Title image – Valencia, Spain Credit; NASA
Flash Droughts and Flash Floods
The theme of the Climate Adaptation Center’s annual conference in 2023, “Climate Warming and The Triple Threat of Water,” foreshadowed an increasingly catastrophic connection between climate change and precipitation. When we published “Extreme Heat, Extreme Rainfall” later that year, 2023 was on the way to becoming the hottest year on record, and extreme rainfall had caused devastating flooding around the Mediterranean. Unfortunately, 2024 offers a disturbing new chapter in the story…
Record Heat Driving Extreme Events
At this writing, 2024 is on track to displace 2023 as the hottest year on record, and to register the first annual global average temperature more than 1.5°C above pre-industrial levels. As shown in the graph below, the Copernicus Climate Change Service estimates that 2024 will close with an annual global average surface air temperature of 1.55°C – well beyond the 2015 Paris Agreement’s goal of limiting global warming to a maximum of 1.5°C. Of course, the Paris Agreement goal refers to a multi-year average, but the symbolism of 2024 breaking the 1.5°C ceiling just nine years later was not lost on the international community attending the COP29 Climate Change Conference in Azerbaijan.
What about the oceans? As we wrote earlier this year, all that heat is accumulating in the oceans, as well. The map below shows that sea surface temperatures in October 2024 are much hotter than the 1991-2020 October average over most of the global ocean. However, the 2023 El Niño that heated the equatorial Pacific Ocean has now ended, replaced by the cooler La Niña, and 2024 is unlikely to match the record 2023 annual average. Nonetheless, the 2024 global average for the ice-free ocean is still likely to be warmer than 2022 and all previous years in the ERA5 dataset.
What do these temperature trends tell us about extreme events?
Well, we know that a warmer atmosphere can hold more water: 7% more for every centigrade degree of warming. That water originates as evaporation from land areas and oceans, with about 86% of evaporation occurring over the oceans. Not only can a warmer atmosphere hold more water vapor, but at the same time evaporation from oceans (i.e., water vapor creation) is accelerating as the oceans heat up.
The result over the oceans is clear—a thirstier atmosphere absorbing rapidly evaporating water. Storms forming over warm oceans can carry a stunning quantity of water, leading to extreme rainfall as the storms move over land. Our most recent examples? Hurricanes Helene and Milton. in September, Helene dumped 12 to 20 inches of rain in two days over a large area of North Carolina, leading to catastrophic flash flooding, while in October, Milton drenched St. Petersburg, Florida, with more than 18 inches of rain in just 24 hours.
At the other end of the spectrum, sustained heat waves and dry air over land can cause a flash drought. While sustained drought plagues the Southwestern US, relatively brief flash droughts can have a disastrous impact on normally productive agricultural areas, rapidly extracting moisture from soil and vegetation.
So, climate change has set the stage for flash floods and flash droughts. But what triggers individual extreme events?
Climate Change and the Jet Stream
Discussions of climate change tend to focus on long-term trends over broad areas. However, climate change is also boosting localized short-term weather events which are becoming at once more persistent and more extreme. Altering the jet stream is one mechanism by which climate warming is driving extreme weather.
Atmospheric circulation is driven by the temperature difference between the tropics and the poles. The resulting movement of air over a rotating globe forms the prevailing easterly and westerly winds that circle the earth. The earth’s rotation stretches out the north-south movement of the atmosphere, and narrow bands of rapid flow–jet streams–form within the general circulation. In the northern hemisphere, we have the high altitude sub-tropical jet stream in the tropics and the lower altitude sub-polar jet stream in the mid latitudes. The two jet streams are constantly in motion, often shifting north or south, sometimes merging other times separated at different latitudes.
In the northern hemisphere, the jet stream is the key driver of regional weather, forming a moving boundary between relatively cold air to the north and warmer air to the south. Mid-latitude weather systems move from west to east along the jet stream, thus the changing position of the jet stream has a tremendous impact on regional weather.
Climate change is slowly but surely changing atmospheric circulation, because climate warming is not evenly distributed over the globe. Since 1979, the Arctic has warmed nearly four times faster than the rest of the world, a phenomenon called Arctic Amplification. By reducing the temperature difference between the pole and the tropics, Arctic Amplification slows atmospheric circulation, thus slowing the jet stream.
Flash Droughts
A weakened jet stream begins to weave from north to south as it circles the globe, becoming progressively wavier as it slows. As the jet stream weakens, the north/south undulations deepen, forming ridges and troughs, as shown in the figure below. These meanders in the jet stream also move slowly from west to east. However, as the jet weakens these meanders are more likely to stall in place, leading to persistent high pressure weather systems (or anticyclones) forming in the warmer air of the ridges. We discussed in an earlier post how this process affects the intensity and duration of heatwaves.
The figure above shows another feature of a wavier, stalled jet stream—trapping cold, lower pressure storm systems in deep troughs. The two low pressure systems effectively block movement of the high pressure system in the ridge between them, a pattern called an “omega block,” often resulting in a persistent heat wave. The normal eastward progression of the high pressure anticyclone is obstructed, leading to prolonged extreme weather conditions.
This is exactly the situation that developed over the US in October 2024, resulting in abnormally high temperatures and virtually no rain for the entire month—a flash drought. Like flash floods, flash droughts appear suddenly with little warning, leaving little time to prepare before it’s too late.
