Observations of the central Arctic in general, and the atmosphere in particular, are rare. The onset of the summer melt in the high Arctic – when and how it happens – has not been studied in detail since the late 1990s as very few research expeditions have ventured into the region in the spring. Most detailed observations come from icebreaker expeditions usually carried out during the late summer and autumn when the ice is easier to navigate.

We have a lot of measurements from when the freeze starts in the autumn but we don’t have enough measurements from the onset of the ice melt, says Michael Tjernström, Professor at the Department of Meteorology and Chief Scientist for ARTofMELT2023, who with the help of Paul Zieger, Associate Professor at the Department of Environmental Science and Co-Chief Scientist, will lead the expedition.

“We know with great certainty that the melting season will begin while we are there. With the help of advanced weather forecasts, we will also see exactly where we need to be five to seven days prior,” he adds.

Atmospheric rivers behind the abrupt transition to summer?

While summer comes gradually on land, the transition between the seasons in the Arctic Ocean may be more abrupt. The researchers’ hypothesis is that this transition may be triggered by strong inflows of warm and moist air coming from further south that feed into feedbacks between the frozen sea-ice-snow surface and the atmosphere. These so-called atmospheric rivers (ARs) occur at all times of the year but are most common in the winter.

“We know almost nothing about what these ARs look like, especially their vertical shape, and in this expedition we want to study them and their effects on the surface. For example, how the sea ice and snow change, but also how the frozen surface affects the warm air coming from south – how the air becomes ‘Arctic air.’ Primarily, the air is cooled from below, and therefore clouds or fog often form. But the clouds, in combination with the warm air, also contribute to the surface warming,” explains Michael Tjernström.

Some studies suggest that ARs have become more frequent, stronger and longer lasting in a warmer climate. To more reliably predict future climate – in the Arctic and globally – current computer models must factor in both the ARs and their effects on the surface.

“The ultimate goal is to increase our knowledge of the Arctic climate system, and we do that by going there during a time of year with very few direct observations,” says Michael Tjernström.

Atmospheric rivers transport climate-relevant particles to the Arctic

Rs can also bring large amounts of tiny particles in the air to the high Arctic. These particles, known as aerosols, can be of natural origin, such as sea salt, pollen or bacterial spores, or they can be manmade, such as soot, formed from the incomplete combustion of biomass and fossil fuels.

Aerosols can act as nuclei for ice crystals in the atmosphere, “seeding” the formation of Arctic clouds, which, in turn, affects the amounts of sunlight and thermal radiation that reach the sea ice. They can also be deposited on the snow and ice, which darkens the surface and accelerates melting by absorbing more sunlight. During ARTofMELT2023, the researchers hope to learn more about the properties, size and distribution of different aerosol particles transported by ARs, as well as the clouds these particles help form during these events.

"By measuring the physical and chemical properties of aerosols, we can obtain information about their sources. Are they natural- or manmade pollutants, and how much of each type is there?” says Paul Zieger. He continues: “In the high Arctic, aerosol concentrations can be extremely low. Since aerosols are essential for cloud formation, if we were to change their concentrations there, we could potentially be changing cloud properties which, in turn, would affect how much energy can be transported to and from the ice. The latter could impact the onset of the sea ice melt,” says Paul Zieger.

According to the latest climate report by the IPCC, clouds and aerosols contribute the largest uncertainty to estimates and interpretations of the Earth's changing energy budget. Zieger hopes that the data from this year’s expedition could provide some of the answers.

"Our observations will hopefully help reduce the uncertainty by providing much needed field data to better understand how aerosols and clouds influence Arctic climate. This information would help us improve our climate models, which are essential for future climate predictions both in the Arctic and globally,” says Paul Zieger.

The weather determines the route

To document the transition between winter and summer in the high Arctic and any possible links to ARs requires flexible planning and, based on weather forecasts, being able to move the icebreaker Oden to places where warm air will enter the Arctic. But moving the ship according to the weather presents challenges.

“Getting access to advanced weather forecasts from our land-based partners poses a communication challenge as we do not have access to the internet this far north. Because we can only move quite slowly, we also need to have a working decision-making strategy to avoid losing time. Finally, we also need to be able to move the ship to where we want to go. Before the ice starts to melt, it is the most difficult time to navigate, says Michael Tjernström. He continues: “Keeping our spirits high may become challenging if nothing happens and we remain in the same place for several weeks. After all, it’s the atmosphere that decides if there will be ARs. This is something we must also be ready for,” he admits.

The icebreaker Oden will be located somewhere inside a "triangle" between northeast Greenland, northwest Svalbard, and the North Pole. There, the researchers have the highest chance of capturing ARs of different origin.

Preparing for the unexpected

The scientists expect that they will experience at least a few episodes of ARs during the six weeks they will spend in the Arctic Ocean. At the same time, they stress the importance of being flexible and of adapting according to the conditions they will encounter whilst constantly optimizing their planning so that they capture as much as possible of what they set out to observe. But surprises could be lurking!

As always in field research, one must expect the unexpected, so spontaneously, we expect surprises,” concludes Michael Tjernström.

Some facts

The Swedish icebreaker Oden will set sail from Longyearbyen on Svalbard, Norway, on 7 May carrying 40 scientists from Sweden (10 from Stockholm University), Finland, Switzerland, Germany, United Kingdom and USA, and 20 crew members for a six-week research cruise in the Artic Ocean.

ARTofMELT2023 is organised by the Swedish Polar Research Secretariat and is mainly financed through the Secretariat's own grants but also through international collaborations. The work of the teams of Professor Michael Tjernström and Associate Professor Paul Zieger is financed primarily by The Knut and Alice Wallenberg Foundation and the Swedish Research Council. In addition, crucial funding towards setting up instrumentation for measuring air particles was provided by the Carl Trygger Foundation and the European Research Council (ERC).

The icebreaker Oden is owned by the Swedish Maritime Administration but is used by the Swedish Polar Research Secretariat during the summer for expeditions to the Arctic Ocean.

More about ARTofMELT2023

ARTofMELT: Atmospheric rivers and the onset of sea ice melt 2023

ARTofMELT2023 expedition will study the arrival of summer in the Arctic

Text: Melina Granberg (Swedish Polar Research Secretariat) and Stella Papadopoulou (Department of Environmental Science)