Running AMOC

By November 19, 2022Environment, Science

A scientific paper by researchers at the University of New South Wale (UNSW), recently published in the journal Nature Climate Change, undertakes modelling of the weakening or collapse of the Atlantic Meridional Overturning Circulation (AMOC)1.

The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that carry warm water from the tropics northwards into the North Atlantic. It acts like a conveyor belt, driven by differences in temperature and the water’s density (in part caused by changes in the salt content). As warm water flows northwards on the surface, it cools and some evaporation occurs, which increases the concentration of salt. A lower temperature and a high salt content make the water denser, and this sinks deep into the North Atlantic ocean. The cold, dense water slowly spreads southwards, several kilometres below the surface. Eventually, it gets pulled back to the surface to replace the warm water flowing northwards, in a process called “upwelling” and the circulation is complete. This global process makes sure that the world’s oceans are continually mixed and oxygenated, and that heat and energy are distributed around the earth. This contributes to the climate we experience today2.

The AMOC has often in the past been referred to as the Atlantic Conveyor and the surface current moving northward as the Gulf Stream3, the latter being one of the factors making northeastern Europe warmer than its latitude would indicate it should be. 

Recent research has indicated that this AMOC has slowed and is currently (excuse pun) the weakest it has been in the last 1,000 years1,4. This is believed to be partly due to natural variation and partly due to climate change, although the current is not yet outside the envelope of its natural variation. However, climate models indicate that human-induced global warming will lead to a continued slowing of the current such as has not been seen before4.

The UNSW study found that an AMOC collapse will drive a profound global-scale reorganisation of atmospheric circulation, initially triggered by a strong North Atlantic cooling and a southward migration of the Intertropical Convergence Zone (ITCZ)1

The ITCZ used to be known by sailors as ‘the doldrums’ and is the zone where the northeast and southeast tradewinds converge. It is mostly near the equator, but varies from place to place because of topographic features and ocean currents, and its position also varies seasonally5.

An AMOC collapse will also drive a significant strengthening of the Pacific’s Walker circulation, something not previously found in the scientific literature1

The Walker Circulation refers to the pattern of air rising in the western Pacific and falling in the eastern Pacific with concomitant westward moving air at the surface. Warm sea surface temperatures in the western Pacific pump heat and moisture into the atmosphere above. This warm air rises high into the atmosphere and, if the air is moist enough, causes towering cumulonimbus clouds and rain. This now-drier air then travels east before descending over the cooler eastern tropical Pacific6.

The strengthening of the Walker Circulation is triggered by enhanced low-level easterlies across the Panama Isthmus and is reinforced by anomalous atmospheric subsidence in the eastern Pacific from enhanced convection over the tropical South Atlantic. The stronger Pacific trade winds in turn lead to tropical central Pacific cooling and further Walker circulation acceleration. So, a collapsed AMOC will force the tropical Pacific into a more La Niña-like state1.

La Niña typically increases the chance of above average rainfall for northern and eastern Australia during spring and summer and the chance of warmer days and nights in northern Australia during spring. This is what we have seen in Australia over almost the last three years7, and the fact that this could become the norm, will make so many inland towns not only uninsurable, but probably uninhabitable. In this new norm, the top ten most at-risk federal electorates are Nicholls and Indi in Victoria, Richmond and Page in New South Wales, Hindmarsh in South Australia, and Maranoa, Moncrieff, Wright, Brisbane and Griffith in Queensland. It is estimated that 15% of the properties (some 165,000) in these electorates will be uninsurable within the next 7 years8.

As an example, Lismore, the town that has suffered an enormous amount of damage from flooding this year, had severe flooding in late February and again in late March. The flood peak in February was the highest ever seen in Lismore, with up to a metre of rain falling in a couple of days in the area9. And yet you still have some members of the federal Liberal and National parties still denying that climate change is actually happening10-13, or if it is happening there is nothing we can do about it. The latter are probably largely correct now, in that the chance to do anything about it was lost during the years of government climate change denial14. This government denial was in part due to bribes paid by fossil fuel companies to these political parties. Another factor was the idiocy of so many of the politicians in the Coalition parties. It is they who have absolutely no understanding of how science works; they just think they do15-17.

The behaviour of the climate change deniers and their purchased politicians is criminal. They need to be made to pay.



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