To enhance our understanding of clouds in the Arctic climate system, the project Arctic Summer Cloud Ocean Study (ASCOS) left Longyearbyen on Svalbard in early August 2008 on the Swedish icebreaker Oden, heading for the central Arctic Ocean. The mission was to spend as much time as possible in the central Arctic pack ice, drifting with the ice while making detailed observations of how clouds form and dissipate. ASCOS is a project within the International Polar Year 2007–2008 (IPY) and perhaps the most extensive Arctic IPY-project for atmospheric observations. The concept of the expedition was born already in 2003 and now it was finally happening.

Without an understanding of how clouds form and dissipate, we have no hope of understanding climate itself. This is especially true in the Arctic; the presence of clouds and their optical properties is the single most important factor in controlling the surface energy balance, which in turn determines the melting or freezing of the Arctic Ocean perennial sea ice. It is especially difficult to understand how clouds respond to a changing climate and how this response itself will then again affect the climate – this is called the “climate feedback” and constitutes a major element of uncertainty in climate science. In the summer, Arctic cloud formation occurs under natural pristine conditions with very little impact from man-made pollution. Our understanding of how this system functions is very limited, due to a paucity of observations from the central Arctic, which is to a large extent a result of difficulties in performing observations here. We have found indications of links between cloud droplet formation and marine biogenic production of the tiny particles that cloud drops can form on. This marine biology has a peak in summer when solar radiation is available and the melting process liberates nutrition frozen into the ice. This makes the late summer and the early autumn freeze-up a target period for ASCOS.

Improved cloud modelling

Climate changes faster in the Arctic than elsewhere and a main feature where this has become obvious is the rapidly decreasing area of the summer ice. Part of the Arctic Ocean sea ice melts every summer and the melted area, with a maximum in September, has increased dramatically over the last decades. While our goal is to be able to improve modelling of clouds in the Arctic climate system, to provide better decision support for policy makers and politicians. Quite obviously it remains impossible to model a phenomenon we do not understand. One objective of ASCOS is therefore to provide observations that will facilitate such understanding.

To understand cloud formation, a combination of different scientific specialties is required, which consequently makes ASCOS genuinely interdisciplinary. Temperature and moisture conditions – the weather – have to be optimal,  thus meteorologists are needed. However, even with optimal meteorological conditions, formation of the droplets or ice crystals that make up a cloud requires the presence of tiny airborne particles, so-called aerosols. Understanding how these form and affect the optical properties of the clouds requires experts on atmospheric chemistry and on the physics and chemistry of the aerosol. During summer Arctic pack ice conditions, natural particles originating from sea or ice algae, bacteria, and viruses in the open water between ice f loes are important for cloud formation. To study these processes marine chemists and biologists are required. Oceanographers are required to study the effects on the interface between the ice and the upper ocean. And most importantly, to integrate all this knowledge and estimate the climate feedbacks, all scientists from the different disciplines have to communicate; this is what ASCOS is all about.

Finding a suitable ice sheet

The observations in ASCOS covered a column from a depth of 400 metre into the ocean, up through the troposphere, the lowest 8–12 km of the atmosphere where clouds and weather occur. We aimed to conduct sampling in the central Arctic Ocean pack ice during the end of the summer melt and the start of the autumn freeze up. On the way to the central Arctic Ocean we also stayed a few days in the open ocean south of the ice edge and in the so called “marginal ice zone” (MIZ) – where the pack ice meets the open ocean.

A requirement for the fieldwork was finding solid ice where we could moor the icebreaker, allowing us to work on the ice. This was to minimize the contamination of the samples caused by the presence of the icebreaker itself. As we progressed up through the first days of pack ice, we encountered relatively thick ice, but with plenty of melt ponds affecting the integrity of the ice (picture 2); the large number and depth of melt ponds caused the ice to break easily. We also had periods of vigorous weather, which is somewhat unusual in summer. As we travelled north of 87˚N, we started to encounter stronger ice and we found “our ice floe” on 12 August, a 3 x 5 km large relatively solid ice floe with a sturdy outside corner against which we could moor Oden (picture 1). This is an important aspect of ASCOS as it was necessary to be able to turn Oden to face into the wind, to minimize sampling of its exhaust. It took us a few days before the ice camp was deployed, a process complicated by strong winds that made it difficult to erect the meteorological masts. We remained anchored to this ice floe until early on the morning on 2 September, when the return journey back towards Svalbard started; the “ice camp” thus lasted almost three weeks.

Our stay by the ice floe commenced with some 10 days of typical melting conditions: air temperatures near or slightly above zero and plenty oflow clouds. Snow that fell during a storm that persisted through the first day on the ice filled many melt ponds with slush that never quite melted. Although the melt ponds remain dark and wet until the last days of the ice drift, open freshwater on the surface was limited. This melt period was followed by a period with intermediate conditions: temperatures between -3 and -1˚C, i.e. below freezing, although it should be noted that seawater in the Arctic freezes at about -1.8˚C. From around 29 August, the start of the autumn freeze-up became very evident and as we left the temperature had fallen to well below -10˚C and no open water was present anywhere around us. Our goal to be on location for the duration of the late summer melt and the early freeze-up was thus fulfilled. The return trip through the pack ice back to Svalbard was easier than expected and allowed for two more research stations, one in the MIZ and one in the open ocean, before arriving again in the fjord off Longyearbyen early in the morning of 9 September.

New findings

ASCOS was a great success. We monitored the vertical structure of the atmosphere and the ocean below the ice, with both in situ and remote sensing instruments. We also monitored the structure and phase of precipitation and clouds as well as the energy fluxes at the surface. Furthermore we were able to conduct comprehensive sampling of chemical and physical properties of aerosols in a specially built laboratory on Oden’s fourth deck; atmospheric trace gases were also sampled here. Excitingly, micro-gel precursors to aerosols were abundant both in the so-called ocean micro-layer, the uppermost ocean surface water, as well as in the fog water. The bubbles that bring these from the water into the air were also abundant. We brought back large amounts of unique data on such aspects as multi-phase clouds and related aerosol properties, and links between aerosols and the marine biology that will keep us busy for years.