How Saharan dust regulates hurricane rainfall

Giant plumes of Sahara Desert dust that gust across the Atlantic can suppress hurricane formation over the ocean and affect weather in North America.

But thick dust plumes can also lead to heavier rainfall -- and potentially more destruction -- from landfalling storms, according to a July 24 study in Science Advances. The research shows a previously unknown relationship between hurricane rainfall and Saharan dust plumes.

"Surprisingly, the leading factor controlling hurricane precipitation is not, as traditionally thought, sea surface temperature or humidity in the atmosphere. Instead, it's Sahara dust," said the corresponding author Yuan Wang, an assistant professor of Earth system science at the Stanford Doerr School of Sustainability.

Previous studies have found that Saharan dust transport may decline dramatically in the coming decades and hurricane rainfall will likely increase due to human-caused climate change.

However, uncertainty remains around the questions of how climate change will affect outflows of dust from the Sahara and how much more rainfall we should expect from future hurricanes. Additional questions surround the complex relationships among Saharan dust, ocean temperatures, and hurricane formation, intensity, and precipitation. Filling in the gaps will be critical to anticipating and mitigating the impacts of climate change.

"Hurricanes are among the most destructive weather phenomena on Earth," said Wang. Even relatively weak hurricanes can produce heavy rains and flooding hundreds of miles inland. "For conventional weather predictions, especially hurricane predictions, I don't think dust has received sufficient attention to this point."

Competing effects

Dust can have competing effects on tropical cyclones, which are classified as hurricanes in the North Atlantic, central North Pacific, and eastern North Pacific when maximum sustained wind speeds reach 74 miles per hour or higher.

"A dust particle can make ice clouds form more efficiently in the core of the hurricane, which can produce more precipitation," Wang explained, referring to this effect as microphysical enhancement. Dust can also block solar radiation and cool sea surface temperatures around a storm's core, which weakens the tropical cyclone.

Wang and colleagues set out to first develop a machine learning model capable of predicting hurricane rainfall, and then identify the underlying mathematical and physical relationships.

The researchers used 19 years of meteorological data and hourly satellite precipitation observations to predict rainfall from individual hurricanes.

The results show a key predictor of rainfall is dust optical depth, a measure of how much light filters through a dusty plume. They revealed a boomerang-shaped relationship in which rainfall increases with dust optical depths between 0.03 and 0.06, and sharply decreases thereafter. In other words, at high concentrations, dust shifts from boosting to suppressing rainfall.

"Normally, when dust loading is low, the microphysical enhancement effect is more pronounced. If dust loading is high, it can more efficiently shield [the ocean] surface from sunlight, and what we call the 'radiative suppression effect' will be dominant," Wang said.

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New 'time travel' study reveals future impact of climate change on coastal marshes

A new Tulane University study published in Nature Communications offers a glimpse into the possible impact of climate change on coastal wetlands 50 years or longer into the future.

Scientists are usually forced to rely on computer models to project the long-term effects of rising seas.

But an unexpected set of circumstances enabled a real-world experiment along the Gulf Coast.

An extensive network of nearly 400 monitoring sites was established along the Louisiana coast after hurricanes Katrina and Rita.

Then the rate of sea-level rise in the region surged to more than 10 millimeters (half an inch) per year -- at least three times the global average.

That exposed the region to the kind of ocean rise not expected until around 2070.

The accelerated rise created a unique opportunity to determine whether the marshes can survive that pace of coastal flooding.

"It is the dream of every field researcher who does experiments -- we can basically travel 50 years into the future to get a peek at what's in store," said Torbjörn Törnqvist, Vokes Geology Professor in the Tulane Department of Earth and Environmental Sciences.

The researchers used new techniques developed by European scientists to measure sea-level rise right off the coast with satellite data, something that was previously not available.

The team then compared the rate of water-level rise at each monitoring site with the rate of wetland elevation change determined by other instruments and found that almost 90% of the sites were in deficit.

"To our knowledge, this is the first time that a climate impact experiment has been carried out over a region this large, based on hundreds of monitoring stations that have collected data for about 15 years," said Guandong Li, a PhD candidate in Earth and Environmental Sciences at Tulane who led the study.

"This has also allowed us to study the climate impact on a heavily human-influenced landscape, rather than a more resilient pristine ecosystem."

Li was investigating the role of land subsidence in coastal Louisiana when a team led by Sönke Dangendorf, the David and Jane Flowerree Professor in Tulane's Department of River-Coastal Science and Engineering, demonstrated the unprecedented rates of sea-level rise along the Gulf and Southeast U.S. coasts since 2010.

"Guandong immediately dropped everything he was working on to take advantage of this unique opportunity," Törnqvist said.

"He set out to answer the key question of whether coastal marshes can keep up with this rate of sea-level rise, as some earlier modeling studies had suggested they can."

If the current climate scenario persists, the rate of sea-level rise by 2070 is expected to be about 7 millimeters (one quarter inch) per year.

The study projects that approximately 75% of wetland sites will be in deficit by that time, potentially resulting in a rate of wetland loss much higher than what has already occurred in the past century.

However, the researchers emphasize that there is hope for a more favorable outcome if immediate action is taken.

By meeting the targets set by the Paris Agreement and reducing carbon emissions, it is possible to shift to a more sustainable climate trajectory that would reduce the rate of wetland loss.

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