Transcript PowerPoint file of poster presented at Sea Ice Extent Workshop

Harry L. Stern
Polar Science Center, Applied Physics Laboratory
University of Washington
1013 NE 40th Street
Seattle, WA. 98105-6698 USA
[email protected]
Variability of Sea Ice in West Greenland
Presented at the Workshop on Sea Ice Extent and the Global Climate System, 15-17 April 2002, Toulouse, France
A major part of the West Greenland ecosystem is
covered with sea ice for about half the year. This skin of
sea ice, called the West Ice, seasonally prevents fishing
activities and is an important habitat for numerous birds
and sea mammals and a variety of ice-associated
biological activity. Parkinson et al. [1] found a trend in
the wintertime sea ice extent of +7.5% per decade for
1978-1996 for the region encompassing Baffin Bay,
Davis Strait, and the Labrador Sea. The wintertime
surface air temperature shows a trend of -2oC per
decade for 1979-1997 along the west coast of Greenland
north of 69oN [2]. These regional changes in ice extent
and temperature are linked to the North Atlantic
Oscillation (NAO) - the difference in surface
atmospheric pressure between Portugal and Iceland [3].
We examine the sea ice area in March in three
overlapping study regions centered at 69oN on the west
coast of Greenland (Figure 1). The objectives are to
estimate the trends in the ice cover as far back in time as
possible, and to develop guidelines for a future annual
assessment of the ice conditions in West Greenland.
Figure 1. The three overlapping
study regions. The small region is
centered on Disko Bay. The areas
of the small, medium, and large
regions (not including land) are:
12,400 km2, 118,000 km2, and
439,000 km2.
Satellite data. We use monthly gridded sea ice
concentration derived from the Scanning Multichannel
Microwave Radiometer (SMMR, 1978-1987) and the
Special Sensor Microwave Imager (SSMI, 1987present). Figure 2 shows an example for March 1996.
Chart data.
From 1952 to 1980 the Danish
Meteorological Institute published an annual volume
called "The Ice Conditions in the Greenland Waters".
Each volume contains maps of the ice concentration at
the end of each month, as in Figure 3. Data for the
charts came from ship and aircraft observations. We
scanned the charts, digitized the contours, and computed
the area of ice within each study region.
Figure 4 shows the time series of ice area in March in
each study region from the combined satellite and chart
data sets. The ice area in the medium study region
shows a significant positive trend, as well as an 8.2-year
cycle (Figure 5). The winter (Dec-Mar) NAO (Figure
4, fourth panel) is highly correlated with the ice area
during the satellite period, but not during the chart
period (Table 1). This is explained by the relationship
between the NAO and the sea temperature: positive
correlation (+0.23) during the chart period and negative
correlation (-0.51) during the satellite period.
Figure 6 (left panel) shows the correlation of winter
NAO with sea ice concentration over the whole Arctic.
Blue is positive, red is negative. Correlations are
moderately positive in West Greenland. In the right
panel, the previous year's NAO value is correlated with
the ice concentration. This boosts the correlation above
0.7 over a large area from Disko Bay toward the
southwest, and suggests that next year's ice conditions
in West Greenland can be predicted to some extent by
this winter's NAO index.
Figure 3.
Sample chart from the Danish
Meteorological Institute for March 1968 showing
the ice concentration in West Greenland, as
follows: Cross-hatching: 10/10; diagonal stripes:
7/10 to 9/10; vertical stripes: 4/10 to 6/10; diagonal
dashes: 1/10 to 3/10; no shading: less than 1/10.
The colored dots (connected by colored lines)
show points that have been digitized. They
delineate areas of constant ice concentration. The
outlines of the three study regions are in purple
(they have been overwritten in some places). Thus,
for example, the green curve in Disko Bay outlines
that part of the small study region for which the ice
concentration is 10/10. The red curve in the lower
right outlines that part of the medium study region
for which the ice concentration is 1/10 to 3/10. In
this way, upper and lower bounds on the ice
concentration (or ice area) in each study region can
be computed. This has been done for 23 charts (23
Marches) from 1953 through 1979 (four missing
years: 1954, 1959, 1965, 1966).
