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in regional extent (and concentration). This is evident in
Figure 7.16
, which shows
September ice extent and concentration for the years 2007-2012; these six years
have the lowest extent in the satellite record based on data through 2012. Although
they share the general pattern of extensive open water along the shores of Siberia
and Alaska, each year has unique features. Arctic wide and regional sea ice variabil-
ity are strongly tied to variations in atmospheric circulation patterns. In some cases,
it appears that the ice cover has multiyear and even decadal “memory” of forcing
by anomalous atmospheric circulation patterns. There is also growing evidence that
the general climate warming viewed as driving the basic downward trend ice extent
has altered the response of the ice cover to natural climate variability. Changes in
ocean heat transport, including both the import of warmer Pacific waters through
the Bering Strait and the import of warmer Atlantic waters to the Arctic, have played
potentially important but still unclear roles in sea ice variability and trends. The fol-
lowing sections illustrate some of these points.
7.4.2
Case Study for 1990
At the time, September 1990 had the lowest September sea ice extent in the satellite
record. It was examined by Serreze et al. (
2003b
) as a case study. The low extent was
mostly attributed to extensive open water in the Chukchi, East Siberian and Laptev
seas. The onset of the anomaly was observed in spring. Over the May-June period,
positive near surface temperature anomalies encompassed all of the Arctic Ocean,
but largest along the Siberian coast. The sea level pressure field for May was charac-
terized by a mean low of 996 hPa centered at about 80°N, 100°E (
Figure 7.17
). This
corresponds to a local anomaly of about 20 hPa. This significant atmospheric event
was associated with strong southerly winds along the Siberian coast. The location of
the mean low is furthermore consistent with the temperature anomalies (especially
strong positive anomalies were found east of the low) and the observation of early
coastal melt. The strong winds also explain the development during this month of
areas of open water and low concentration ice in the Laptev and East Siberian Seas.
This resulted in an early reduction of the surface albedo, further enhancing the melt
process through the albedo feedback effect (see
Chapter 5
). Increased cloud cover
in June may have also played a role (the radiative cloud forcing is positive in this
month). Even though the ice extent anomaly was already large by July, it grew
rapidly during August. This resulted from the development of a strong anticyclone
centered north of Alaska, associated with strong easterly winds in Chukchi and East
Siberian seas. As the ice motion tends to be to the right of the geostrophic wind, the
result was a poleward motion of ice away from the coast.
7.4.3
The SHEBA Anomaly
We next examine a pronounced regional anomaly that developed in summer 1998,
examined as a case study by Maslanik, Serreze, and Agnew (
1999
). It is sometimes
referred to as the “SHEBA anomaly” as it occurred during the SHEBA field year,
