On the Response
of Sea Ice to the Arctic Oscillation
1,2,
2, and
3
1Applied Physics Laboratory,
University of Washington, Seattle, Washington
2Department of Atmospheric Sciences,
University of Washington, Seattle, Washington
3 Frontier Research System for Global
Change, International Arctic Research Center, University of Alaska,
Fairbanks, Alaska
[The full text of this paper can be obtained from J. Climate, v. 15, no. 18, pp. 2648 - 2668, 2002.]
Introduction
Dramatic changes in Arctic climate have been noted during
the past two decades. For example:
1.) Sea level pressure decreased by 5
mb. (Walsh et al. 1996) between the periods 1986-1994 and 1979-1985 (Fig. 1).
2.) Surface air temperature warmed from
1979 – 1997 (Rigor et al. 2000)
during winter and spring, with values as high as 2°C/decade in the eastern
Arctic during spring (Fig. 2).
3.) Sea ice area and extent decreased
(Parkinson et al. 1999, Fig. 3), and
4.) Sea ice thinned (Rothrock et al.
1999, Fig. 4).
In this study we address the question “Did the warming of air temperature melt the sea ice, or did the thinner sea ice allow more heat to pass from the ocean to warm the air?”
Many of the changes in Arctic climate have been linked to
changes in the Arctic
Oscillation (AO, Thompson and Wallace 1998), whose index is defined as the
leading principal component (PC) of Northern Hemisphere SLP. The AO can be
characterized as an exchange of atmospheric mass between the Arctic Ocean and
the surrounding zonal ring centered ~45°N. The observed trend in the AO toward
its “high index” polarity (i.e. toward stronger westerlies at subpolar
latitudes and lower SLP over the Arctic) is a way of interpreting the observed
decrease in SLP over the North Pole and the associated cyclonic tendency in the
surface winds over the Arctic.
In this study we show how the changes in surface wind associated
with the fluctuations and trend in the AO affect sea ice motion (SIM) in the
central Arctic and how the changes in SIM, in turn,
affect the thickness and concentration of sea ice, and the distribution of SAT
over the Arctic.
Climatology and Climate Change
Data collected by the International Arctic Buoy Programme
from 1979–1998 are analyzed to obtain statistics of sea level pressure (SLP, Fig. 5) and sea ice motion (SIM, Fig. 5). The annual and seasonal mean fields agree
with those obtained in previous studies of Arctic climatology (Fig. 6). The data show a decrease of 3 hPa in decadal
mean SLP over the central Arctic Ocean between 1979–1988 and 1989–1998 (Fig. 7, left column). This decrease in SLP drives a
cyclonic trend in SIM, which resembles the structure of the AO (Fig. 7, right column).
Regression maps of SIM on the wintertime (January – March)
AO index (E.g. Fig. 8) show (1) an increase in ice
advection away from the coast of the East Siberian and Laptev seas, which
should have the effect of producing more new, thin ice in the coastal flaw
leads, (2) a decrease in ice advection from the western Arctic into the eastern
Arctic, and (3) a slight increase in ice advection out of the Arctic through
Fram Strait. Taken together, these changes suggest that at least part of the
thinning of sea ice recently observed over the Arctic Ocean can be attributed
to the trend in the AO toward the high index polarity. Maps of the circulation
of sea ice (regimes of ice motion, Fig. 9), and the
residence time of sea ice (Fig. 10), also show these
changes.
Seasonal Memory of the Prior Winter Arctic
Oscillation
Rigor et al. (2000) showed that
year-to-year variations in the wintertime AO imprint a distinctive signature on
surface air temperature (SAT) anomalies over the Arctic, which is reflected in
the spatial pattern of temperature change from the 1980's to the 1990's (Fig. 11). Here it is shown that the memory of the
wintertime AO persists through most of the subsequent year: spring and autumn
SAT (Figs. 12 and 13)
and summertime sea-ice concentration (Fig. 14)
are all strongly correlated with the AO-index for the previous winter.
It is hypothesized that
these delayed responses reflect the dynamical influence of the AO on the
thickness of the wintertime sea-ice, whose persistent 'footprint' is reflected
in the heat fluxes during the subsequent spring, in the extent of open water
during the subsequent summer, and the heat liberated in the freezing of the
open water during the subsequent autumn (Fig. below).
References
Parkinson, C. L., D. J. Cavalieri, P. Gloersen, H. J. Zwally,
and J. Comiso, 1999: Arctic sea ice extents, areas, and trends, 1978–1996. J. Geophys. Res., 104(C9), 20 837–20 856.
Rothrock, D. A., Y. Yu, and G. A. Maykut, 1999: Thinning of
the Arctic sea-ice cover. Geophys. Res.
Lett., 26(23),
3469–3472.
Thompson, D. W. J., and J. M. Wallace, 1998: The Arctic Oscillation signature
in the wintertime geopotential height and temperature fields. Geophys. Res. Lett., 25(9), 1297–1300.