In summer 2021 tropical cyclones brought rainfall to both the west and the East coasts of Australia.
The grape vine is a Mediterranean plant that leaf’s out in summer and matures its fruit in autumn. Enhanced summer rainfall is associated with disease that attacks the leaves and fruit. To counter this, fruit may be harvested ahead of the rain. Growers are caught between a rock and a hard place.
Summer rainfall is associated with stronger surface pressure in Tahiti and lower surface pressure in Darwin and across the north of the Australian continent. This is termed the La Nina phenomenon. This paper examines the origin of that phenomenon.
The location of Tahiti is shown within the green circle (SLP1013mb) and that of Darwin (SLP 1008mb) in the red circle. The date is April 11 2021. Along the coast of Western Australia, air flows from north to south bringing torrential rainfall. A tropical cyclone moves south to merge with an intense low-pressure zone centred near the Antarctic coast.
Air pressure reflects the number of molecules in the column at a particular location. A zone of extremely low surface pressure surrounds the Antarctic continent. High pressure cells (much lighter colour) are centered on about 35S latitude, notably in the Great Australian Bight and to the east of New Zealand and over the Antarctic continent itself. The air that rises in the ‘Antarctic trough’ descends in the adjacent high-pressure cells warming as it descends, engendering a cloud free atmosphere.
On a global basis this turnover is the most persistent and vigorous because the surface pressure difference between the mid latitude highs and the Antarctic Trough is extreme. What is happening at the equator is just a sideshow to this main event. This paper will outline how the tropical sideshow depends on the dynamics in the Antarctic and the North Pacific Trough, the former active all year round, the latter for a few months in northern winter. Along the way it will be established that the agents responsible for change in weather and climate are to be found in high, not low latitudes.
The Southern Oscillation Index is the simplest method of tracking the El Nino La Nina phenomenon. It is based on the difference in the surface pressure between Tahiti and Darwin. High index values relate to relatively high surface pressure in Tahiti and/or lower surface pressure in Darwin.
In the figure below the right-hand axis is inverted. Superficially the temperature of the Ocean in the Eastern Pacific in the ENSO 3-4 zone appears to depend on the difference in surface pressure between Tahiti and Darwin. However, close observation indicates that in some instances the change in water temperature leads the change in pressure.
This diagram is based on the simple difference in sea level pressure between Tahiti and Darwin. It is different to the Southern Oscillation Index that is based on departures from the average pressure in each location. Later I examine the differences between these two indices in some detail.
On a global basis the Trade winds wax and wane with the Westerly winds in high southern latitudes, driving the West Wind Drift that is responsible for the anticlockwise circulation in the South Pacific. However, the wind in high latitudes is much stronger than it is in low latitudes and it peaks at a different time of the year. The circulation of water in the South Pacific Ocean moves like a swing that is attached to a post, like a Maypole, moving around the post, pushed from two extremities. The post is a high-pressure cell about which the air swings anticlockwise to all points of the compass with a greater attractor located in the south than the north. Surface pressure is much lower in the Antarctic Trough than it is at the equator. Accordingly, the surface of the Ocean is driven harder in high latitudes than in low latitudes.
The temperature of the surface of all tropical waters follows that in the Nino 3-4 region. Because the tropical latitudes have the widest circumference, the tropics amount to 40% of global surface area. Accordingly global surface air temperature follows that in the Nino 3.4 region and the tropics generally. This evolution of surface temperature in low latitudes is due to a stop/start mixing process.
As wind intensity increases, very cold water from high latitudes is driven north towards the Galapagos Islands and westwards across the Pacific. The Antarctic Trough is the major sink generating a flow of air and water from the west to the East. So vigorous is the circulation within and above it, that the entire atmosphere rotates faster than, and in the same direction, as the Earth. But, the intensity of the circulation in the Antarctic trough changes on all time scales. The trough occupies a very large area, stretching from 40S latitude to 70S latitude. Change in atmospheric pressure in the trough manifests as a shift in atmospheric mass to other parts of the globe, modifying surface pressure relationships and the planetary winds globally.
