc., 77, 437-471.

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