A novel temperature anomaly source diagnostic: method and application to
the 2021 heatwave in the Pacific Northwest
Abstract
Quantitative methods to pinpoint the origin of atmospheric temperature
anomalies (T’) associated with heatwaves are pivotal for the
construction of physically plausible synoptic storylines of heatwave
formation and their evaluation in models. Here, we combine a Lagrangian
T’ decomposition with concepts from moisture tracking techniques to
identify where and when the principal physical processes generate T’ and
to attribute these sources to synoptic weather systems. Applying this
framework to near-surface and free-tropospheric T’ associated with the
record-shattering 2021 heatwave in the Pacific Northwest shows that
ascending, diabatic air streams in North Pacific cyclones contribute
more than 50% of free-tropospheric T’, whereas near-surface T’ is
mainly produced by local subsidence and diabatic heating with only
marginal upstream contributions. Since free-tropospheric T’ facilitates
near-surface accumulation of locally produced T’ by rendering the
atmosphere stable to moist convection, our findings corroborate the
notion of top-down induced heatwave formation fuelled by upstream
diabatic processes.