Drought map for October 29, 2024, showing increasing drought intensity in progressive shades of yellow to red Credit: NASA Earth Observatory/U.S. Drought Monitor
The October flash drought affected over 78 percent of the American population—the highest percentage in the U.S. Drought Monitor’s 25-year record. Over 70 weather stations recorded the driest October on record. Many locations reported no rain at all, including the cities of Philadelphia, Atlanta, Birmingham, Dallas, Las Vegas, and Sacramento.
Research by Yuan et al (2023) showed that droughts have been intensifying more rapidly since the 1950s, attributed to greater evapotranspiration and dwindling precipitation caused by climate change. At the same time, flash droughts have become more common over much of the world.
Extreme Rainfall and Flash Floods
While the U.S. experienced a record flash drought in the fall of 2024, at the same time Europe experienced a rapid fire series of heavy precipitation/flash flood events in France, Italy, Austria and Czech Republic, culminating in a catastrophic event in Valencia on the east coast of Spain.
Like the flash drought in the U.S., these flash flood events are the consequence of the same extremely wavy jet stream. In this case, a cold low pressure system in a deepening trough in the jet stream sinks south and the trough pinches off, leaving the low pressure system cut off from the jet stream. Not surprisingly, the system is now classified as a “cut-off low.” The following image shows a notional cut-off low over the Eastern U.S.
Typical cut-off low Credit: NOAA
Without the guidance of the west-east flow of the jet stream, the cut-off low moves very slowly, staying in one spot for several days, often accompanied by unsettled weather, heavy rainfall, thunderstorms and flash flooding. In Europe, warm, moist air from the Mediterranean Sea can get drawn under the cold air of a cut-off low higher up in the atmosphere—an unstable situation. Huge storms form, amplified by mountainous topography, and heavy rainfall ensues—sometimes lasting for days.
Climate change affected the cut-off low flash flooding events in two ways. First, via a meandering jet stream spawning cut-off low systems. Second by extreme heat in the Mediterranean Sea, which set a record average surface temperature of 28.9°C (84.02°F) in August. As an example, the Czech Republic recorded three months of rain in just five days in September.
Catastrophic Flash Flooding in Spain
October 29-30, 2024, a cut-off low led to intense flash flooding in the province of Valencia, on the east coast of Spain. 222 people lost their lives, almost half over 70 years of age. Floods destroyed buildings, shut down train transport and more than 150,000 homes are still without power, a month later.
The city of Valencia received a year’s worth of rainfall in just eight hours, with the nearby town of Chiva in the hills west of Valencia recording a jaw-dropping 19 inches of rainfall in 8 hours—more than fell in the preceding 20 months.
Fall storms like this (but less extreme) are not uncommon in Eastern and Southern Spain, where they go by the Spanish acronym DANA. However, this event is the deadliest in Spanish history and the worst flooding event in Europe in over 50 years.
As you can see in the satellite image below, warm moisture-laden winds blew in from the Mediterranean Sea, where they ended up beneath the stagnant pool of cold air sitting in the cut-off low—a highly unstable situation. A group of intense thunderstorms formed in the hills west of the city of Valencia, accompanied by heavy rainfall. Although the wind off the Mediterranean pushed the thunderstorms westward, the static system kept generating more thunderstorms near Valencia. The process was fueled, of course, by record heat in the Mediterranean Sea. In late August 2024, the semi-closed, shallow sea posted a record average surface temperature of 28.9ºC.
The cut-off low system over Spain. Valencia is on the east coast. Note the warm air over the Mediterranean being pulled onshore by the anticlockwise circulation around the low. Credit: UMETSAT
The cut-off low over Spain, unseasonably warm weather, and record heat in the Mediterranean Sea launched the storm, but an atmospheric river turbocharged it. Analysis by Climate Central revealed an atmospheric river pumping moisture from an unusually warm tropical Atlantic to the cut-off low over Spain. Further analysis showed the warm temperatures in the tropical Atlantic were fueled by climate change, as shown in the following figure.
The satellite image below shows the intensity of rainfall during the Valencia event. Precipitation rates in excess of 50 mm/hour (about 2 inches/hour) in the hills above Valencia sent muddy water cascading down rivers and valleys into the seaside city.
Credit: EUMETSAT
But what made 2024’s DANA the deadliest weather event in Spanish history? The short answer is climate change and unusually high sea surface temperatures. A preliminary rapid analysis by World Weather Attribution found October 2024’s heavy rainfall was about 12% heavier and twice as likely than an equally intense storm in a cooler pre-industrial climate.
The Landsat image below shows the city of Valencia and its neighboring towns before the flash flood. (Note: in order to show a cloud-free view, the image shown is from October 2022.) Note that previous flooding events led the city to “channelize” the Turia River, visible in the center of the image, south of central Valencia. (Similar channelization was carried out by the U.S. Army Corps of Engineers in the Los Angeles basin.)
Credit: NASA
The next image was acquired October 30, 2024, immediately after the flash flood. Widespread flooding of urban and agricultural lands in and around Valencia is clearly visible. Mud-laden floodwaters fill the channelized Turia river, which empties into the Mediterranean, and the L’Albufera coastal wetlands south of the city.
Credit: NASA
Credit: r-europe/jasz
As you can see in the image above, even where buildings seem to be intact, the damage was extensive. Estimates of insurance losses alone are in excess of $3.8 billion. Total damage will run well over $10 billion.
The extensive damage and loss of life could have been reduced. Although the storm was forecast, by the time warnings were broadcast flood waters were already running through the streets.