Article Available Online
Journal of Climate
"The Early Twentieth-Century Warming in the Arctic - A Possible Mechanism"
The article is available at:
http://www.nersc.no/MACESIZ/ben04.pdf
An article entitled "The Early Twentieth-Century Warming in the Arctic -
A Possible Mechanism", recently appeared in the Journal of Climate
(Volume 17, pp. 4045-4057) and is available on the Nansen Environmental
and Remote Sensing Center (NERSC) web site at:
http://www.nersc.no/MACESIZ/ben04.pdf
ABSTRACT
The huge warming of the Arctic that started in the early 1920s
and lasted for almost two decades is one of the most spectacular climate
events of the twentieth century. During the peak period 1930-40, the
annually averaged temperature anomaly for the area 60º-90ºN amounted to
some 1.7ºC. Whether this event is an example of an internal climate mode
or is externally forced, such as by enhanced solar effects, is presently
under debate. This study suggests that natural variability is a likely
cause, with reduced sea ice cover being crucial for the warming. A
robust sea ice-air temperature relationship was demonstrated by a set of
four simulations with the atmospheric ECHAM model forced with observed
SST and sea ice concentrations. An analysis of the spatial
characteristics of the observed early twentieth-century surface air
temperature anomaly revealed that it was associated with similar sea ice
variations. Further investigation of the variability of Arctic surface
temperature and sea ice cover was performed by analyzing data from a
coupled ocean-atmosphere model. By analyzing climate anomalies in the
model that are similar to those that occurred in the early twentieth
century, it was found that the simulated temperature increase in the
Arctic was related to enhanced wind-driven oceanic inflow into the
Barents Sea with an associated sea ice retreat. The magnitude of the
inflow is linked to the strength of westerlies into the Barents Sea.
This study proposes a mechanism sustaining the enhanced westerly winds
by a cyclonic atmospheric circulation in the Barents Sea region created
by a strong surface heat flux over the icefree areas. Observational data
suggest a similar series of events during the early twentieth-century
Arctic warming, including increasing westerly winds between Spitsbergen
and Norway, reduced sea ice, and enhanced cyclonic circulation over the
Barents Sea. At the same time, the North Atlantic Oscillation was
weakening.
Authors:
Lennart Bengtsson (Max Planck Institute for Meteorology, Hamburg,
Germany, and Environmental Systems Science Centre, University of
Reading, Reading, United Kingdom)
Vladimir A. Semenov (Max Planck Institute for Meteorology, Hamburg,
Germany, and Obukhov Institute of Atmospheric Physics, Moscow, Russia)
Ola M. Johannessen (Nansen Environmental and Remote Sensing
Center/Geophysical Institute, University of Bergen, Bergen, Norway)