Concurrent extreme events of atmospheric moisture transport and
continental precipitation: the role of landfalling atmospheric rivers
Abstract
An analysis of concurrent extreme events of continental precipitation
and Integrated Water Vapour Transport (IVT) is crucial to our
understanding of the role of the major global mechanisms of atmospheric
moisture transport, including that of the landfalling Atmospheric Rivers
(ARs) in extratropical regions. For this purpose, gridded data on CPC
precipitation and ERA-5 IVT at a spatial resolution of 0.5º were used to
analyze these concurrent events, covering the period from Winter
1980/1981 to Autumn 2017. For each season, and for each point with more
than 400 non-dry days, several copula models were fitted to model the
joint distribution function of the two variables. At each of the
analysed points, the best copula model was used to estimate the
probability of a concurrent extreme. At the same time, within the sample
of observed concurrent extremes, the proportion of days with landfalling
ARs was calculated for the whole period and for two 15-year sub-periods,
one earlier period and one more recent (warmer) period. Three metrics
based on copulas were used to analyse carefully the influence of IVT on
extreme precipitation in the main regions of occurrence of AR landfall.
The results show that the probability of occurrence of concurrent
extremes is strongly conditioned by the dynamic component of the IVT,
the wind. The occurrence of landfalling ARs accounts for most of the
concurrent extreme days of IVT and continental precipitation, with
percentages of concurrent extreme days close to 90% in some seasons in
almost all the known regions of maximum occurrence of landfalling ARs,
and with percentages greater than 75% downwind of AR landfall regions.
This coincidence was lower in tropical regions, and in monsoonal areas
in particular, with percentages of less than 50%. With a few
exceptions, the role of landfalling ARs as drivers of concurrent
extremes of IVT and continental precipitation tends to show a decrease
in recent (warmer) periods. For almost all the landfalling AR regions
with high or very high probabilities of achieving a concurrent extreme,
there is a general trend towards a lower influence of IVT on extreme
continental precipitation in recent (warmer) periods.