Abstract
‘Super’ Storm Desmond broke meteorological and hydrological records during a record warm year in the British–Irish Isles (BI). The severity of the storm may be a harbinger of expected changes to regional hydroclimate as global temperatures continue to rise. Here, we adopt a process-based approach to investigate the potency of Desmond, and explore the extent to which climate change may have been a contributory factor. Through an Eulerian assessment of water vapour flux we determine that Desmond was accompanied by an atmospheric river (AR) of severity unprecedented since at least 1979, on account of both high atmospheric humidity and high wind speeds. Lagrangian air-parcel tracking and moisture attribution techniques show that long-term warming of North Atlantic sea surface temperatures has significantly increased the chance of such high humidity in ARs in the vicinity of the BI. We conclude that, given exactly the same dynamical conditions associated with Desmond, the likelihood of such an intense AR has already increased by 25% due to long-term climate change. However, our analysis represents a first-order assessment, and further research is needed into the controls influencing AR dynamics.
Highlights
Such high winter British–Irish Isles (BI) precipitation totals and flood events are usually associated with enhanced horizontal water vapour transport from the Atlantic Ocean (Allan et al 2016, Lavers et al 2011)
Storm Desmond was no exception in this regard, with satellite-derived column-integrated water vapour estimates during the passage of the weather system revealing a plume of moist air connecting the BI to the sub-tropical western Atlantic (figure 1(b))
The algorithm detected the well-known atmospheric river (AR) in November 2009, an event that was associated with flooding in both Ireland and the United Kingdom (UK) (Adger et al 2013, Lavers et al 2011)
Summary
Such high winter BI precipitation totals and flood events are usually associated with enhanced horizontal water vapour transport from the Atlantic Ocean (Allan et al 2016, Lavers et al 2011). Given that the saturation specific humidity increases by ∼7% ◦C−1 according to the Clasius–Clapeyron relation, there are concerns that AR-related flooding could worsen in a warming climate This view is supported by global climate model projections which suggest that AR frequency and intensity, driven by enhanced atmospheric moisture content, could increase for BI and coastal northwest Europe as the climate warms (Lavers et al 2013, Ramos et al 2016). Storm Desmond occurred during the warmest year globally since observations began, in a December that was the warmest on record for the UK (Kennedy et al 2016) In light of these conditions, and physical reasoning—both from first-order physics and modelling—that higher air temperatures drive more intense ARs, it is tempting to invoke climate change as contributing to Desmond’s severity (Oldenborgh et al 2015). Data In the subsequent sections, while all notation is introduced within the text, we include table 2 in the supplementary information (hereafter SI) for convenience when referring to terms featuring within equations
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