Posted: July 28, 2026

Some of the largest, longest, and most intense marine heatwaves on record have occurred in the Northeast Pacific Ocean, the open ocean area from California to Alaska which most notably experienced “The Blob” marine heatwave that started in late 2013 and lasted a little over two years.

A marine heatwave occurs when the ocean’s near-surface waters are unusually warm for days to months. These elevated temperatures can have devastating impacts on the marine ecosystem, blue economy, and coastal communities. Because of these negative effects, better understanding of what makes marine heatwaves in this region tick from season to season is key to improving their prediction.

Tongtong Xu of the NOAA Physical Sciences Laboratory and the Cooperative Institute for Research In Environmental Sciences (CIRES) at CU Boulder has studied Northeast Pacific marine heatwaves for years. Building off of her previous work, Xu wanted to understand how marine heatwaves in this region responded to tropical dynamics versus local ocean processes, particularly from season to season.

The question

Marine heatwaves result from a number of different drivers.

Globally, one major driver is ENSO, the El Niño Southern Oscillation. This large climate pattern of warm (El Niño) and cool (La Niña) ocean temperature in the tropical Pacific follows a seasonal timeline, usually starting in the spring and peaking in the winter. It additionally has “flavors” - sometimes happening in the central Pacific and sometimes farther east along the equator.

More locally, the Northeast Pacific also has an “ocean memory” through a process called "reemergence." In the winter, the ocean mixes deeply and warm surface water gets pushed down deep where it can get trapped during the summer months. As winter returns, the ocean begins to mix again, bringing the trapped warm water back to the surface and reestablishing the warmer conditions.

Xu and her colleagues wondered to what extent tropical ENSO dynamics versus local reemergence and internal ocean processes determine the type of marine heatwave in the Northeast Pacific.

The result is their study published recently in Communications Earth & Environment.

The approach

• The historical record itself has provided relatively few marine heatwaves. To discover how marine heatwaves behave in different seasons, the team needed more marine heatwaves to study. The authors turned to an emulator based on a Cyclostationary Linear Inverse Model (CS-LIM) to generate hypothetical marine heatwaves.
• A CS-LIM is a data-driven model that uses real-world observations to simulate global conditions over different time periods. In this case, the researchers’ CS-LIM used 64 years of real-world sea surface temperature and height data.
• Using their specific CS-LIM, the researchers generated thousands of “synthetic” marine heatwaves, creating a large sample by which they could better analyze how marine heatwaves starting in one season evolve differently than those starting in another, and what potentially influenced them.
• The CS-LIM generated simulations spanning the Pacific Ocean so that researchers could analyze ENSO while also looking at the Northeast Pacific local patterns that would indicate the presence of processes such as reemergence.
• Why was this approach different? A CS-LIM specifically factors in seasons, whereas a traditional LIM, known as a Stationary Linear Inverse Model, assumes background conditions are constant throughout the year.

The results

• The researchers found that different regions of the Northeast Pacific respond to the different ENSO “flavors.” For example, an El Niño in the Central Pacific can help trigger a marine heatwave in the Gulf of Alaska, while a La Niña in the Eastern Pacific can contribute to one in the Central North Pacific.
• Apart from ENSO influence, local processes such as “ocean memory” reemergence also play a role in generating warm ocean conditions.
• Marine heatwaves that start in the winter last longer due to the combination of ENSO impact and the reemergence process, which gives the heatwave staying power. Ones that start in the summer can be much more intense, but they generally end quickly.

Figure: Warmer and cooler than average ocean waters linked to (a) Eastern and (b) Central tropical Pacific ENSO patterns. The colored boxes highlight the three specific regions looked at in the study: the Central North Pacific (yellow), the Northeast Pacific (blue), and the Gulf of Alaska (orange).

