The Pacific Ocean is not a single, uniform climate system. It contains several interacting regions where winds, ocean currents, temperature patterns and atmospheric pressure influence one another. Changes in the tropical Pacific are well known for producing El Niño and La Niña events, which can alter rainfall, storms and temperatures across the planet. Yet the North Pacific also has its own slowly evolving patterns, including variations in sea-surface temperature that can persist for many years. The new research focuses on the coupling between these regions at “decadal timescales”—meaning climate fluctuations that unfold over roughly ten years or longer.

This timescale is crucial because it occupies the space between short-term weather events and long-term climate change. A marine heatwave may develop over weeks or months, but the background conditions that make such an event more likely can be established years in advance. Warmer or cooler ocean states can affect atmospheric circulation, modify ocean currents and change the way heat is transported across entire basins. By investigating tropical and North Pacific coupling, the researchers are addressing a central question in climate science: how can gradual, basin-wide changes prepare the ocean for sudden and extreme temperature events?

The study’s central finding, reflected in its title, is that interactions between the tropical Pacific and the North Pacific help shape variability across the Pacific basin. This coupling means that climate signals do not remain confined to the region where they first emerge. A change in tropical Pacific temperatures can influence atmospheric winds, while those winds can alter surface currents and ocean mixing farther north. In turn, conditions in the North Pacific may feed back into the broader ocean-atmosphere system, affecting how heat is stored, redistributed or released.

The physical mechanism involves several linked processes. Sea-surface temperatures influence the atmosphere by changing the amount of heat and moisture transferred from the ocean into the air. These changes can modify pressure patterns and wind fields. Winds then push surface waters, alter evaporation and control the depth of the ocean’s mixed layer—the upper zone where winds stir heat, salt and nutrients. When the mixed layer becomes unusually shallow, solar energy can be concentrated in a smaller volume of water, allowing surface temperatures to rise rapidly. When it deepens, heat can be mixed downward, temporarily shielding the surface from extreme warming.

These processes are especially important for marine heatwaves, which are prolonged periods of unusually warm ocean temperatures. Marine heatwaves can disrupt fisheries, stress coral reefs, reduce oxygen levels and force marine species to move into unfamiliar habitats. They can also affect coastal economies and intensify heat and drought on nearby land. Although individual events may appear sudden, their intensity and persistence can depend on pre-existing ocean conditions. The study highlights how decadal Pacific variability can influence that background state, potentially changing the likelihood, duration or geographical reach of future marine heatwaves.

The research also challenges the idea that Pacific climate variability can be understood by examining one region in isolation. Climate models and forecasting systems often divide the ocean into specialized zones, but the atmosphere and ocean do not respect those boundaries. Signals can travel through atmospheric teleconnections, ocean currents and changes in the distribution of heat below the surface. A pattern that begins in the tropics may later appear as altered temperature conditions in the North Pacific, while northern changes may modify the atmospheric circulation that connects back to the tropics.

Understanding these connections could improve climate prediction beyond the familiar seasonal outlook. Forecasts of El Niño and La Niña typically focus on timescales of months to a few years, while decadal prediction aims to anticipate broader shifts in the climate system. If the state of one Pacific region provides information about what may happen in another, scientists could use those relationships to improve early warnings for marine heatwaves and other ocean extremes. Such predictions would not determine the exact location or timing of every event, but they could identify periods when ecosystems and coastal communities face elevated risk.

The findings also carry implications for interpreting recent ocean warming. Human-driven climate change is raising global ocean temperatures, but natural variability can temporarily amplify or mask that long-term trend in particular regions. Decadal coupling between the tropical and North Pacific may help explain why some parts of the ocean experience unusually rapid warming while others warm more slowly for a time. Separating these natural fluctuations from the underlying rise caused by greenhouse gases is essential for detecting climate change accurately and planning effective adaptation.

By showing that tropical and North Pacific dynamics are closely linked on decadal timescales, Capotondi, Xu, Newman and their co-authors provide a broader framework for understanding Pacific climate behavior. The message is both technically important and highly relevant to a warming world: the next major marine heatwave may be influenced not only by conditions at the location where it develops, but also by ocean-atmosphere interactions unfolding thousands of kilometers away years earlier. As marine extremes become more consequential, mapping these hidden connections could become one of the most valuable tools for anticipating how the ocean will change next.

Subject of Research: Tropical and North Pacific coupling, decadal climate variability, basin-scale ocean dynamics, and marine heatwaves.

Article Title: Tropical and North Pacific coupling at decadal timescales shapes basin variability and marine heatwaves.

Article References: Capotondi, A., Xu, T., Newman, M. et al. “Tropical and North Pacific coupling at decadal timescales shapes basin variability and marine heatwaves.” Communications Earth & Environment (2026). https://doi.org/10.1038/s43247-026-03864-7

Image Credits: AI Generated

DOI: 10.1038/s43247-026-03864-7

Keywords: Pacific Ocean, tropical Pacific, North Pacific, decadal variability, climate coupling, marine heatwaves, ocean-atmosphere interaction, climate prediction.

For over a century, climatologists have tracked the cyclical patterns of the PDO, an ocean-atmosphere phenomenon characterized by shifts in North Pacific sea surface temperatures roughly every 20 to 30 years. These oscillations vacillate between a warm, positive phase and a cool, negative phase, the latter marked by cooler waters along the U.S. West Coast. During these negative periods, storm tracks move poleward and inland rainfall declines significantly, often heralding drought conditions across the western United States. Until recently, the scientific consensus held that these phases were predominantly natural oscillations driven by internal variability in the ocean-atmosphere system.