Returning to the figure above its apparent that T_D declines between 1948 and 1978 and increases after 1978. The trend line in Nino 3.4 temperature declines with T-D until the decade of the 1960s, rises gently until the turn of the century and declines again after 2005 indicating warming in the Nino 3-4 region.
Why does the temperature of the waters in the trade wind zone not reflect the enhanced tendency for more cold water from high southern latitudes to cool the tropics as the difference between surface pressure between Tahiti and Darwin increases? The answer follows.
In the southern Hemisphere high- pressure cells dominate the expansive surface of the Southern Ocean in the mid latitudes. As surface pressure falls in the Antarctic trough it rises in the high-pressure zone in the mid latitudes where descending air is warmed by compression. Accordingly this zone tends to be cloud free. Increasing pressure widens the zone of influence of these cells directly reducing the Earths albedo, allowing more solar energy to reach the Ocean where it is absorbed to depth.
What is causing the current trend increase in the difference in surface pressure between Tahiti and Darwin? To answer this question, one needs to attend to the process of change in surface pressure. The situation is complex, and a methodical approach is required.
We see above that surface pressure in Darwin falls away more than it does in Tahiti in the summer season. For this reason the difference in surface pressure between Tahiti and Darwin is greatest in January and least in July. The reduced surface pressure in summer indicates reduced air density associated with enhanced kinetic energy as the land mass of the Australian continent heats the atmosphere.
The atmosphere is global. Like one big bathtub. Seasonally, there is a shift of atmospheric mass from the summer to the winter hemisphere. The shift from south to north peaks in January. This is documented in the maps below. The cooling of the Eurasian continent assists the increase in atmospheric pressure in the northern hemisphere. The generation of low pressure zones over the north Atlantic and the North Pacific Oceans retards this process. These two low pressure cells, forming over the ocean, are the northern Hemisphere equivalent of the Antarctic trough.
The location of Tahiti is indicated with a white circle.
In January, surface pressure falls away in the region of Australia. Notice the low surface pressure in the Coral sea to the east of New Guinea, an area associated with the generation of tropical cyclones, as is the zone of low surface pressure to the south of Indonesia.
Summer rainfall across the north, the west and the East of the Australian continent is associated with the monsoonal flow and in particular the incidence of tropical cyclones. Cyclone tracks are mapped in the figure above. Some of these cyclones, particularly on the West Coast, travel far enough to merge with low pressure systems in the Antarctic trough. It is probable that as the trough has deepened, this merging occurs more frequently.
The incidence of tropical cyclone activity increases strongly from November reflecting the evolution towards low surface pressure in summer. Peak month for cyclone activity is January. But in the next two months, while the number of cyclones falls away slightly, the number of stronger category 5 cyclones increases.
If we want to understand the evolution of the planetary winds and all forms of cyclone activity, we must understand the dynamics driving surface pressure.
Surface pressure dynamics
The lowest surface pressure on the globe is found in Antarctica between November and January (yellow line). But on a year-round basis, the Antarctic Trough (grey line) generates the lowest surface pressure. The Antarctic Trough is where polar cyclones form.
Polar cyclones dominate the latitude band 40 to 70 degrees south. They have their genesis at jet stream altitudes where the strongest winds occur. Above the tropopause ozone reduces the number of molecules in the column of air by transferring energy to oxygen and nitrogen that has been acquired from the Earths own infrared emissions, by day and by night. In low pressure cells the upper two thirds of the atmospheric column is less dense and the tropopause, where the lapse rate of temperature with increasing altitude falls to zero, is two to three kilometers lower in elevation, than in high pressure cells. The uplift is intense, extending to the top of the atmospheric column at 1hPa or 45 kilometers in elevation, to the stratopause. The corresponding downdraft in the mid latitude high pressure cells introduces ozone into the troposphere to such an extent that at 200 hPa, the temperature of the air peaks in winter. That indicates the heating power of ozone as it is energized by infrared radiation from the Earth itself.
