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Kapsch: Springtime atmospheric energy transport and the control of Arctic summer sea-ice extent

2013 May 6


Figure S5: Radiative and turbulent flux anomalies at the surface for LIYs from
NCEP-DOE R2. The black line shows the sea-ice concentration (ERA-Interim reanalysis).
a, displayed is the net longwave radiation plus the turbulent fluxes (latent
and sensible; in red) and the net shortwave radiation (green). b, the radiative fluxes
are split into their components but only downwelling longwave (red) and shortwave
(green) radiation are shown together with the latent (dark blue) and sensible (light
blue) heat flux. All time series are based on daily anomalies of LIYs and averaged
over the area indicated by the red box in Supplementary Fig. 2. A 30-day runningmean
filter is applied to all time series.

Springtime atmospheric energy transport and the control of Arctic summer sea-ice extent

The summer sea-ice extent in the Arctic has decreased in recent decades, a feature that has become one of the most distinct signals of the continuing climate change1, 2, 3, 4. However, the inter-annual variability is large—the ice extent by the end of the summer varies by several million square kilometres from year to year5. The underlying processes driving this year-to-year variability are not well understood. Here we demonstrate that the greenhouse effect associated with clouds and water vapour in spring is crucial for the development of the sea ice during the subsequent months. In years where the end-of-summer sea-ice extent is well below normal, a significantly enhanced transport of humid air is evident during spring into the region where the ice retreat is encountered. This enhanced transport of humid air leads to an anomalous convergence of humidity, and to an increase of the cloudiness. The increase of the cloudiness and humidity results in an enhancement of the greenhouse effect. As a result, downward long-wave radiation at the surface is larger than usual in spring, which enhances the ice melt. In addition, the increase of clouds causes an increase of the reflection of incoming solar radiation. This leads to the counter-intuitive effect: for years with little sea ice in September, the downwelling short-wave radiation at the surface is smaller than usual. That is, the downwelling short-wave radiation is not responsible for the initiation of the ice anomaly but acts as an amplifying feedback once the melt is started.

Springtime atmospheric energy transport and the control of Arctic summer sea-ice extent
Marie-Luise Kapsch, Rune Grand Graversen & Michael Tjernström
Nature Climate Change
doi:10.1038/nclimate1884
http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1884.html

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