Northwestern North America has experienced an exceptional heatwave in
late June 2021 with many new temperature records across western Canada,
Oregon and Washington states. Here we use a recent atmospheric
reanalysis and a conditional approach based on dynamical adjustment to
assess and quantify the influence of atmospheric circulation and other
driving factors to the heatwave magnitude during the June 28–30 period.
A blocking anticyclone, enhanced low-level moisture and clear-sky
downward long-wave radiation are shown to be the main factors of the
heatwave persistence and magnitude. The heatwave magnitude is mainly
attributable to internal variability with climate change being an
additional factor (10%). Consequences of a similar atmospheric
circulation anomaly in different phases of the Atlantic Multidecadal and
Pacific Decadal Oscillations and in a warmer world at different global
warming levels (1, 2, 3 and 4°C) are explored based on a single model
initial-condition large ensemble.