Relating to figure immediately above:
- Black line: Northern hemisphere surface pressure peaks in northern winter, falling away as the air warms in summer.
- Dashed blue line. There is a small increase in atmospheric mass between 0-15S latitude in the winter months.
- The difference between Tahiti (dotted green line) and Darwin (dotted blue line) is greatest in January.
- Orange Line. This is where high pressure cells are located at 15 to 40 degrees of latitude. Atmospheric pressure builds strongly in the winter season, peaking in August. Elevated atmospheric pressure is related to cloud free skies and therefore reduced albedo. Pulses of higher atmospheric pressure (and geopotential height) are associated with an increase in surface temperature. The presence of ozone elevates the amplitude of swings in temperature above 500 hPa by comparison with the contemporaneous swings below 500 hPa and at the surface. In very cold air this strongly affects ice cloud density, opacity and reflectivity. It changes the Earth’s albedo.
- Yellow line. There is strong increase in atmospheric pressure in Antarctica in winter, absorbing some of the movement of atmospheric mass from the northern hemisphere. High surface pressure over the Antarctic continent is associated with the descent of relatively ozone deficient, mesospheric air inside the polar vortex, deeper contrasts in air density across the vortex and enhanced polar cyclone activity. In this way, the Antarctic Trough, despite its proximity to the continent, avoids the increase in air density and surface pressure that manifests over the continent in winter.
- Grey line. The activity of polar cyclones keeps surface pressure in the Antarctic Trough to a planetary minimum all year round.
- Green line. Tahiti sees peak pressure in September at a time when ozone partial pressure peaks in the high latitude southern stratosphere.
The evolution of ENSO since 1992
In this diagram the 61day smoothed SOI (left axis) is compared with the simple 61day smoothed difference in surface pressure between Tahiti and Darwin (right axis). The year marked on the y axis indicates the first day of January in that year. The columns are inserted as an aid to judging the incidence of peaks and troughs in the data.
To remove the Madden Julian Oscillation from surface pressure data a 61day average is required, centred on the 30th day.
From Wikipedia: The Madden–Julian oscillation is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall generally becomes nondescript as it moves over the primarily cooler ocean waters of the eastern Pacific but reappears when passing over the warmer waters over the Pacific Coast of Central America. The pattern may also occasionally reappear at low amplitude over the tropical Atlantic and higher amplitude over the Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where thunderstorm activity is suppressed. Each cycle lasts approximately 30–60 days. Because of this pattern, the Madden–Julian oscillation is also known as the 30- to 60-day oscillation, 30- to 60-day wave, or intraseasonal oscillation.
Relating to figure above
- T-D (orange) is the simple difference between atmospheric pressure between Tahiti and Darwin). This statistic is rarely negative, unlike the SOI (Blue). The default arrangement is for winds to blow east to west driving cold surface waters westwards. This relates to a positive value for T-D except briefly, in the winter season, manifesting in only 9 of the 29 winters.
- T-D exhibits an annual cycle, not always evident in the SOI, with a consistent peak in December-January-February. Because the SOI (left axis) is computed to reveal anomalies in relation to the average, this peak is suppressed, a January peak occurring only when the anomaly is abnormally large. Another difficulty with this statistical treatment (departure from the average) is that trends are distorted as the average changes. The average is always changing because the climate of the Earth changes over time naturally. ENSO has been present for thousands of years.
- The peak in T-D rarely strays from early January.
- The timing of the trough in T-D, that occurs in the winter season, is irregular.
- The year-to-year variation in T-D in January is much larger than the variation in the trough in winter. This points towards a dominant Northern Hemisphere influence driving change in surface pressure in January because shifts of atmospheric mass primarily emanate from the winter hemisphere. In the winter season the Antarctic trough dominates.
- The SOI is noisy by comparison with T-D. It conceals rather than reveals the surface pressure dynamic that drives the winds.
