Extreme Associated Functions: Optimally Linking Local Extremes to Large-scale Atmospheric Circulation Structures

We present a new statistical method to optimally link local weather
extremes to large-scale atmospheric circulation structures. The method
is illustrated using July-August daily mean temperature at 2m height
(T2m) time-series over the Netherlands and 500 hPa geopotential height
(Z500) time-series over the Euroatlantic region of the ECMWF reanalysis
dataset (ERA40). The method identifies patterns in the Z500
time-series that optimally describe, in a precise mathematical
sense, the relationship with local warm extremes in the
Netherlands. Two patterns are identified; the most important one
corresponds to a blocking high pressure system leading to subsidence
and calm, dry and sunny conditions over the Netherlands. The second
one corresponds to a rare, easterly flow regime bringing warm, dry air
into the region. The patterns are robust; they are also identified in
shorter subsamples of the total dataset. The method is generally applicable and
might prove useful in evaluating the performance of climate models in
simulating local weather extremes.