Simulations with the new AWI climate model reveal the influence of global warming
To do so, the researchers simulated the evolution of the current systems, e.g. with the new AWI climate model. In the first simulations, the starting conditions were equivalent to those for a world with the same level of atmospheric carbon dioxide content as in 1850, the dawn of industrialisation. The experts then gradually raised the amount of CO2 in the Earth’s atmosphere until it was twice the 1850 level, and calculated the potential current development for a range of initial climatic conditions.
Thanks to their advanced modelling efforts, the team succeeded in precisely discriminating between changes caused by global warming, and those produced by natural variations. “Our calculations for a world with high carbon dioxide values produced the same trends that we saw in the satellite data. And we’re seeing similar changes in the analyses we run with other available model runs from around the globe. In this way, we can show that global warming is a major motor for these shifting currents,” says co-author and AWI climate modeller Prof Gerrit Lohmann.
In addition, the climate simulations offer clues as to which processes in the interaction of the ocean and atmosphere are contributing to the shift. According to Lohmann, “We can see, in various types of observational data, and in our model runs, that the winds powering the currents are now moving poleward. How the individual components of the climate system are interlinked in this regard is an aspect we’re now investigating in follow-up projects.”
The team’s new findings build on those from previous studies, which indicated that especially the gyres’ eastern and western boundary currents were moving toward the poles. “For example, the climate data confirm that during the last ice age, the Agulhas Current was seven degrees of latitude closer to the Equator than it is today,” says Hu Yang. In order to more precisely measure the speed and the drivers of the shift, the long-term satellite data would need to be combined with historical climate data of the water temperature near the gyres’ borders.
The beginnings of a fundamental change in the ocean?
But one thing is already certain: these shifts in the major current systems will have far-reaching consequences for human beings and the environment alike. “For example, as the western boundary currents continue to shift, the courses of winter storms and of the jet stream are following suit. At the edges of the eastern boundary currents, we’re now seeing the rich ecosystems begin to shrink, because the shifting currents are changing the living conditions too quickly for marine organisms to adapt,” Yang explains. Dramatic temperature changes have been observed e.g. in the Gulf of Maine (due to the shifting Gulf Stream), resulting in a migration of the cod stocks. Researchers have observed similar changes off the Atlantic coasts of Uruguay and Argentina, where the Brazil Current is gradually moving south.
In addition, when boundary currents penetrate higher latitudes, the local sea level rises disproportionately – a problem that especially communities on the northeast coast of North America are now confronted with. And to make matters worse, the displacement of the major subtropical gyres is causing the nutrient-poor regions to expand, reducing the productivity of the ocean as a whole. Accordingly, the shift in the gyres could represent the beginning of a fundamental change in the ocean.
Information for members of the press
The satellite data used in the study covers the timeframe from 1982 to 2018 for the ocean surface temperature, and from 1992 to 2018 for the sea level height.