From warm seas to flooded streets: The impact of sea surface temperature on cutoff low and extreme rainfall in Valencia, Spain

Abstract

The catastrophic October 2024 flood in Valencia, Spain, highlights the increasing risk of extreme weather due to global climate change. The frequency of extreme weather events, including floods, has significantly risen in recent decades, with climate change as a primary driver (Bolan et al., 2023; Easterling et al., 2000; Newman & Noy, 2023). Rising global temperatures and atmospheric energy have intensified rainfall events (Al-Ghussain, 2018; Karl & Trenberth, 2003), with cutoff low-pressure systems (COLs) playing a notable role. COLs are cold-cored systems in the mid-troposphere, often marked by a sharp temperature gradient on their eastern side (Nieto et al., 2008). Key regions prone to COLs include southern Europe, the eastern Atlantic, the eastern North Pacific, and areas from northern China through Siberia (Nieto et al., 2005). In particular, southern Europe and the eastern Atlantic are highly vulnerable to COL-driven heavy rainfall. The El Niño–Southern Oscillation (ENSO) further influences COL activity by altering large-scale atmospheric circulation and sea surface temperatures (SSTs) (Pinheiro et al., 2022). During ENSO events, SST shifts increase the probability of COL formation, leading to extreme rainfall (Ferreira, 2021). Climate projections suggest that COL-induced precipitation could rise significantly with continued warming, increasing flood risks in regions like eastern Spain, including Valencia. Warmer SSTs enhance moisture convergence in COLs, fueling convective cloud formation and intense precipitation (Pinheiro et al., 2022). Thus, changes in SSTs directly link warming oceans to the intensification of flood events, as exemplified in Valencia. This study underscores the need for improved flood forecasting and urban resilience planning to address escalating climate-induced hazards.

On October 29, 2024, a cold drop event involving COLs led to severe flash flooding across southern and southeastern Spain, with Valencia experiencing significant impacts. Rainfall exceeded historical records, overwhelming flood infrastructure. In the Júcar Basin, a crucial water resource in eastern Spain, accumulated rainfall reached 107.43 mm between October 29 and November 1, 2024, with a maximum of 610.9 mm recorded by GSMaP data. The basin received an estimated 4.453 billion cubic meters of water, surpassing drainage capacities and causing widespread inundation and structural damage. Extensive human and economic losses affected homes, businesses, and infrastructure across eastern, southeastern, and southern Spain, severely impacting transportation, agriculture, and daily life.

Figure 1a presents Sentinel-2 satellite images illustrating the conditions in southern Valencia before and after the recent flood, with the city itself located in the map's northern section. A false-color combination (Bands 12-8-2) enhances visualization, highlighting the flooded Lagoon of Valencia and the Turia River, which channels muddy waters into the Baltic Sea. Figure 1b, presents Sentinel-1 satellite radar image images from October 31 and November 1, captures areas initially obscured by clouds, clarifying flood boundaries. However, analyzing flood impacts within urban areas requires high-resolution imagery. Current maps lack the detail necessary to assess urban infrastructure and residential neighborhood damage accurately. More detailed images would enable precise evaluation of critical infrastructure impacts and guide targeted recovery efforts, underscoring the importance of enhanced spatial resolution in urban flood assessments.

Severe floods often arise from distinct atmospheric patterns, exacerbated by increasing global temperatures that accelerate the hydrological cycle, resulting in greater irregularity and unpredictability in precipitation (Merz et al., 2021). A warmer atmosphere retains more moisture, thereby intensifying rainfall events (Tabari, 2020; Zhang et al., 2021). According to the World Meteorological Organization, every degree of temperature increase allows saturated air to hold ~7% more water vapor. Such shifts affect environmental systems, water management, and global livelihoods. During recent floods in Spain, some areas experienced rainfall levels equivalent to their annual precipitation within a few hours (Figure 2). Spain's geographical positioning between the Atlantic Ocean and the Mediterranean Sea is a contributing factor, with air masses originating from the North Atlantic frequently driving extreme precipitation events. Figure 3a illustrates the long-term trends in October air and SSTs across the North Atlantic (latitudes 30–60°N). Since October 2018, SSTs and air temperatures have exhibited an increasing trend, peaking in October 2022 at SSTs of 18.7°C and air temperatures of 17.4°C. Historical El Niño events, noted in Figure 3a, align with increases in SST and air temperatures, indicating a potential linkage between El Niño phases and Atlantic warming. The 2023–2024 El Niño event has contributed to elevated SSTs, as shown in Figure 3b, which also highlights increased North Atlantic atmospheric moisture. Despite a slight reduction in atmospheric humidity in October 2024, levels remained above the 1981–2021 average, underscoring the persistent flood risk driven by moisture in regions like Spain. Moisture-rich air over the North Atlantic increases flood risks for regions like Western Europe, Eastern North America, and Canada. Resilient infrastructure, advanced flood management, and proactive climate adaptation—supported by emissions reduction, forest restoration, and decreased fossil fuel reliance—are essential to counteract climate-driven increases in flood frequency and severity. Finally, we stress the need to improve predictive models incorporating SST anomalies, ENSO phases, and COL dynamics to support advanced flood risk management and urban resilience planning.