A new study published in Science Advances overturns a long-standing paradigm in climate science that stronger Northern Hemisphere summer insolation produces stronger tropical rainfall. Instead, a precisely dated 129,000-year rainfall reconstruction from a Cuban cave shows that the Caribbean often did the opposite, drying during intervals of intensified summer insolation.
The research reveals a new unrecognized primary driver: The North Atlantic Subtropical High (NASH), a powerful and ever-present high-pressure system, that surprisingly has been the dominant force shaping the region's hydroclimate on millennial to orbital timescales. When NASH strengthens and expands westward, it suppresses convection and reduces rainfall across the Atlantic Warm Pool, a region that includes the Caribbean Sea, the Gulf of Mexico, and parts of Central America.
"We found that the story of Caribbean rainfall is more about the push and pull of this massive atmospheric anticyclone and is not really related to the direct influence of the sun as in other global location," said lead author Hanying Li at Xi'an Jiaotong University in China. "When the NASH strengthens and bulges westward, it essentially squashes convection over a huge area, leading to drier conditions."
"We built climate history by analyzing the geochemical signatures locked within stalagmites from the Santo Tomás cave in western Cuba. Like reading tree rings, they measured stable oxygen isotope in the growth layers to reconstruct a detailed timeline of rainfall patterns. " says co-corresponding author Amos Winter, a professor at Indiana State University.
"The stalagmites act as a natural rain gauge, preserving a remarkable record of hydroclimate variability through multiple ice ages and warm periods," explains co-author Hai Cheng, professor at Xi'an Jiaotong University.
The team discovered that well-known periods of high summer solar output, when the conventional paradigm would predict wetter conditions, were instead linked to drier periods in the Atlantic Warm Pool. The data, combined with climate model simulations, showed that this counterintuitive pattern occurs because high insolation intensifies and expands NASH causing the high pressure system to move westward causing drying.
"It is the combination of stronger land heating and cooler tropical waters that amplifies the westward push of the Subtropical High," says Amos Winter and co-author Sophie Warken from the University of Heidelberg in Germany. "When these conditions coincide, the Caribbean can experience prolonged and severe drying."
The research also identified two of the most extreme dry spells of the last 129,000 years, occurring around 126,000 and 105,000 years ago. These "megadroughts" occurred when the combined effects of peak summer insolation and cool Atlantic sea surface temperatures were at their strongest.
"Interestingly, the dynamics underlying these ancient "megadroughts" resembles the 'midsummer dry spell' that affects the Caribbean today", says Ashish Sinha, a professor of Earth science at California State University Dominguez Hills and a co-author of the study. "In the past, however, this pattern was much stronger and persisted for centuries."
The study further reveals a climatic dipole, linking the fate of the Caribbean to that of eastern North America. When the NASH was strong and expanded westward, the Caribbean dried out. Instead moisture was funneled toward eastern North America, causing wetter conditions there—a pattern clearly visible in the geological records from both regions.
"Our findings place the North Atlantic Subtropical High in a leading role for driving orbital-scale climate change in this critical region," the researchers conclude. "Given that future climate projections point to a strengthening and westward shift of the NASH, this study suggests a realignment of climate patterns with the potential for more frequent or prolonged summer drying in the Caribbean."
