Oceanographer Benjamin Rabe plans to study the phenomenon of upwelling of warm water and nutrients already observed at the edges of the Arctic Ocean to see if it will extend towards the North Pole over the next decade. His project is based on the Tara Polar Station and its series of Polaris expeditions. An Interview.
Benjamin Rabe, you are a physical oceanographer at the Alfred Wegener Institute, and have been studying the high latitudes of the Arctic since 2006. For the past five years, you’ve been involved in preparing the missions of the Tara Polar Station, a research vessel christened last April to drift through pack ice and observe the Arctic Ocean. However, the German research icebreaker Polarstern is capable of conducting scientific operations in polar environments. What do you find interesting about Tara’s polar station for conducting your research, and how do these two vessels complement each other?
When Nansen went to the Arctic in 1893 with Fram, he paved the way for other expeditions. Tara’s drift in 2007 and the MOSAiC mission in 2019 are examples of this, both increasingly scientific, using the principle of drift in the transpolar current. What we can learn from all this is that continuity of measurement is essential, especially in times of marked changes in the way the ocean works.
When Polarstern conducted its observations in 2019, it was a major effort, involving many scientists and ships. Even today, new scientific publications are based on this expedition, but these operations are difficult to repeat every year.
Tara Polar Station has a completely different format. With a reduced staff, it was designed to operate over a longer period. This way, we can manage programs and work over a decade of repeated Arctic crossings.
During the MOSAiC expedition, you studied the evolution of freshwater in the upper Arctic Ocean and its links with inputs from the continents and other oceans, as well as interactions between the ocean and sea ice. You are also interested, more broadly, in the decadal changes associated with global warming. What do you hope to discover with Tara Polar Station and its Polaris I, II, III missions?
We plan to observe the evolution of the ocean over the seasons, over several years. We will deploy autonomous vehicles and instruments on the ice next to the station to gain access to internal waves, temperature, salinity, etc., and use the opportunity to develop new sensors.
We’re going to focus on the first 100 meters of depth from the surface, and deeper down to the layer of warm Atlantic Water arriving through the Fram Strait. Normally, this warmer layer remains fairly isolated from the surface due to stratification, i.e. the layers of different salinity above it – the halocline – but this stratification is changing. It is weakening along the continental slopes, and this is already happening near the Laptev Sea.
I’d be interested to see if it extends towards the center of the ocean. At the starting point of our drift, we may observe it, and we’ll see how it evolves on our way to the North Pole.
In winter, the formation of sea ice reinforces the presence of salt in the surface water, triggering vertical exchanges in the upper ocean. In this way, various properties of the water rise to the surface, which could potentially be the case for the heat contained in the Atlantic’s layer of warm water. But we’ll need at least ten years, if not more, to distinguish the underlying trend from inter-annual variability, i.e. one-off phenomena.
I also wonder whether the upwelling of nutrients will be sufficient to feed phytoplankton throughout the light season, since stratification normally strengthens in summer and the supply of nutrients diminishes.
However, in some parts of the Arctic, it has been observed that stratification in the upper ocean has weakened and near-surface mixing during winter has intensified. It is likely that more nutrients will be available by accessing layers that are usually out of reach.
Even if stratification, boosted by meltwater from the pack ice, remains important in the central Arctic, the upwelling of heat and nutrients could begin to be observed. Indeed, another phenomenon could come into play: the arrival of waves, with the overall reduction in ice thickness.
With the Polaris missions, you’ll be increasing observations of the central Arctic in winter for at least ten years, in the run-up to International Polar Year 2032-2033, a major scientific event for the study of the poles. How do you see the Arctic Ocean in less than ten years’ time?
Some parts of the Arctic should approach the normal conditions of the northern Atlantic Ocean. This will not affect the entire Arctic Ocean. Predictions show that an ice-free ocean in summer could appear within a few decades – it could also happen sooner. But we need to see what this means in concrete terms. It could be true one year, but not necessarily the next.
Researchers are on the launch pad for Polar Year, and this is generating a lot of projects. Tara Polar Station will make a major contribution to this scientific climax. As part of this joint international effort, the experience we’ll be acquiring will be useful for increasing the scale of observations.
In the meantime, the collaboration between Tara and the Alfred Wegener Institute should help identify specific phenomena in the ocean on which Polarstern could focus from time to time. Just as this summer, the oceanographic vessel will escort the station into the ice for the launch of the first tests in icy conditions.
Dr Benjamin Rabe is a physical oceanographer at the Alfred Wegener Institute in Bremerhaven, Germany, where he also teaches. A specialist in the Arctic Ocean, he studies the circulation, stratification and evolution of water masses. He has taken part in over ten expeditions, most of them aboard the Polarstern icebreaker, and coordinates projects linked to the MOSAiC and Polaris expeditions.