The newest findings challenge this long-held assumption by presenting robust evidence that human activity, specifically emissions of greenhouse gases and industrial aerosols, has gained substantial influence over the PDO since the mid-twentieth century. Leveraging an innovative array of more than 500 climate model simulations, the research team demonstrated that anthropogenic factors account for over 50% of PDO variability from the 1950s onward, a dramatic shift from the primarily natural drivers observed prior to that period. This revelation forces a reevaluation of the PDO’s role in modern climate dynamics and water resource forecasting in drought-prone regions.

What sets this study apart is the application of enhanced modeling techniques that address the notorious “signal-to-noise paradox” in climate science—a challenge wherein models tend to overestimate natural variability while underplaying the impact of human-induced external forcings. By implementing corrections for this bias, researchers achieved greater fidelity in simulating past climate patterns and unraveling the dominant forces shaping the PDO. According to Jeremy Klavans, lead author and postdoctoral researcher in CU Boulder’s Department of Atmospheric and Oceanic Sciences, this methodological breakthrough allowed them to confidently attribute the prolonged negative phase of the PDO—and consequently the enduring megadrought—to anthropogenic emissions rather than natural cycles.

Since the 1990s, the PDO has been anomalously stuck in its negative phase, a duration that far exceeds its usual oscillation period. This stuck phase has amplified the already severe hydrological deficits across the American Southwest by rerouting precipitation-laden storm systems northward and suppressing moisture availability. Such prolonged dry conditions have culminated in the region’s current status as one of the driest in over 1,200 years, with approximately 93% of the western United States experiencing drought and 70% confronted by severe levels. The persistence of this dry trend poses significant challenges for water management, agriculture, ecosystems, and urban planning in an area already experiencing heightened climate vulnerability.

Intriguingly, the team found that natural climatic resets, such as the strong El Niño event in 2015—which typically would shift the PDO back toward a positive phase—failed to produce lasting change. Instead, the PDO briefly flipped positive only to revert swiftly to its negative state, signaling an underlying yet poorly understood anthropogenic forcing mechanism effectively anchoring the oscillation. This insight suggests that the PDO’s behavior in the coming decades cannot be assumed to follow historical natural rhythms, posing a formidable obstacle to conventional climate forecasting and resource management strategies.

By uncovering the extent of human influence on the PDO, the study invites a broader reconsideration of decadal climate variability as not merely internal fluctuations but as signals increasingly shaped by external forcings. This reframing holds profound implications for climate science, underscoring the importance of refining global climate models to better capture the sensitivities of regional climates to anthropogenic drivers. Co-author Pedro DiNezio emphasizes that existing models historically underestimated the sensitivity of regional systems like the PDO to such forcing, thus highlighting the necessity for improved simulation techniques in predicting climate extremities.

Beyond the American Southwest, the findings carry potential global significance. The research posits that similar oceanic modes, such as the North Atlantic Oscillation—which influences drought patterns in Europe including Spain—may likewise be modulated by human activity to a greater extent than previously recognized. If so, this might herald a new paradigm in understanding the anthropogenic imprint on ocean-atmosphere variability across multiple basins, with far-reaching consequences for regional climate risk assessments and adaptation planning worldwide.

The American Southwest’s water crisis today must therefore be viewed not as a transient meteorological anomaly but as a manifestation of a fundamental climate-driven reconfiguration of the hydrological cycle. As Klavans cautions, the ongoing drought is effectively a lasting transformation of the region’s water system, necessitating urgent, science-informed policy responses. Planners and decision-makers need to incorporate this expanded understanding into water resource management, infrastructure development, and conservation initiatives to mitigate the escalating risks posed by this anthropogenically locked PDO phase.

Looking forward, the study’s methodological innovations also provide a powerful toolset for improving long-term predictions of regional climate impacts, including precipitation patterns crucial for agriculture, ecosystems, and urban water supply. Amy Clement, co-author and professor at the University of Miami, highlights that enhanced model accuracy in capturing human-driven external forcings will enable more reliable climate projections, a critical capability for societies worldwide facing the multifaceted challenges of climate change. The integration of these new insights into climate modeling thus promises to reshape how scientists, policymakers, and communities prepare for and respond to the intensifying extremes of a warming planet.

In summary, the CU Boulder-led research marks a seminal advance in climate science, upending previously entrenched beliefs about the origins and persistence of the Pacific Decadal Oscillation. It definitively connects human emissions to the prolonged negative phase of the PDO, a key driver of the longest and most severe megadrought in over a millennium in the American Southwest. Far from temporary natural variability, the region’s drought is deeply rooted in human-induced changes to ocean-atmosphere dynamics, casting a long shadow over water security in the coming decades. This realization demands that climate mitigation and adaptation efforts account for these altered baseline conditions as our planet’s climate system increasingly responds to anthropogenic influence.

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News Publication Date: August 13, 2025
Web References: http://dx.doi.org/10.1038/s41586-025-09368-2
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Keywords: Pacific Decadal Oscillation, megadrought, anthropogenic forcing, greenhouse gas emissions, aerosols, North Pacific Ocean, climate modeling, signal-to-noise paradox, climate variability, water crisis, American Southwest, El Niño, climate adaptation