3.3 Observed changes in WWEs
Our new method of assessing WWEs begins with identification of
long-duration events (LDEs) in reanalysis fields. We find that the most
common patterns (Fig. 1 ) also tend to exhibit a higher
occurrence of LDEs per year (Fig. S6 ), which is expected given
that a larger number of days in one pattern increases the chances of
three or more consecutive days occurring. Significant
(>95% confidence) increasing trends in LDEs exist in node
#1, while nodes #10 and #12 exhibit declining trends (while the
f-test detected significant (>90%) trends in nodes #2 and
#6, the number of LDEs is too small to be consequential). These
findings are consistent with our earlier work on weather-regime
persistence (Francis et al., 2018; Vihma et al., 2019; Francis et al.,
2020). Specifically, Francis et al., 2018 found an increased occurrence
of LDEs particularly in patterns featuring positive height anomalies in
high latitudes of the North American domain, the same as that used in
the present study. Increased LDEs in warm-Arctic nodes are consistent
with the counterintuitive downward trend in extreme summer heat over
much of the upper Midwest during recent decades, and the finding
suggests that amplified Arctic warming favors more persistent weather
conditions that can lead to extreme weather events.
The matrix of time series shown in Fig. 6a displays the
frequency per year with which a WWE originates from an LDE in each node.
The infrequency of WWEs (13.2 per year on annual average) and large
interannual variability challenge the detection of frequency changes
over the observational record, but a few nodes exhibit statistically
significant trends. Nodes #1 and #12 initiate substantially more WWEs
than do other nodes, and significant trends are evident for node #3
(increasing) and node #12 (decreasing). These trends are attributable
mainly to the changes in frequency of these nodes (Fig. 3 ), in
that LDEs are more likely to occur when a node becomes more prevalent,
and vice versa.
Winter: Changes over time are more evident when assessed as
differences in frequency between the two 20-year periods at the
beginning and end of the observation record. During winter months
(Fig. 6b ), only node #9 exhibits a significant increase in
WWEs, while those initiated in node #12 have declined. The pattern in
node #9 features a strong meridional ridge/trough pattern across the
continent, which according to Fig. 2 , is most likely to shift
to node #1 or #12 after a WWE (note node #10 is adjacent, thus does
not qualify as a WWE), both of which are more zonal patterns. This
transition would suggest that persistent warm, dry weather in the
western states and cool conditions across central North America would
likely transition to a more progressive synoptic regime in a WWE. The
significant decrease in WWEs originating in node #12, in contrast,
would suggest fewer abrupt transitions from broad western
troughing/eastern ridging to patterns similar to either node #4 or
#10, the two most likely nodes following an LDE in node #12. A WWE
originating in the strong zonal jet conditions depicted by the pattern
in node #12, featuring cold (warm) high (middle) latitudes, would most
likely transition along one of two paths: 1) the cold migrates westward
over the northeast N. Pacific and Alaska while warmth invades much of
North America (node #4), or 2) the cold migrates eastward over
northeastern North America, while warmth bulges northward into Alaska
(node #10).
Summer: Our results suggest that WWEs originating in node #3
during JAS have increased (Fig 6c ). An LDE in node #3 is most
likely to shift to node #1 in a WWE (Fig. S8 ), which would
replace anomalous troughing (ridging) in the northwestern (northeastern)
parts of the continent with opposite height anomalies. In terms of
sensible weather, the increased frequency in WWEs from node #3 to node
#1 shift anomalously high temperatures in the Hudson Bay region to
Alaska and British Columbia, accompanied by increased negative anomalies
throughout most of the western interior. Anomalously wet conditions
shift from southeast Alaska and the British Columbia coast to southern
California (Fig. 5 ). Note that a transition from node #3 to
#1 in summer would be consistent with the reported increased
precipitation extremes in western North America (Swain et al., 2018). A
decrease in WWEs originating in nodes #4 and #10 has also occurred. An
LDE in node #4 is most likely to shift to node #6 or #12 during a
WWE. The shift in temperatures and precipitation from node #4 to #6
would reduce the likelihood of hot days over central Canada, reduce the
likelihood of abnormally cool days over Alaska, and decrease the chances
of heavy precipitation in western Canada while increasing chances in
southeastern North America. A WWE that started in node #4 and shifted
to #12 would also decrease hot days in central Canada but would also
increase cold days over the northwest part of the domain, while heavy
precipitation would increase from the Gulf of Mexico to New England A
WWE that was initiated in node #12 is most likely to shift to node #10
or # 4. The most notable changes in extremes during a transition to
#10 will be an increase in high-temperature extremes in the Gulf of
Alaska, more widespread cool spells over the Rocky Mountains and High
Plains, and a transition from anomalously wet to dry conditions along
the coast of British Columbia. A shift from node #12 to #4 is likely
to bring abnormally warm conditions to central Canada, a lower
probability of anomalously cold days in the Arctic, and more focused
heavy precipitation in the Pacific Northwest. The decreased frequency of
WWEs originating in node #12 will mean fewer of these types of abrupt
transitions.