In the previous introduction and article on cycles I covered ground that will be familiar to many. In this article I want to look at Hadley cells and start to consider other ways the planet might operate.
- Criteria for Cycles
- Global warming and earthquakes
- Thermal crust expansion, decomposition and the Carbon cycle
- Overview of feedbacks
- Climate stability
- Hitting the buffers
- How CO2 could control climate
- Drying of climate
- 5million years of cycles
Most people reading this, whatever their view on climate, will be familiar with the concept of an ice-age and most will understand how the Vostok ice cores (fig 20 right) shows details of the last four cooler periods with their warmer interglacials.
However, where we may start to find differences is looking within this Vostok data where CO2 and temperature appear to rise and fall together and whether this is cause and effect and which if either is the cause. However, few will disagree that whilst CO2 might on its own cause modest heating, this in no way explains the change from ice-age to interglacial. So, to fit this square peg of a very small impact of CO2 into the round hole of massive temperature change a variety of forms of massive positive feedback mechanisms have been postulated to explain how a relative small change in CO2 might just have caused enough temperature change to produce the above graphs. But is this the dog waggling the dog? Could it be that CO2 changes in response to temperature. But if change of CO2 during the ice-ages were responsible for the change in temerpature, then this certainly would suggest (at face value) that rising CO2 today would have a much greater effect.
I’m now going to present a theory that in large part supports that view
[As I detailed in my previous article “Now I’m a CO2 denier“, I failed to fulfil this intention because CO2 is not correlated with temperature around 16,000 years after the inter-glacial peak.]
But please read on, because far from agreeing with this theory that says that further warming is highly unlikely as a result of CO2 rising I will be concluding [true] that we are very unlikely to see further warming!
But before doing that I will look at Hadley cells:
Hadley Cell Structure
To the right is the typical views of the “Hadley” cell structure of the world’s climate. Looking at the northern hemisphere, we see that air rises at the equator due to the strong solar heating where the sun is overhead. (Also this air is humid and less dense). This makes the equator a low pressure region which draws air in from the North (and south) at surface level, this rises, the air cools and water is precipitated as rain over the tropical equatorial jungles, and then the cooler drier air is pushed out from the equator at the top of the troposphere by new rising air.
This air, now moving above or at the cloud level, is above most of the blanketing atmosphere and so relatively rapidly loses heat to space (aided by so called “greenhouse gases” which at high altitude have a cooling effect). Having lost heat firstly by precipitation and then by IR emission and also because drier air is heavier per volume, eventually this colder-dry air begins to descend. In the three cell Hadley structure this occurs over the Sahara and other desert regions located equidistant to the north and south from the equator. This descending cold-dry and so dense air creates a region of high pressure. High pressure as we know is cloudless, so as it moves out from these desert regions it picks up warmth, then as it reaches wetter areas, it starts to pick up moisture until at the equator the air rises, the water precipitates and the cycle repeats.
Going up to the Poles, we see areas with very little solar heating. So, heat is lost by IR emissions predominantly from the top of the atmosphere where greenhouse gases help cooling. This colder, denser air descends making the poles a high pressure zone. This high pressure pushes air out at surface level allowing high altitude air which is warmer that the descending air to be drawn in until it in turn cools.
So, poleward moving surface air cycles are created at the poles and equator driven by solar heating at the equator and IR cooling at the poles.
Finally, because we have two areas of high pressure, one at the pole and one over the desert regions, the air from each is pushed toward the latitude of the UK. As it crosses over the earth’s surface, the Sahara air mass picks up humidity and becomes warm-wet, and the Polar air flow picks up only modest amounts of heat and moisture to become cold-dry. When they meet, we get turbulent weather systems. Polar air gains heat and rises, Sahara air looses moisture and gains enough heat to rise allowing a three Hadley cells to develop.
Are other Cell Configurations possible?
There are two main constraints on the Hadley cell structure:
- Air is heated most of the equator so there must be rising air at the equator (or more exactly where the sun is most intense as this moves with the time of year)
- Air is heated least at the poles so, this is where we must see descending air.
But in between the pole and equator, there doesn’t seem to be any obvious fundamental reason why there could not be other configurations:
As a physicist I could not help notice the similarity of lasers or indeed electrons in an atoms. Whereby the laser or atom is in one state, but can sometime spontaneous change its configuration. So why not with the Hadley cells? It seemed to me that the three Hadley cell structure was an entirely arbitrary number and therefore it was entirely conceivable that there could be five, seven or nine cells, or indeed 1. The only basic requirement was that air should descend at the pole and rise at the equator.
So, I began to wonder whether there might indeed have been other configurations in the past and the most obvious one seemed a single Hadley cell.
