Abstract
Persistent atmospheric high pressures can lead to long-lasting droughts and heatwaves with severe societal and environmental impacts, as evident in summer 2018 in Europe. It is known that oceanic and atmospheric features connected with the tropical Pacific influence the atmospheric circulation regimes over the North Atlantic/European sector leading to blocking high pressures in the cold season. Here we show that in the warm season, different combinations of sea surface temperatures in the North Pacific and the North Atlantic are associated with distinctly different atmospheric circulation patterns over northwest Europe some three months later. While most studies are restricted to atmospheric variables, for the UK we also investigate the hydrological impact and find that the effect of the preferred seasonal storm tracks is more clearly seen in regional streamflow observations than in precipitation, presumably because streamflows integrate the influences of precipitation and evapotranspiration. These relationships open up the possibility of skilful statistical forecasts for much of spring to autumn, which will usefully complement the currently available skilful winter forecasts based on general circulation models. Our results deliver new understanding of the truly global driving processes of UK droughts and highlight the potential for improved early warning for the wider European domain.
Highlights
Long-range dependencies between oceanic, atmospheric and hydrological variables over space and/or time have been extensively explored for Europe for the cold season, when the inter-annual variability of influential forcings such as the North Atlantic Oscillation (NAO) is large
In the present study we investigate the association between North Pacific sea surface temperatures and northwest European climate and hydrology for different temperature states of the North Atlantic
Droughts affecting the lowland southeast of the UK generally extend into the neighbouring lowland parts of continental Europe
Summary
Long-range dependencies between oceanic, atmospheric and hydrological variables over space and/or time (teleconnections) have been extensively explored for Europe for the cold season, when the inter-annual variability of influential forcings such as the North Atlantic Oscillation (NAO) is large. An ‘atmospheric bridge’ concept has been proposed, whereby the extratropical atmosphere responds to perturbed tropical forcings during El Niño/Southern Oscillation (ENSO) events, and in turn influences the underlying extratropical ocean (e.g. Alexander et al 2002, Graf and Zanchettin (2012)). Exploring features of this bridge mechanism in all seasons, Lau and Nath (2001) found the strongest atmospheric response to occur in January-February, followed by peaks in the North Pacific and North Atlantic sea surface temperature (SST) signals one to two months later. The negative ENSO-NAO correlation is significant only when the ENSO and the Atlantic Multidecadal Oscillation (AMO) are in phase
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