- Because T-D is positive almost all the time, the circulation of the Pacific Ocean in the Southern Hemisphere is always anti-clockwise. The warmest water is always in the Western Pacific located in South East Asia and the Indonesian archipelago. This is the case even when the trade winds falter, because the trades reverse for only a tiny fraction of the time and it is the constant north westerlies that are the dominant driver of the circulation of the waters of the Pacific Ocean. There are parts of the Pacific Ocean that have not warmed with the tropics. In fact they are cooler today than when records began.
- Negative T-D implies a reversal of the trades to blow West to East. This is the El Nino condition when the centre of convection moves from the western to the Eastern Pacific. This is facilitated by the eastward movement of local centres of convection that originate in the warm waters of the Indian Ocean documented as the Madden Julian Oscillation that has a frequency of 30-60 days. In addition, the Earth’s atmosphere moves from West to East, rotating faster than the Earth itself with the speed of travel increasing with latitude. This opposes the east to west movement of the trade winds. The MJ oscillation and the generalized movement of the atmosphere to the east, tends to move the centre of convection over cooler waters in the eastern Pacific regardless of the T-D surface pressure differential. Convection, once started, is self-reinforcing via the release of latent heat. To the extent that the movement of cold surface waters to the west is slowed as the Trades and the Westerlies weaken, the waters of the Eastern Pacific will warm up, assisting the movement of the centre of convection to the east.
- There is a 10-13year long wave in T-D. The peaks in T-D are punctuated by anomalous deep troughs. These anomalous troughs occurred in (1997, 1998) and (2009,2010), by far and away the years with the strongest El Nino tendency. The abrupt reversal from cold to warm is associated with a build up of warmer waters in the mid latitudes that spill into the tropics to the exclusion of colder waters from higher latitudes. Energy gain in the mid latitudes relates to elevated surface pressure there, driven by low surface pressure in the Antarctic trough. So far as the ocean is concerned, energy is absorbed to depth and its distribution is affected by mixing processes. It follows as a matter of logic that one cannot assume that an average of global surface temperature actually reflects the process of energy acquisition and emission by the Earth as a whole. To assess the energy balance, one should endeavour to measure the energy stored in the atmosphere and in the ocean, at least to the depth of the ocean that light penetrates. To speak of the hottest year since records began, while it adequately describes surface conditions tells us nothing about the underlying process of energy acquisition and emission.
- The trough in the 10-12year long wave in T-D rarely manifests negative values. But this occurred notably in (1993,1994), (2002,2004,2006) and 2015.
- Any weakness in the Antarctic trough will manifest as a slowing in the anticlockwise movement of the Pacific Ocean in the southern hemisphere. Weakness in the trough is associated with a decline in atmospheric pressure in the high-pressure cells of the mid latitudes and a consequent increase in cloud cover reducing the uptake of solar energy in the mid latitudes. It follows that the energy balance in the climate system will turn to the negative (emission exceeding acquisition) when surface pressure in the Antarctic trough begins to increase.
- The surface temperature dynamic was much affected by a marked increase in the temperature of the stratosphere in the late 1970s, a reflection of its ozone content, the origin being a relative collapse in the intake of mesospheric air due to a fall in atmospheric pressure over the Antarctic continent at that time. The temperature change propagated globally affecting temperatures in the upper troposphere, reducing the Earths albedo.
Dynamics affecting the evolution of the system.
The data in the figure immediately above suggests that the annual cycle in the differential pressure between Tahiti and Darwin is part of a longer 10-12year cycle, cause unknown.
The annual cycle involves change in cloud cover related to the central pressure within and the change in the area occupied by high pressure cells.
The strongest zone of uplift on the planet is not located at the equator. It is located on the margins of Antarctica. The change in cloud cover in the mid latitudes is directly related to the vorticity and volume of air uplifted within the Antarctic trough. This should focus our attention away from the Hadley cell, that is characterized by weak and fluctuating trade winds towards high latitudes.