The effect of a single Hadley Cell
To understand how changing the number of Hadley cells would affect the climate we first need to understand the relationship between ocean currents and trade winds.
Below is a map of trade winds over which I’ve marked the predominant flows.
Comparing that diagram to the Hadley Cells, we see it has the same tropical winds and the Hadley cell also as westerly winds at the latitude of the UK. And whilst less clear, there is a general movement of air toward the equator and toward the latitude of the UK. If now we superimpose that broad wind with the ocean currents:
Looking at the fig 24, we can see that the main trade winds (red arrows) closely match the main ocean currents (green arrow) except where the presence of land-masses (e.g. China) make it impossible for the ocean current to follow the trade wind.
There is a broad east to west current flow at the equator corresponding with the east-west equatorial winds and there is also a second group at the latitude of the UK both in the pacific and Atlantic where warm water heads westward. But if the current’s path is blocked by land, the current turns away from the equator. This happens along the equator on the east coast of America, Australia and the shallow seas to the north and on the east coast of Africa.
But at the latitude of the UK, the currents move not eastward but westward and here it also turns north when it hits America (Pacific) or the UK (Atlantic). Similarly in the southern hemisphere, there is a westerly current flowing around the Antarctic, but with no landmass in its path and Antarctica preventing southerly flow, the current continues its circular path.
However, whilst the winds and currents tend to match it is not as simple as that. If we look at the Pacific currents both winds and currents are strong moving eastward. But whereas the winds are strongest where they head toward the equator, the strongest currents continue moving away from the equator.
Generally the currents flow around in a circular flow. This should mean the current is equally strong all around the circuit, but it is not. In fact, the strongest currents are always warm water currents heading away from the equator . These occur on the east coast of Africa, China and America both south and north of the equator where water flowing eastward heads away from the equator along the shores. The least pronounced are cold water currents heading toward the equator
Warm water floats
The reason the warm water currents moving away from the equator are strongest is simple: warm water is less dense and so floats. The best known example of such a warm water current is the Gulf steam which is readily seen on an IR Satellite image to the right as a toothpaste like squiggle of read against the darker colours of the Atlantic. The Gulf stream (not to be confused with the general North Atlantic Drift) is in fact a very small current. It arises because of easterly winds pushing hot equatorial water into the Gulf of Mexico and this forces this “toothpaste” like current out between Cuba and Florida. Being much hotter than the Atlantic waters in this area, the hotter Gulf stream readily floats at the surface until it looses its heat and disappears into the general North Atlantic drift.
To the right is an overview of this North Atlantic current showing how the warmer waters float on the surface as they move northward, and float over the colder returning water. The result is that the the warmer northward heading currents tend to dominate the sea surface.
In the mid latitudes, in a three Hadley cell climate, the northerly direction of the mid latitude winds forces warmer water from the equator northward which tends to float.
So, the mid-latitude Hadley cell has northerly going surface winds which push along the northerly going sea currents. And if the wind should die or tend to change directly, the momentum of northerly going sea currents will tend to push the wind along. So the two work in the same direction and tend to enhance each other.
However, it all falls apart in a single Hadley cell climate.
If the climate were to start operating as a single Hadley cell, now the winds at the pole and equator are the same (so we ignore the currents here), but the winds in the mid-latitudes are from pole to equator. However, because the warm water is at the equator, and it tends to float on top of the colder return currents, the current flow will tend to have warm surface currents heading poleward which are in opposition to the winds. Now, instead of the currents and winds tending to work together, the two are in opposition. The wind tends to slow down the current, and the current tends to slow down the wind.
The switch between 3 and 1 cell
And therefore if the climate started to operate as a single Hadley cell, the situation would be self re-inforcing. The mid-latitude currents would dramatically reduce in intensity because the warm water is no longer flowing in its preferred direction away from the pole and the result would be to dramatically reduce the heat flow toward the latitude of the UK where this heat is needed to keep the 3-Hadley cell going. Because it both warms the polar air allowing it to return to the pole and it warms Sahara latitude air allowing it to return to the Sahara.
Therefore the single Hadley cell climate will cut the heating in the critical area of the UK causing substantial cooling at these latitudes and enhancing the likelihood of a single Hadley cell climate continuing.
And if the system were to (relatively) sudden switch, we would see a very different distribution of heat. (And likewise if we went from a 3 to a 5 cell Hadley structure for the climate.)
As such the combination of trade winds and ocean currents could provide the necessary conditions for positive feedback in the climate leading to a bi-state (or even tri-state) climate where “catastrophic” cooling might occur going into an ice and “catastrophic” warming coming out.