[Bolincentret-at-su.se] PhD defense - Marie Kapsch - Oct 16

[Cecilia Wesslén]

PhD defense on October 16

*Name*
Marie Kapsch
Department of Meteorology, Stockholm University, Sweden

*Title*
The atmospheric contribution to Arctic sea-ice variability

*Time and place*
Friday 16 October 2015, 10.00
Nordenskiöldsalen, Geo building, U house, floor 3

*Abstract*
The Arctic sea-ice cover plays an important role for the global climate 
system. Sea ice and the overlying snow cover reflect up to eight times 
more of the solar radiation than the underlying ocean. Hence, they are 
important for the global energy budget, and changes in the sea-ice cover 
can have a large impact on the Arctic climate and beyond. In the past 36 
years the ice cover reduced significantly. The largest decline is 
observed in September, with a rate of more than 12% per decade. The 
negative trend is accompanied by large inter-annual sea-ice variability: 
in September the sea-ice extent varies by up to 27% between years. The 
processes controlling the large variability are not well understood. In 
this thesis the atmospheric contribution to the inter-annual sea-ice 
variability is explored. The focus is specifically on the 
thermodynamical effects: processes that are associated with a 
temperature change of the ice cover and sea-ice melt. Atmospheric 
reanalysis data are used to identify key processes, while experiments 
with a state-of-the-art climate model are conducted to understand their 
relevance throughout different seasons. It is found that in years with a 
very low September sea-ice extent more heat and moisture is transported 
in spring into the area that shows the largest ice variability. The 
increased transport is often associated with similar atmospheric 
circulation patterns. Increased heat and moisture over the Arctic result 
in positive anomalies of water vapor and clouds. These alter the amount 
of downward radiation at the surface: positive cloud anomalies allow for 
more longwave radiation and less shortwave radiation. In spring, when 
the solar inclination is small, positive cloud anomalies result in an 
increased surface warming and an earlier seasonal melt onset. This 
reduces the ice cover early in the season and allows for an increased 
absorption of solar radiation by the surface during summer, which 
further accelerates the ice melt. The modeling experiments indicate that 
cloud anomalies of similar magnitude during other seasons than spring 
would likely not result in below-average September sea ice. Based on 
these results a simple statistical sea-ice prediction model is designed, 
that only takes into account the downward longwave radiation anomalies 
or variables associated with it. Predictive skills are similar to those 
of more complex models, emphasizing the importance of the spring 
atmosphere for the annual sea-ice evolution.

*Welcome!*

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