This important planetary wind system is changing

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Analysis of sedimentary cores over 8 meters long seems to confirm that a major characteristic of atmospheric circulation will be disrupted due to climate change. The results were summarized in a study recently published in the journal Nature.

Understanding how the atmospheric circulation of mid-latitudes reacts to an increase in planetary temperature is a difficult task. However, it is crucial. Indeed, this circulation largely controls the distribution of climatic regimes on the continents. As such, Western Europe – tempered and moistened by its oceanic westerly flow – is an emblematic example.

West winds deported to the north

Observations have shown that in recent decades, the belt of westerly winds typical of our latitudes flowed back on average towards the pole. In other words, the track of the depressions and accompanying precipitation has shifted to more northern latitudes. In this context, the regions initially located at the limit of the disturbed flow – such as the Mediterranean – find themselves exposed to increasing arid influences.

Numerical simulation of general traffic. Credits: JAMSTEC / DCOP.

This development is no surprise to scientists. Indeed, it was anticipated by climate models from the 1980s and is supported by strong theoretical arguments. However, the observational series mentioned in the previous paragraph are still too short to find out the portion actually attributable to climate change of that linked to internal multi-decadal fluctuations. It therefore remains difficult to say whether actual climate change is consistent with what the models simulate.

To take a step back, researchers recently took an interest in in the Pliocene climate about 3 million years ago. Why exactly at this time? The answer lies in concentrations of CO2 similar to those we are currently experiencing. In addition, the temperatures reached values ​​that we should know in the second half of the century if we do nothing. That is to say between +2 ° C and +4 ° C on average overall.

The Pliocene, an analogue of the future climate?

The geological scale considered makes it possible to study a climate which has had time to balance all its components. The authors therefore looked at how the mid-latitude westerly winds had evolved. To do this, two sediment cores taken at 36 ° and 45 ° in the North Pacific were analyzed. In these areas, the winds deposit large amounts of dust sucked over the Asian continent. By measuring the dust concentration and the differences between the two samples, it is thus possible to deduce the strength and location of the temperate flow.

warming structure
Latitudinal structure of warming in winter. Note the maxima at the surface in the Arctic and at altitude in the tropics. It is the latter which seems to deport the disturbed flow to the north in a warmer climate. Credits: Y. Peings et al. 2019.

The results confirm the suspicions. During the hot peak of the Pliocene, westerly winds have broadly weakened and shifted towards the pole. On the contrary, when transitioning to colder conditions, it was a strengthening and a more southerly position that the researchers found. “I think it’s important … because scientists will be able to incorporate this information into climate models to more accurately represent what we can expect in the future.”, notes Sarah Aarons, a geochemist who did not participate in this study.

In summary, with global warming, the belt of westerly winds may well no longer blow over part of the temperate latitudes. Nevertheless, a migration of this magnitude will take more than a few decades to develop. The characteristic scale seems to be of the order of several centuries, but with very serious implications in terms of water supply for the affected regions. “Our work in the Pliocene is really important, but being able to limit the uncertainties even more is what we hope to do next”, says Jordan Abell, main author of the paper.

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