"Large, wind-driven ocean currents shift towards the pole"

New publication from the SLOSH project

Figure 1 from Yang et al., 2020: The major ocean gyres and associated climatological conditions. Contour lines represent the barotropic stream function. Solid lines indicate clockwise flow, and dashed lines indicate anticlockwise flow. (a) Background shading shows the sea surface height (SSH). Black arrows illustrate the significant features of the currents (Yang et al., 2020, GRL, 47(5), doi:10.1029/2019GL085868.

The large, wind-driven current systems of the oceans have shifted towards the pole at high speed in the past 40 years. Model studies also demonstrate the significant influence of global warming. This is the conclusion reached by our SPP SeaLevel colleagues from the SLOSH project based at the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI), after evaluating global long-term satellite data of surface temperature and sea level.
Conclusions on the course of the large surface currents can be drawn from both datasets. Accordingly, both in the northern and southern hemisphere, the boundaries of the ocean vortices and their important marginal currents shift by 800 meters per year towards the pole. This shift in gigantic water masses is largely driven by global warming, as calculations, for example, with a new AWI climate model prove. The researchers are feeling the effects of change, people and nature report: Among other things, sea levels are rising in the affected regions, species are migrating and storm areas are moving in new directions. The study appears today in the Geophysical Research Letters magazine.

In simple terms, the water moves in a circle in the large, wind-driven ocean currents of the earth. There are eight such giant gyres worldwide - three in the Atlantic, three in the Pacific and one each in the Indian and the Southern Ocean. These rotating flow systems largely determine the weather and marine productivity in the coastal regions of our planet. The five subtropical ocean vortices, for example, transport heat and moisture from the tropics to the middle and higher latitudes on their western edge (e.g. Gulf Stream) and thus influence local air temperatures and precipitation. In contrast, nutrient-rich water rises from the depths on its eastern edge (e.g. Canary River). Algae, microorganisms and fish thrive in such large quantities that the upwelling areas are also known as the pantries of the oceans.

Monitoring the ocean vortices worldwide and over long periods of time involves enormous costs for long-term oceanographic measurements. The AWI researchers from the SPP SeaLevel SLOSH project have now found a new approach that makes it comparatively easy, by analyzing long-term satellite data on the surface temperature and altitude of the world's oceans, they were able to use the temperature and altitude differences to reconstruct the location of the large flow systems and their spatial extent. "Comparing the data showed that all eight large wind-driven surface flow systems have shifted in the past 40 years," says lead author and AWI oceanographer Hu Yang, a member of our SPP SeaLevel Early Career Scientists (ECS) Community.

The new method can also be used to determine the speed of the flow shift. Accordingly, the ocean vortices shift on average by 800 meters per year. “These changes can be seen particularly clearly in the southern hemisphere. In the northern hemisphere, on the other hand, factors such as the location of the continents and the development of sea ice in the Arctic influence the course of the current; we saw strong natural fluctuations here that motivated us to find out which processes drive this shift and to what extent, ”explains Hu Yang.

Simulations of the new AWI climate model show the influence of global warming
To this end, the researchers simulated the development of the flow systems using the new AWI climate model, among other things. In the first calculations, the starting conditions corresponded to a world with a carbon dioxide content as in 1850, the beginning of industrialization. They then increased the carbon dioxide content in the earth's atmosphere to twice the level and calculated the possible flow pattern for different climatic starting conditions.

Thanks to these complex models, the research team was able to differentiate exactly which changes were caused by global warming and which by natural fluctuations; calculations based on a world with high carbon dioxide values ​​revealed the same trends as seen in the satellite data, similar changes with other model runs available worldwide, showing that global warming is a key driver of this shift in flow.
Moreover, the climate simulations also provide clues as to which processes in the interplay of ocean and atmosphere cause the ocean eddies to shift. Both different observational data and model runs showed that the winds that drive the ocean currents shift towards the pole, however further investigation is underway to examine how the individual building blocks of the climate system work together, as commented by the co-author and AWI researcher Gerrit Lohmann.

The new research results expand knowledge from previous studies, according to which the eastern and western marginal currents of the large ocean vortices in particular are shifting towards the poles. "Climate data show that, for example, the Agulhas current during the last ice age was seven latitudes closer to the equator than is the case today," says Hu Yang. In order to be able to determine the speed and drivers of the shift even more precisely, long-term satellite data and historical climate data on the water temperature in the marginal areas of the ocean currents would now have to be combined.

The beginning of a fundamental sea change?

It is already clear today that the shift in large flow systems will have far-reaching consequences for people and the environment. “With the western marginal streams, for example, the paths of the winter storms and the jet stream are shifting. In the marginal areas of the eastern marginal streams, we observe that the rich ecosystems are shrinking because the shifting currents are changing the living conditions for the sea dwellers too quickly”, explains Hu Yang.

So far, there have been strong temperature changes in the Gulf of Maine (shifting the Gulf Stream), as a result of which cod stocks have migrated. Scientists made similar observations off the east coast of Uruguay and Argentina, where the Brazilian current is gradually shifting south.

Where marginal currents reach higher latitudes, the local sea level also rises above average, an aspect that citizens on the northeast coast of North America in particular are facing. In addition, the shifting of the large subtropical ocean vortices means that the nutrient-poor marine regions are expanding and the productivity of the world's oceans is declining overall, emphasizing further the significance of the shifting of the large flow rings, which could thus be the beginning of a fundamental change in the world's oceans.

The full publication can be accessed here:

Yang, H. G. Lohmann, U. Krebs-Kanzow, M. Ionita, X. Shi, D. Sidorenko, X. Gong, X. Chen, and E.J. Gowan (2020), Poleward Shift of the Major Ocean Gyres Detected in a Warming Climate, G.R.L., 47(5), doi: 10.1029/2019GL085868.

The article is based on the original press release news article which was first published at the Association of Biology, Life Sciences and Biomedicine (Verband Biologie, Biowissenschaften und Biomedizin, VBIO) website.

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