Lake Melville

Likelihood of Thaw Seasons, as Return Periods (yr)
300
Temperature Anomaly (oC)
−4 −2 0 2 4 6
Raw Anomaly
NAO Attributed
Return Period (yrs)
2010 2011
200
100
0
Goose Bay Nain Cartwright Churchill Falls Wabush
Figure 3.7. The likelihood of the number of thaw days observed
in the winters (December–February) of 2009/10 and 2010/11 for
five Labrador communities.
2010 1958 1969 2011 1960 1992 1991 1973 1972 1957
Figure 3.4. Observed temperature anomalies for the five
warmest (left) and coldest (right) winter temperature
anomalies, and the anomaly attributed to the NAO.
contributed relatively little. Results suggest that NAOrelated
shifts in atmospheric circulation reduced the
flow of cold air from the Arctic and Canadian interior
towards Labrador, and may have increased the flow of
relatively warm air from the Atlantic, leading to warmer
mean temperatures. To the extent that such winters
pose a hazard to Labrador residents (e.g. promoting
dangerous across-ice travel conditions), the state of
the NAO then presents a concern to the region. Given
enough time and continuing warming trends, similar
warm winters may become more common, requiring
less input from a favourable NAO to produce adverse
conditions.
These NAO-related changes also encouraged frequent,
persistent thaw events in most of the region. The
number of thaw days in a given winter is a useful
measure of winter severity in Labrador, as frequent or
prolonged thawing of snow and ice cover is responsible
for many negative outcomes for residents; e.g. melting
snowmobile trails or failing sea ice cover present
transportation hazards. To determine the expected
number of thaw days, we employed a point process
statistical model of thaw duration and frequency,
which was used to estimate the number of thaw days
expected in a season (Furrer et al., 2010). Figure 3.7
shows the likelihood of thaw events in Labrador, given
as a return period or the typical length of time between
events of this magnitude or greater. For example, the
100-year event is expected to be met or exceeded once
a century, on average. Results show that the 2009/10
event in Labrador was very unusual in terms of thaw
days, expected at most once every 100 years in 4 of 5
communities examined (Figure 3.7). The 2010/11 event
was less severe, exceeding the 100-year event in only
one location (Nain). These provide some context for
these unusual winters; both were extremely unusual in
Nain, but only the one was unusual in most of Labrador
(becoming relatively common in western Labrador).
The timing of ice cover across Lake Melville
demonstrates strong connections with climate.
Figure 3.8 shows statistical test scores (‘z’ scores) for
differences in mean ice break-up and freeze-up dates
during the three Labrador climate regimes. Boxplots
show the range of scores across the fourteen ice
analysis locations, with one value calculated at each
point (Figure 3.2). Comparisons to Regime 1 were not
possible for freeze-up dates, as too few observations
were available. Results demonstrate that no two
regimes give universally significant differences in
mean ice formation/break-up dates across all points.
However, the most significant difference is seen
between break-up dates in Regimes 1 (near-normal)
and 3 (warm). Regimes 2 (cold) and 3 (warm) show
similar, though weaker, changes. This is somewhat
surprising and counter-intuitive; if climate were the
only important driver, we would expect that Regime 2
as the coldest and 3 as the warmest would show the
strongest difference. The inference is that some other
non-climate factors are exerting an influence that
further enhances differences between the mid-20 th
and early 21 st century climates, and thus contribute
to recent reductions in the length of the ice season.
For example, gradual climate change may enhance
differences between the early and late ice record;
alternately, various non-climate factors (e.g. Churchill
River discharge) may be impacting ice variability.
However, it is also possible that these changes are an
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