Figure 2. Left: Example of sea ice concentration (%)
derived from passive microwave satellite data (SSMI)
for March 1996. Right: Blow-up of the rectangle
centered on Davis Strait, showing the three study
regions. Each sea ice concentration grid for the
Northern Hemisphere is 304 x 448 pixels with a
nominal pixel size of 25 x 25 km.
Acknowledgements
We thank the Danish Cooperation for the
Environment in the Arctic (DANCEA) for
funding this project. Satellite data were
obtained from the National Snow and Ice Data
Center in Boulder, Colorado (nsidc.org).
References
[1] Parkinson, C. L., et al., J. Geophys. Res.,
104, C9, 20837-20856, 1999.
[2] Rigor, I. G., et al., 1979-97, J. Climate, 13,
896-914, 2000.
[3] Hurrell, J. W., Science, 269, 676-679, 1995.
Figure 4. The top three panels show
the ice area (in thousands of km2) in
March in the three study regions vs.
time (1952-2001). The blue dots
indicate data derived from ice charts,
as in Figure 3; the black dots and red
dots are from satellite data, as in
Figure 2. The trends for the large,
medium, and small regions are
(respectively): 1.5, 3.0, 1.7 %/decade.
The estimated uncertainty in the trends
is about 1 %/decade. The trend in the
medium region is very significant.
The variability in the medium region
explains most of the variability in the
large region. For those two regions,
notice the decadal cycle with low
points in 1962, 1970, 1979, 1986, and
1996. (See spectrum in Figure 5).
The fourth panel shows the winter
(Dec-Mar) NAO index. Its spectrum
has a weak peak at the same period as
the peak in the ice area (Figure 5).
The fifth panel is the near-surface sea
temperature measured at Fylla Bank
(63.6oN, 52.4oW) in early summer. It
helps to explain the correlations
between ice area and NAO in Table 1.
Region
Large
Medium
Small
1953-1978 1979-2001
0.09
0.20
0.15
0.60
0.55
0.44
Mads Peter Heide-Jørgensen
Greenland Institute of Natural Resources
c/o National Marine Mammal Laboratory
7600 Sand Point Way NE
Seattle, WA. 98115 USA
[email protected]
Figure 5. Top: Power spectrum of the
ice area in the medium region (Figure
4, second panel). The largest power is
at the longest period, but the spike at
8.2 years stands out prominently.
Bottom: Power spectrum of the winter
NAO (Figure 4, fourth panel),
showing a weak peak at the same
period. The vertical scale is linear.
Figure 6. Left: Correlation of wintertime (Dec-Mar)
sea ice concentration from SMMR & SSMI (19791999) with wintertime NAO index (1979-1999). Right:
Correlation of sea ice concentration (1979-1999) with
the previous year’s NAO value (1978-1998). Colors:
dark red < -0.5; light red -0.5 to -0.3; white -0.3 to
+0.3; light blue +0.3 to +0.5; dark blue > +0.5. The
correlations are generally positive in Baffin Bay, Davis
Strait, and the Labrador Sea. In the un-lagged case
(left), correlations are around 0.3 across a wide swath
of the medium study region, while the same area in the
lagged case (right) has correlations above 0.7. This
suggests that last year’s NAO has predictive value for
this year’s ice conditions in West Greenland.
Table 1. Correlation of ice area with winter NAO for the chart period (left, 1953-1978) and the
satellite period (right, 1979-2001). The difference in the correlations can be understood by
considering the NAO and the sea temperature (Figure 4), which are positively correlated during the
chart period (+0.23) and negatively correlated during the satellite period (-0.51). A high NAO
(strong Icelandic low pressure system) was associated with low sea temperature during the later
period (1979-2001), allowing ice to advance farther south - hence high ice area and high correlation
with NAO. But during the early period (1953-1978) a high NAO was weakly associated with a high
sea temperature, damping the tendency of the high NAO toward high ice area.