Recent Trends in Arctic Sea Ice and the Evolving Role of Atmospheric Circulation Forcing, 1979–2007 - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 6

Recent Trends in Arctic Sea Ice and the Evolving Role of Atmospheric
Circulation Forcing, 199-200

Clara Deser and Haiyan Teng

National Center for Atmospheric Research, Boulder, Colorado, USA

This study documents the evolving trends inArctic sea ice extent and concentration
during 199-200 and places them within the context of overlying changes in
the atmospheric circulation. Results are based on 5-day running mean sea ice
concentrations (SIC) from passive microwave measurements during January 199
to October 200. Arctic sea ice extent has retreated at all times of the year, with
the largest declines (0.65 × 10 6 km 2 per decade, equivalent to 10% per decade in
relative terms) from mid July to mid October. The pace of retreat has accelerated
nearly threefold from the first half of the record to the second half, and the number
of days with SIC less than 50% has increased by 19 since 199. The spatial patterns
of the SIC trends in the two halves of the record are distinctive, with regionally
opposing trends in the first half and uniformly negative trends in the second half. In
each season, these distinctive patterns correspond to the first two leading empirical
orthogonal functions of SIC anomalies during 199-200. Atmospheric circulation
trends and accompanying changes in wind-driven atmospheric thermal advection
have contributed to thermodynamic forcing of the SIC trends in all seasons during
the first half of the record and to those in fall and winter during the second half.
Atmospheric circulation trends are weak over the record as a whole, suggesting
that the long-term retreat of Arctic sea ice since 199 in all seasons is due to factors
other than wind-driven atmospheric thermal advection.

1. INTRODUCTION predict that Arctic sea ice will decline in response to atmo-

spheric greenhouse gas increases [ Holland et al. , 2006], the
current pace of retreat at the end of the melt season is ex-
ceeding the models' forecasts by approximately a factor of 3
[ Stroeve et al. , 200]. Long-term records of summer sea ice
extent within the central Arctic Ocean dating back to 1900
exhibit large multidecadal variations [ Polyakov et al. , 2003],
a factor which must be taken into account when interpreting
the recent sea ice retreat.
The physical mechanisms underlying the Arctic sea ice
decline are not fully understood but include dynamical pro-
cesses related to changes in winds and ocean currents and ther-
modynamic processes involving changes in air temperature,
radiative and turbulent energy fluxes, ocean heat storage,
and ice-albedo feedback [ Serreze et al. , 200; Stroeve and

The accelerating retreat of Arctic sea ice in recent dec-
ades, evident in all months of the year, is one of the most
dramatic signals of climate change worldwide (see Serreze
et al. [200], Meier et al. [200], and Stroeve et al. [200]
for recent overviews; ongoing updates on Arctic sea ice may
be obtained from the National Snow and Ice Data Center
(available at http://nsidc.org)). Although climate models