Summary and Conclusions
Abrupt shifts from one anomalous weather condition to another
substantially different one are disruptive to agriculture (e.g.,
thaw/freeze episodes that can cause early budding on fruit trees),
winter recreation enterprises, management of municipal utilities, animal
behavior, and a variety of human activities. The term “weather
whiplash” recently entered the public discourse to describe these types
of shifts, but a commonly accepted definition has yet to emerge. In this
study we propose a characterization that differentiates normal frontal
passages, which often usher in marked but short-lived weather changes,
from more disruptive events in which a persistent and anomalous weather
regime is abruptly replaced by another strikingly different one.
Whiplash events typically involve a persistent winter cold spell that is
supplanted by a winter heatwave, a prolonged drought followed by days of
storminess, or the reverse order of these transitions.
In this study, we demonstrate a new method to identify and trace weather
whiplash events (WWEs) based on characteristic large-scale patterns of
daily upper-level (500 hPa) geopotential height anomalies over the
domain encompassing the northeast North Pacific Ocean and North America.
Patterns are objectively determined from 72 years of daily height fields
(from NCEP/NCAR reanalysis), and each day is classified into a matrix of
representative clusters or nodes by a neural-network-based tool called
self-organizing maps (Figs. 1 and S9 ). Persistent long-duration
events (LDEs) are identified when a string of four or more consecutive
days occurs in a single cluster. A WWE is then diagnosed when the
pattern two days following a LDE transitions to a cluster that is
sufficiently different from the one where the LDE occurred, as measured
by the Euclidean distance between clusters in the matrix. Extreme
temperatures and heavy precipitation associated with each pattern in the
matrix are also characterized. This information is used to describe the
expected shift in weather conditions before and after a WWE.
We assess the number of WWEs that originate from LDEs occurring in each
node during all months (Fig. 6a ), as well as changes in WWE
frequency during winter (JFM; Fig. 6b ) and summer (JAS;Fig. 6c ). The clusters dominated by high or low height
anomalies spanning high latitudes tend to spawn the most WWEs (nodes #1
and #12). Patterns featuring a distinct meridionally oriented dipole of
height anomalies (primarily nodes #3/4 and #9/10) also produce a
relatively large number of WWEs.
Changes in WWEs over time are more robust in future model projections
assuming minimal abatement of carbon emissions (Fig. 7 ) than
during past decades. WWEs originating in nodes #1 and #3 increase
significantly from 2006-2030 to 2076-2100 during both winter and summer,
while those originating in node #12 decrease significantly. Increasing
WWEs are projected for nodes with positive height anomalies in high
latitudes, while WWEs will decrease when the Arctic is anomalously cold.
Using extreme temperature and precipitation patterns associated with
each node, we analyzed the sensible weather shifts that are likely to
occur in nodes where WWEs become more or less likely. In addition to
illuminating specific weather transitions in particular regions, we find
more generally that a WWE originating in node #1 (large positive height
anomalies in high latitudes) is most likely to shift to nodes #3 or #9
(Fig. 2 ), both of which are substantially more meridional in
character. This finding suggests that a persistent episode of strong AAW
is likely to be followed by a more amplified upper-level circulation
pattern. Interestingly, WWEs originating in nodes #1 and #3 are likely
to jump between these two nodes, resulting most notably in repetitious
cold spells in midlatitudes of North America and fluctuating wet/dry
periods in west-central regions. During summer months, WWEs originating
in nodes #1 and #3 will produce similar shifts in temperatures and
precipitation, but with somewhat less distinct contrasts (Fig.
5 ). Shifts in temperature and precipitation extremes associated with
WWEs initiated in each node for any season can be assessed using this
methodology.
Our objective in this paper was to demonstrate a new approach to measure
the occurrence of WWEs, which large-scale atmospheric patterns are more
or less likely to initiate them, the extreme weather conditions
associated with the patterns before and after a WWE, and how their
frequency may change in the future as greenhouse gas concentrations
continue to rise. Future efforts will examine WWEs in other sectors of
the globe and investigate relationships between WWE frequency and
changes in the climate system, such as rapid Arctic warming, disruptions
of the stratospheric polar vortex, and fluctuations in natural climate
oscillations. Because WWEs tend to cause abrupt and disruptive shifts in
weather conditions, a better understanding of these events – and the
extremes associated with them – will better enable preparations by
decision-makers that will lessen their impacts.