A Shifting Atlantic High Quietly Steers American Summer Rainfall

Every summer, a vast dome of high pressure parks itself over the western North Atlantic Ocean, and its slow breathing helps decide whether farms in Georgia flood or fields in Iowa bake. Known to meteorologists as the western North Atlantic subtropical high, or WNASH, this sprawling anticyclone is one of the most persistent features of the Northern Hemisphere circulation, yet its fine structure has long been treated crudely in climate studies. A new analysis published in Climate Dynamics by Jeremy E. Diem of Georgia State University argues that scientists have been measuring the wrong thing. By focusing on the detailed architecture of the high’s western ridges rather than its basin-wide intensity, the study finds a far sharper connection to summer precipitation across the eastern and central United States than earlier indices could reveal.

The research examined the summers of 1980 through 2025, a 46-year window long enough to capture both year-to-year variability and slower structural change. Diem applied principal component analysis to sea-level pressure fields from the ERA5 global reanalysis, a state-of-the-art dataset produced by the European Centre for Medium-Range Weather Forecasts that blends millions of observations with a physical model of the atmosphere. Principal component analysis is a statistical technique that compresses many correlated variables, in this case pressure values across a grid of locations, into a small number of independent patterns that explain most of the variance. The result is a set of circulation modes that describe how the subtropical high strengthens, weakens, and shifts from one summer to the next.

What makes this study distinctive is its treatment of the high’s western edge. Earlier work often summarized the WNASH with a single metric, such as the longitude of its westernmost ridge or its overall intensity averaged across the basin. But the anticyclone is not a smooth, symmetrical blob. It frequently breaks into multiple ridges, with separate lobes of high pressure over the western Atlantic and the Gulf of Mexico. Diem constructed indices specifically designed to track the positions and separation of these western ridges, then tested how well each representation of the circulation explained precipitation recorded at 1,747 rain gauges scattered across the eastern and central United States.

The answer was unambiguous: the structure of the western ridges matters more than the coarse basin-wide picture. Distinct modes of WNASH and western-ridge variability produced contrasting, regionally heterogeneous precipitation responses, with the strongest and most spatially coherent relationships appearing across the Southeast and portions of the Atlantic seaboard. This makes physical sense. The western flank of the subtropical high is where the clockwise circulation around the anticyclone funnels warm, moist Atlantic and Gulf air toward the continent, feeding the thunderstorm complexes that deliver much of the region’s summer rain. When the ridge positions shift, the moisture conveyor shifts with them, and the gauges record the consequences.

The mechanics of that conveyor are well established in the literature. The Great Plains low-level jet, a river of air a few hundred meters above the ground that surges northward at night, draws its strength partly from the pressure gradient established by the Atlantic anticyclone. Mesoscale convective systems, the organized clusters of thunderstorms that can span several states, depend on that moisture supply. Sea-breeze circulations along the Carolina and Florida coasts, which can trigger afternoon downpours, are also modulated by the position of the high. By reshaping all of these processes, the western ridges of the WNASH act as a kind of hidden thermostat on summertime hydroclimate, one that conventional indices had blurred into the background.

Perhaps the most striking finding is that the high itself has been reorganizing. Over the 1980 to 2025 period, Diem documented a structural change characterized by decreasing separation between the western Atlantic and Gulf of Mexico ridge positions. In other words, the two lobes of the anticyclone have been drifting closer together, altering the geometry of the moisture pathways into the continent. This kind of reorganization is exactly the sort of response climate scientists expect from a warming world, since subtropical anticyclones respond to greenhouse forcing and sea-surface warming in ways that can differ between ocean basins. The study’s trend analyses and comparisons between different epochs of the record allowed these slow structural shifts to be separated from ordinary year-to-year noise.

Crucially, the western-ridge indices outperformed the broader circulation modes when it came to explaining recent precipitation increases across portions of the Southeast. That is a practical result with real forecasting implications. If the geometry of the ridges tracks summer rainfall better than traditional measures, then seasonal outlooks and climate model diagnostics could be sharpened simply by changing how the circulation is described. It also refines the interpretation of earlier studies that linked southeastern rainfall variability to the high’s westward displacement, suggesting that those relationships were real but incompletely characterized, because a single ridge position cannot capture the behavior of a multi-lobed anticyclone.

Yet the study delivers an equally important caution. Substantial residual precipitation increases across the Midwest and Northeast remained unexplained after accounting for WNASH variability, indicating that recent hydroclimatic change across the eastern and central United States cannot be attributed to the Atlantic high alone. The Midwest’s summer rainfall is heavily influenced by soil-moisture feedbacks, convective systems rooted in local land-atmosphere coupling, and moisture streaming in from the Gulf of Mexico and beyond, processes only partially governed by Atlantic circulation. A warming atmosphere also holds more water vapor, roughly seven percent more per degree Celsius, which intensifies heavy rainfall regardless of where the high pressure sits. Disentangling the circulation contribution from the thermodynamic one is one of the central challenges of modern hydroclimate science, and this study draws a clearer boundary around what the WNASH can and cannot explain.

The methodological approach deserves attention as well. Combining rotated principal component analysis of pressure fields, which isolates physically meaningful circulation patterns, with targeted ridge indices and a dense network of ground-based gauges gives the analysis both dynamical rigor and observational grounding. Multiple linear regression modeling allowed the contributions of different circulation modes to be weighed against one another, while inter-epochal comparisons revealed how those relationships have themselves evolved over the decades. The precipitation data used in the study are publicly archived, and the reanalysis products are freely available, making the work reproducible and extensible by other research groups.

The broader significance extends beyond the United States. Subtropical anticyclones shape the climates of every continent bordering the great ocean basins, from the Azores High over Europe to the systems governing the Mediterranean, China, and the West African monsoon. Understanding how their internal structure, not just their average strength, controls regional rainfall is a step toward better projections of where water will be plentiful and where it will become scarce as the planet warms. For the tens of millions of Americans living from the Gulf Coast to the Great Lakes, the message of this research is that the summer rain on their roofs is choreographed, in large part, by the shifting geometry of a high-pressure dome a thousand kilometers out at sea, and that learning to read that geometry more precisely may be one of the keys to anticipating the water cycle of the coming decades.

Subject of Research: Influence of western North Atlantic subtropical high variability on summer precipitation in the eastern and central United States

Article Title: The western North Atlantic subtropical high and summer precipitation across the eastern and central United States

Article References: Diem, J. E. (2026). The western North Atlantic subtropical high and summer precipitation across the eastern and central United States. Climate Dynamics, 64(10), Article 436. https://doi.org/10.1007/s00382-026-08400-z

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08400-z

Keywords: North Atlantic subtropical high, summer precipitation, climate variability, ERA5 reanalysis, sea-level pressure, southeastern United States, Great Plains low-level jet, hydroclimate, Climate Dynamics, atmospheric circulation, precipitation trends, western ridge