Atmospheric rivers are thawing Arctic ice

Extreme currents: The sea ice of the Arctic is not only being decimated by general warming – atmospheric currents are also increasingly affecting it, as a study reveals. Accordingly, these huge streams of warm, humid air are reaching the polar regions more and more frequently, where they inhibit the formation of new sea ice in winter. In the Barents Sea and the central Arctic alone, this phenomenon is responsible for a good third of the sea ice loss, as researchers report in “Nature Climate Change”.

They are thousands of kilometers long, hundreds of kilometers wide and can cause torrential rain for days: Atmospheric rivers sometimes form over the tropical seas, especially in winter – fast streams of water vapor-saturated air that reach into the middle and higher latitudes and rain down huge masses of water there. This extreme weather phenomenon repeatedly causes devastating heavy rain, landslides and floods, especially on the west coast of North America.

The frequency and intensity of such atmospheric fluxes have increased in recent decades, studies show. Climate researchers assume that the warming of the oceans and atmosphere due to climate change is contributing to this increase.

Phenomenon is also becoming more common in the Arctic

But this doesn’t just apply to mid-latitudes, as Pengfei Zhang of Pennsylvania State University and his colleagues have found. For their study, they examined how often atmospheric flows reach into the central and Eurasian Arctic, how the frequency of these events has changed since 1979, and what consequences this has for Arctic sea ice. Because previous studies had already suggested that the atmospheric fluxes could increasingly shift further poleward.

In fact, the analyzes showed that over the past 40 years or so, the frequency of atmospheric fluxes over the central Arctic and the Barents and Kara Seas has increased significantly. This trend is caused on the one hand by natural fluctuations in the tropical climate, including the interdecadal Pacific Oscillation, and on the other hand by man-made climate change: “The increase in atmospheric fluxes corresponds to the increasing water storage capacity of the warming atmosphere,” the scientists explain.

Renewal of sea ice inhibited

The problem with this is that these powerful streams of warm, humid air reach the polar region primarily in winter and thus at the time when new sea ice is forming in the Arctic Sea. As a result, the atmospheric flows disrupt the replenishment of ice considerably: the rain they bring with them and the greenhouse effect of these enormous water vapor flows prevent the formation of new sea ice, as Zhang and his team determined.

While the sea ice area grows rapidly without an atmospheric flow, especially in November, December and January, this growth comes to a standstill almost completely during such a water vapor influx, as the evaluations show. This inhibiting effect also lasts up to ten days after the end of the weather phenomenon. “The sea ice retreat is not only caused by gradual influences such as warming, but also by such episodic weather events,” says co-author L. Ruby Leung from the Pacific Northwest National Laboratory.

Responsible for 34 percent of sea ice loss

Zhang and his team have also determined what this means for the Arctic sea ice as a whole. According to this, the increase in atmospheric fluxes is responsible for around 34 percent of the total winter sea ice retreat in the Arctic. “Although temperatures in the Arctic are still below freezing, the ice retreat is significant even in winter – our results show that the atmospheric fluxes are a cause,” says Zhang.

According to the researchers, this demonstrates that the impact of climate change on Arctic sea ice and the Arctic is more complex and far-reaching than is often assumed. “When this type of moisture transport occurs in the Arctic, the atmospheric flow not only results in heavy rain or snowfall, it also has a strong thawing effect on the ice,” says co-author Mingfang Ting of Columbia University in New York.

In view of the rapid loss of sea ice in recent years and decades, this is an important finding. In addition, there are first indications of a similar accumulation of atmospheric fluxes over Greenland and West Antarctica – these are also polar regions with particularly high ice loss. (Nature Climate Change, 2023; doi: 10.1038/s41558-023-01599-3 )

Source: Pennsylvania State University