The AMOC in trouble
John Nissen
Hi everyone,
In Peter Wadhams’ book “A Farewell to Ice”, he describes a vortex of sinking water in the centre of the Labrador Sea, which produces cold saline “bottom water” and helps to drive AMOC. It is driven by cold winter winds. Another vortex is in the Greenland Sea, driven by sinking brine as sea ice forms and producing a dramatic chimney of sinking water; see plate 28 in his book. He reports that the former has weakened and the latter has disappeared altogether. The AMOC is in deep trouble.
Veli Albert Kallio has shown us a map of
the North Atlantic from the IMO, seeming to show a
failure of the AMOC [1]. I believe that
the cold anomaly south of Greenland is due to surface water originating as meltwater
and flowing from the Labrador Sea across the
Atlantic southward and southwest-ward. I
saw an animation of this surface effect at the US
stand at the COP20 in Lima
with Paul Beckwith.
The AMOC circulation continues underneath this colder surface water, but its strength is diminished because there is no wind to drive it easterly across the Atlantic towards Norway. There is also less ice formation in the Greenland Sea to create the sinking brine, which is the main salinity driver of the AMOC. Wind and salinity are the two principal drivers of the AMOC. Thus I suspect that AMOC is weakening beyond its 40% reduction since around 1980. This is having an ocean warming effect throughout the AMOC’s circulation, which is global. For example this could be contributing to the accelerated discharge from some major Antarctic glaciers.
One might hope that any decline in AMOC would help the Arctic sea ice to recover. But what we are seeing is increased incursion of Atlantic water into the Arctic. For example the Barents Sea now has seasonal ice, with hardly any left by the end of summer.
What to do? I believe that refreezing the Arctic is our best hope for re-energising the AMOC.
Cheers, John
[1] I tried to find Albert’s diagram, but the best I could find is from June 2023, which also had a huge anomaly of heat across to the Straights of Gibraltar. There is some useful commentary, e.g. as regards reduced wind speed causing less surface mixing.
EU Copernicus (July 2023)
Record-breaking North Atlantic Ocean temperatures contribute to extreme marine heatwaves
During June, the atmospheric circulation over the North Atlantic basin was unusual. The Azores High, a semi-permanent area of high atmospheric pressure over the Atlantic Ocean, was much weaker than average – the weakest in the ERA5 data record for June, by a very large margin, as shown in the map and the chart above. This follows 10 years during which the Azores High was close to or above average at this time of year.
A weakened Azores High is associated with a weakening of the winds that are typical of the region. The ERA5 data show that surface wind speeds were below average over most of the northeastern Atlantic during June. Wind speed values that were 20-30% below average often coincided with the largest positive sea surface temperature anomalies, particularly in the subtropical north Atlantic. Averaged over the northeastern Atlantic region, June 2023 saw the largest negative wind speed anomaly in the ERA5 record.
Surface wind speeds are closely linked to sea surface temperatures because reduced wind speeds lead to a reduction in the mixing of surface water with the cooler water below, allowing the sea surface temperatures to increase. Lower wind speeds also weaken the upwelling of deeper, colder water along the Canary current, an ocean current flowing southward along the coast of West Africa and then westward into the North Atlantic near the Cape Verde islands.
Then dust and SO2 pollution are mentioned.
A further consequence of weaker surface winds is a reduction in the movement of Saharan dust westward over the north Atlantic. The lack of vertical mixing in the ocean may have contributed to the rapid increase in the sea surface temperatures, while the reduction in Saharan dust over the North Atlantic also reinforced the increasing temperatures. Typically, Saharan dust has the effect of scattering solar radiation back into space before it reaches the ocean surface. [Snip]
On longer timescales, there are several other factors that may have influenced or reinforced the warm anomalies. One is of course the influence of climate change and global warming due to increasing greenhouse gas concentrations. Another factor is the decreasing particulate pollution over the northern hemisphere, in particular from Europe and North America.
While the reduced pollution is beneficial for the environment and human health, less pollution could also have an impact on the amount of solar radiation being scattered back into space, leading to increased warming.
alb...@thefarm.org
Given the many independent lines of evidence, there is overwhelming evidence for a long-term weakening of the AMOC since the early or mid-twentieth century.
***
The current cold blob is already affecting our weather, though not in the way that might be expected: a cold sub-polar North Atlantic correlates with summer heat in Europe (Duchez et al., 2016). The cooling of the sea surface is enough to influence the air pressure distribution in a way that encourages an influx of warm air from the south into Europe.
In other words, Europe becomes like a loaf of bread baked in a skillet—burnt on the bottom and cold and gooey on top. The last time this happened, toward the end of last Ice Age, it was caused by the breakup of the thick Laurentide Ice Sheet that covered northern America, sending out “iceberg armadas.”
This led to even more dramatic climate changes, linked to a complete breakdown of the AMOC. So much ice entered the ocean that sea levels rose by several meters (Hemming, 2004). Evidence that this amount of freshwater entering the northern Atlantic shut down the AMOC is found in the fact that Antarctica warmed while the Northern Hemisphere cooled (Blunier et al., 1998), indicating that the AMOC’s huge heat transport from the far south across the equator to the high north had essentially stopped.
The Equatorial region retained heat while northern parts of Europe experienced extreme cold. Tropical rainfall belts shifted, leading to warm and humid conditions as far away as Asia, and at intervals, an absence of the Asian monsoons and catastrophic drought. The danger, at least for Anglophiles and Europhiles, is that Iceland, Ireland, Scotland, Denmark and the Netherlands could literally freeze while Southern Europe and the Mediterranean, North Africa, México, the Caribbean Basin and the US Gulf Coast bake. Some models spare Northern Europe, including the Baltics, Norway and Sweden, due to countervailing warming migrating up from Africa and the Mediterranean, but still offer little hope for Ireland and the UK.
Rahmstorf, S. 2024. Is the Atlantic overturning circulation approaching a tipping point? Oceanography, https://doi.org/10.5670/oceanog.2024.501.
Albert Bates
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