How will climate change impact North Atlantic storms?

Abstract

North Atlantic storms present one of the major weather risks for North West (NW) Europe. Their associated strong winds and heavy precipitation can result in wind damage and both coastal and inland flooding. Notable recent examples include the St Jude's Day storm of October 2013 which brought hurricane-force winds to NW Europe, including record-strong winds in Denmark and the loss of 17 lives. In November and December of that year, intense rainfall associated with a series of storms led to widespread UK flooding which was particularly severe over the Somerset Levels, and the timing of storm Xaver on 5 December 2013 generated a storm surge which led to coastal flooding around both the North and Irish Seas. Understanding and predicting how North Atlantic storms might respond to climate change is therefore essential for assessing future weather risks and informing climate change adaptation strategies. This briefing focuses on the North Atlantic storm track which traverses the North Atlantic Ocean and extends over NW Europe, with the strongest impacts usually occurring in autumn and winter. Over most of the Northern Hemisphere, climate models project that the midlatitude storm tracks will, on average, shift further poleward under climate change. Coupled with the shift is a general decrease in the number of storms. If this pattern occurred in the North Atlantic, one might expect a reduction in the number of storms impacting NW Europe. This is indeed the case in summer, but the wintertime North Atlantic storm track does not follow this simple picture; instead, it is projected to extend more strongly eastward over NW Europe resulting in an enhanced storm risk in this region. Comprehensive analyses of wintertime North Atlantic storms across a wide range of global climate models suggest an increase in storm occurrence of 2–4% by the end of the century (Zappa et al., 2013). The quoted range represents scenario uncertainty associated with two different future greenhouse gas emissions pathways, corresponding to global warming levels of approximately 2–4 degC respectively. There are additional uncertainties not accounted for, particularly due to internal variability of the climate system. A full analysis of North Atlantic storms in the most recent generation of climate models (CMIP6) has yet to be completed, but initial studies indicate broadly similar changes per degree of global warming (Harvey et al., 2020). On the level of individual storms, substantial and robust increases in precipitation are expected. The same suite of global climate model simulations quoted above project a 9–18% increase in the average precipitation per storm from wintertime storms impacting NW Europe. Although the strongest winds associated with storms are expected to weaken, wintertime North Atlantic storms again do not follow the simple picture with a modest 0.4–0.9% increase in the strongest near-surface wind speeds expected by the end of the century. Increases in the occurrence and severity of coastal flooding are also expected, predominantly due to rising sea levels (Murphy et al., 2018). North Atlantic storms are generated by the large-scale contrast in temperature that exists between the tropics and the Arctic. Climate change is expected to warm the Arctic more than the tropics through a process known as polar amplification, acting to generally reduce storminess in the midlatitudes. However, the intensification of the wintertime North Atlantic storm track over NW Europe bucks this trend and the reasons for this are not fully understood. It may partly arise from a region of slower ocean warming in the central North Atlantic, associated with changes in the ocean circulation, resulting is a local increase of the north–south temperature gradient over the ocean surface and stronger potential for storm growth there (Gervais et al., 2019). Alternatively, more hemispheric-scale dynamical changes may be playing a role (Ciasto et al., 2016). On the level of individual storms, the projected increases in precipitation are consistent with the increased availability of moisture associated with increased air temperatures. One key research question currently being addressed is the extent to which the current generation of climate models accurately represent North Atlantic storms. For example, small-scale atmospheric processes not fully resolved by these models, including sting jets and embedded convective precipitation, have the potential to locally amplify both precipitation and wind impacts in a manner not currently captured by global climate projections.