Evolution of surface pressure in Tahiti and Darwin by month of year
- Surface pressure has increased most strongly in September pointing towards enhanced cyclone activity in the Antarctic Trough.
- Trends in sea level pressure in Tahiti and Darwin tends to converge in the middle of the period but with a secular increase in all months over time.
- Between November and March, the amplitude of the individual swings in surface pressure increases.
- The T-D Negative status (westerly winds) manifests most strongly in January, February and March.
- From October through to March the variability in Darwin is generally greater than in Tahiti.
- Surface pressure in Darwin and Tahiti tend to be anti-correlated at all times of the year, but not strictly so. This lack of strict correlation indicates that the migration of the zone of convection is influenced by conditions unrelated to the process of convection itself. This invalidates the notion that the ENSO phenomenon is solely due to an interaction between the atmosphere and the ocean in tropical waters.
- Year to year volatility is strongest in September-October, falls away in November then ramping up in December-March.
- In January the thirty years between 1969 and 1999 had the greatest potential for pressure reversals between Tahiti and Darwin.
- From 1980 to 1999 conditions in March favoured El Nino.
The origins of shifts in atmospheric mass
The figure above shows the evolution of the Antarctic and the Pacific troughs over time, its relative depression in relation to the North Pacific trough and the competition for influence on southern hemisphere atmospheric pressure from the North Pacific Trough in December, January, and February. To be more complete this analysis should be extended to compare the evolution of surface pressure in North Pacific, to that in the North Atlantic perhaps a subject for another day.
We see in the figure above that, in January, the shifts in atmospheric mass consequent on changes in the intensity of the North Pacific trough are much more substantial than those emanating from the Antarctic Trough. In the main, a depression in pressure in the Pacific trough coincides with an increase in surface pressure in the Antarctic Trough and vice versa. However, its not a simple case of atmospheric exchange within a closed system. The red arrows mark occasions when both troughs move in the same direction emanating from a global change in the partial pressure of ozone in the upper air or perhaps an electromagnetic jerk affecting the rotation of the atmosphere. Either way, the agent of change is external to the Earth, not the product of an internally generated change within a closed climate system. This system is open to external influences. If we don’t acknowledge this dynamic that affects the planetary winds, the Earths Albedo and ENSO, we have our heads buried in the sand. When I survey the climate science literature and the work of the United Nations all I see is bums in the air.
Due to the complexity of the origins of shifts in atmospheric mass from the high-pressure troughs in both hemispheres, the fact that the troughs vary in their influence according to the month of the year and according to external influences that are currently unrecognized, and currently unpredictable, it is not possible to anticipate the likely evolution of climate from year to year or in the decade to come. Modelers should acknowledge this and back off.
However, given that cloud cover in the mid latitudes is undoubtedly a function of the evolution of the high latitude troughs, we can say that, when the troughs weaken, the beneficial warming trend that commenced in the late 1970s will reverse.
It is well to remember that the temperature of the Earth peaks in July when, due to orbital considerations, solar radiation is 6% weaker than in January. Global cloud cover reaches its minimum in July, its maximum in January. This demonstrates the importance of albedo in determining how much solar energy reaches the ocean, the only medium that can absorb solar energy to depth and retain it for long periods of time. The atmosphere is incapable of retaining energy. Any energy gain is strictly temporary, swiftly countered by convection and decompressive cooling with a compensatory compression in the mid latitudes where most of the radiative energy emitted by the Earth system emanates from high altitudes. Radiation theorists need to acknowledge this geometry and back off.
It is stupid to suggest that there is ‘a climate or any other sort of emergency’ when something bad is expected to happen if we as humans are powerless to influence the course of events.
It’s comforting to remember that warming has been highly beneficial. The bulk of the globe is too cold for comfort, especially in winter. This is what the green tremolos need to fully absorb, and admit. Then back off for the good of humanity.
As for the grapes, the great years have dry summers, frequently associated with an upswing in the solar cycle like the one that is now underway, albeit weakly.