Abstracts
SEARCH Open Science Meeting
October 27, 2003
Seattle, Washington, USA
Possible Feedbacks on Arctic Cloud Formation: Can the Arctic Biosphere Affect the Melting of the Ice?
Michael Tjernström1, Caroline Leck2
1Department of Meteorology, Stockholm University, Arrhenius lab., 106 91 Stockholm, Stockholm , SE-106 91, Sweden, Phone 46-816-3110, Fax 46-815-7185, michaelt@misu.su.se
2Department of Meteorology, Stockholm University, Arrhenius lab., 106 91 Stockholm, Stockholm , SE-106 91, USA, Phone 46-816-4354, Fax 46-815-9295, lina@misu.su.se
Boundary layer clouds are an important factor affecting the energy balance at the surface in the Arctic. In contrast to the mid-latitude oceans, low-level clouds are a warming factor for the Arctic Ocean through most of the year. During winter, the effects of low-level clouds are the single most important local factor determining the stability of the lower troposphere. Clouds modulate the energy balance at the surface with amplitudes far larger than those imposed by an enhanced greenhouse effect. In summer, with larger cloud fractions, changes in the microphysics of clouds - more small, or fewer large droplets - can alter their radiative properties for solar radiation. Formation of clouds requires the presence of small airborne particles, Cloud Condensation Nuclei or CCN. While the amount of water in a cloud is determined by the thermodynamic and dynamic properties of the atmosphere (e.g. temperature, moisture and vertical motions and mixing), the number of droplets is regulated by the abundance of CCN. With many CCN the condensed water is distributed over many small droplets, rather than over a few large. This in turns makes the cloud look “whiter”, thus reflecting more solar radiation back to space.
Where then does these particles come from? There are obviously anthropogenic sources, related to burning of fossil fuels. Such sources are mainly located in industrial areas at large distances from the central Arctic. There are also natural sources for example breaking wind-driven ocean waves generate a spray of sea-salt particles that are effective CCN. These are probably of smaller importance in the central Arctic, since the fraction of open sea is small. A large natural source is due to biological activity; gracing of algae by zooplankton generate a gas called DMS; its sulfur becomes oxidized in the atmosphere to sulfate particles. The latter is a dominant source in the summertime Arctic marginal ice zone. However, as these particles become CCN while travelling in over the pack ice, they will become parts of clouds droplets that eventually deposits at the surface by gravitational settling or precipitation, and the particles are lost for ever. The further in over the pack ice the air gets, the less CCN remain in the air, which affects the properties of Arctic clouds, making them “grayer” than their midlatitude counterparts. These processes are poorly described in current climate models.
Can climate change alter the Arctic system such that more biogenic particles are produced locally by, for example, opening larger areas of open water? Are there other processes that produce biogenic aerosols locally? Will such an enhanced local production of CCN in the central Arctic Ocean act as a negative feedback, producing brighter clouds that reflect more solar radiation back to space? The Arctic Ocean Experiment 2001 (AOE-2001) on the Swedish icebreaker Oden was launched to take in situ measurements of atmospheric chemistry, aerosols and boundary-layer structure during the summer 2001, to help answer these questions. We found clear evidence that local aerosol production at the ocean surface occurred even when the ice fraction was rather large. In addition to formation of new very small particles, we also found evidence of new moderately large aerosols directly from open leads, containing bacteria and virus. These were very similar to particles sampled from the biogenic surface film on the open leads. The boundary-layer structure was also relatively well mixed in the lowest 100’s of meters, but often capped by a very strong inversion. This would facilitate mixing of surface generated aerosols through the boundary layer, but inhibit entrainment of aerosols or aerosol precursor gases from distant sources long-range transported in the free troposphere.
Abstract Categories: Physical Feedbacks
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