Small-scale phenomena impact ocean circulation – 09/14/2023 – Fundamental Science

A recent article published in the journal Nature Communications caught the attention of the media by concluding that the probability of a collapse of the so-called Atlantic Meridional Overturning Cell (AMOC) is high by the end of the century. In other words: the system of marine currents that regulates the temperature of the North Atlantic (and consequently, Europe) would be threatened.

But what exactly is this phenomenon? And why is it crucial to understand small-scale interactions to predict their real impacts?

AMOC is an ocean circulation pattern associated with the formation of deep waters in the Labrador Sea and Nordic Seas, close to Greenland. Due to heat loss to the atmosphere at high latitudes, the waters at the ocean’s surface cool and consequently become heavy and sink.

At high depths, these waters move south to the Antarctic convergence region. Along its path, deep waters slowly return to the surface due to the combined action of strong winds in the Southern Ocean (also called the Southern Ocean) and their mixing with warmer waters that occupy the upper regions of the water column in the tropics and subtropics.

Around 15 million cubic meters of water sink per second in the Greenland region, equivalent to 75 times the average flow of the Amazon River. This volume of colder water that submerges and eventually moves south is supplied by warm, recently resurfaced surface waters that are transported from the South Atlantic to the North Atlantic. Because it is interhemispheric and carries a colossal amount of heat, the AMOC is closely linked to the global climate.

The possibility of a collapse of the upper branch of the AMOC is not exactly new. Thanks to the warming of the planet, the continental ice in Greenland has been melting and reaching the ocean as fresh water. Therefore, surface waters in the Labrador Sea and Nordic Seas are becoming less saline and, consequently, lighter, which makes it more difficult for them to sink and weakens the AMOC.

What stood out most to the media was the divergence of this study in relation to the latest report from the UN Intergovernmental Panel on Climate Change (IPCC), published in 2021, in which direct measurements in the North Atlantic suggested a small weakening of the AMOC in recent decades. . Based on this data and climate projections made with global models, the IPCC concluded that a collapse of the AMOC is unlikely by the end of the century. Despite advances in understanding the AMOC and simulating global climate, these projections are still quite uncertain.

Imagine writing your name in a calligraphy notebook with a four-inch brush, like the one used to paint a wall. You can try hard, but you will hardly be able to leave your autograph on the lines of the notebook: your tool does not have enough resolution to resolve the calligraphic details. This limitation serves as an analogy for the root of uncertainties in long-term climate projections.

Global climate models are a combination of mathematical equations that govern the evolution of fluids, such as water and air, and observational data. Such models divide the Earth into a gridded grid with a certain spatial spacing. Executed on supercomputers, in this global mesh they simulate physical quantities, such as temperature.

The size of the represented phenomena is limited by the mesh spacing, which in turn is determined by the current available computing power. The climate projections analyzed by the IPCC have a spatial resolution of around 100 kilometers, sufficient to represent in some detail phenomena such as the AMOC, which cover tens of thousands of kilometers.

The challenge is that the ocean, like the atmosphere, is a turbulent fluid. This means that there are interactions between phenomena of different sizes. In practice, the evolution of planetary phenomena, such as AMOC, is influenced, perhaps even controlled, by small-scale phenomena, such as fronts, instabilities and waves on the surface and in the interior of the ocean.

With a spatial scale of a few meters to tens of kilometers, these phenomena are far from being captured by global models. Following projections of Moore’s Law, which roughly predicts the doubling of computational capacity every two years, the inability of models to capture these phenomena will be a reality for many decades.

Although global climate projections do not have sufficient resolution to explicitly capture fronts, waves and instabilities, the effects of these phenomena on the evolution of large-scale ocean circulation and global climate need to be taken into account in some way. In scientific parlance, models implement parameterizations, that is, formulas that attempt to capture the effects of small scales on the evolution of large-scale phenomena. An active area of ​​research, the development of parameterizations of small-scale oceanic phenomena requires basic research on physical mechanisms that are still very little understood.

This research, which is overlooked in the media, is fundamental to improving the accuracy of climate projections under the influence of global warming and better predicting the weakening of the AMOC and the timing of its eventual collapse.

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César B. Rocha has a PhD in physical oceanography from the University of California San Diego (USA) and professor at USP.

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