AMOC – the heating of Europe

What is the AMOC?

The Atlantic Meridional Overturning Circulation (AMOC) consists of warm, northward-directed near-surface currents, a subsidence of large water masses at high latitudes and deep bottom currents at the western edge of the Atlantic. The AMOC is part of a global oceanic redistribution system for heat and freshwater, but many people mistakenly equate it 1:1 with the Gulf Stream. However, the Gulf Stream is essentially driven by the trade winds. They carry the surface water from east to west, while the American continent deflects it to the north. In the process, large quantities of warm surface water are transported to higher latitudes. At its maximum, the Gulf Stream transports 150 Sv (1 Sverdrup corresponds to 1 million cubic meters per second, all rivers in the world combined carry approximately this amount of water). The Gulf Stream will not come to a standstill even in a warmer world!

Off the coast of Newfoundland, it meets the Labrador Current and is thus pushed away and branched out. One of these arms becomes the North Atlantic Current, which then splits further at the level of Ireland. Each of these arms is driven by thermohaline circulation. On its way north, a lot of moisture evaporates from the warm surface water, causing the salt content to rise – and salinity to increase. On its way north, the water also releases a lot of heat into the atmosphere above, making it cooler. In combination, the density increases, the water becomes heavier and begins to sink. In the waters off Canada and Norway, large quantities of water sink in this way and begin to flow southwards again at a depth of 2 to 3 kilometers near the bottom. The temperature and salinity are therefore decisive for the effectiveness of this mechanism.

Dilution by fresh water

The system of ocean currents is complex and subject to natural fluctuations, with overlapping causes and effects. The effects on the thermohaline circulation have been discussed in the context of global warming and the increasing amount of meltwater from the Greenland ice sheet and the influx of freshwater from the rivers. This is because the fresh water reduces the salt content and the rising water temperatures further reduce the density. It was therefore expected early on that the sinking process and, as a result, the AMOC would be weakened or at least modified. Data from sediments and ice cores prove that this has already happened several times (Heinrich events).

Saltwater from the Indian Ocean

The influence of an ocean current in the Indian Ocean on the North Atlantic – the Agulhas Current also shows how complex and interconnected the system actually is. It carries warm and salty water along the east coast of Mozambique and South Africa to the southern tip of Africa. There, however, it makes a relatively sharp turn, the so-called retroflection. However, bubbles from the salty Agulhas Current still enter the South Atlantic in batches. These form vortex-like structures, the Agulhas rings.

They migrate northwestwards across the South Atlantic towards the Caribbean and the Gulf Stream. As a result, the latter is virtually inoculated with saltier water. Interestingly, this proportion has actually increased as global warming has progressed.

Approaching tipping point?

It could therefore be assumed that the small portions of saltwater from the Indian Ocean support the thermohaline circulation in the North Atlantic and possibly compensate for the diluting effect of the rising southern water input. This may also be the case to a certain extent, but it does not appear to be sufficient. Increasingly elaborate studies and simulations have led to growing evidence that the AMOC is not simply weakening continuously, but that it can also collapse rapidly once a tipping point is reached. In recent days, a new study (link to original article) has caused a huge stir. Using the most complex simulation to date, a Dutch research team has defined several signals that appear before such a tipping point is reached. Just 2 to 3 years ago, this was much less clear, but we seem to be closer to such an event than we thought. No specific year is mentioned, but we are not talking about centuries, but years (worst case) up to several decades. The AMOC has already seemed weaker and more fragile than before since the middle of the last century, and now climate change is fueling those factors that suggest a further weakening.

Impacts

Much has been written recently about the possible effects. If Europe's heating comes to a standstill, this would have fatal consequences for northern Europe in particular – Temperatures would drop significantly and become more similar to those in Canada at the same latitude. This would not only make it colder, but also drier. By contrast, the poorer heat dissipation would increase warming in the lower latitudes, particularly in the southern hemisphere. Despite the significant regional cooling in Europe, it would continue to get warmer globally. The contrasts are simply intensifying!