The figure contains two side-by-side maps of the Pacific Ocean comparing Sea Surface Temperature Anomalies (SSTA) in degrees Celsius. A shared color scale at the bottom ranges from dark blue (negative anomalies of -1°C or lower) to white (0°C) to dark red (positive anomalies of 1°C or higher). Panel a, on the left, is labeled "EP ENSO" and displays a distinct band of strong positive temperature anomalies concentrated directly along the equator in the eastern Pacific Ocean, extending westward from the coast of South America. Panel b, on the right, is labeled "CP ENSO" and shows the strongest positive equatorial anomalies shifted further west into the central Pacific Ocean. This panel also displays a broader, horseshoe-shaped pattern of warming extending northward along the western coast of North America. Both panels feature three overlapping, colored rectangular boxes outlining specific regions of interest in the Northeast Pacific, off the North American coast: an orange box located further west and south, a central blue box overlapping its top right corner, and a red box located further north and east, nearest to the coastline.

Schematic summarizing seasonal characteristics of Northeast Pacific marine heatwaves

The schematic is divided into four main sections illustrating the drivers of marine heatwaves in the Northeast Pacific. ENSO Influences (Top Left): Three maps show how different tropical ENSO phases link to specific regions. Central Pacific El Niño correlates with heatwaves in the Gulf of Alaska. A neutral state has a weaker influence. Eastern Pacific La Niña correlates with heatwaves in the Central North Pacific. Ocean Memory Processes (Top Right): A diagram illustrates the Reemergence Mechanism. In Winter Year 0, a warm anomaly is at the surface in a deep mixed layer. In Summer Year 0, the mixed layer becomes shallow, hiding the anomaly beneath it. In Winter Year 1, the mixed layer deepens, and the anomaly resurfaces. Seasonality of Northeast Pacific Marine Heatwaves (Bottom Left): A graph compares ENSO amplitude and marine heatwave persistence. Winter and Spring have high persistence due to a favorable ENSO state and surface warming reemergence. Summer and Fall have lower persistence, characterized by short-lived but intense events. Takeaways (Bottom Right): Marine heatwaves occur year-round, but persistence depends on seasons. Different flavors of heatwaves link to different ENSO flavors. Winter persistence reflects both ENSO and North Pacific ocean memory.

The impact

• The study highlighted the importance of the season and the heat hiding below the surface when predicting marine heatwaves. Is it starting in the summer? If so, there is a good chance it will be hotter but fade quickly. Starting in the winter? Then it won’t be as intense, but it could last awhile, even over years, similar to “The Blob.”
• The study results advance the knowledge and understanding of marine heatwave seasonal dynamics in the hotbed region of the Northeast Pacific. The methods used in this study could potentially be applied elsewhere to better understand the dynamics in other regions.
• With improved understanding of the seasonal heatwave drivers in the Northeast Pacific from this and future studies, prediction of marine heatwaves through improved models and tools can give communities and industries more time to prepare for these extreme events, potentially reducing the amount of harm caused.

What about now?

In a blog post accompanying the study, Xu applied the findings to current ocean conditions:

CS-LIM initialized from Spring 2026 points toward El Niño development in Winter 2026/27

A map displaying the Sea Surface Temperature Anomaly (SSTA) forecast for February 2027, initialized from April 2026. The map covers the Pacific Ocean from 20 degrees South to 60 degrees North latitude, and 130 degrees East to 80 degrees West longitude. The color scale indicates temperature anomalies in degrees Celsius, ranging from negative 2 (dark blue, cooler than average) to positive 2 (dark red, warmer than average). Key features of the map include: Equatorial Pacific: A prominent, deep red band indicating strong positive temperature anomalies (over 1.5 degrees Celsius) stretches along the equator from the coast of South America westward past 170 degrees West. North American Coast: Moderate to strong positive anomalies (yellow and orange, 0.5 to 1.5 degrees Celsius) hug the western coast of North America and extend northward into the Gulf of Alaska. Central North Pacific: A distinct, large region of negative temperature anomalies (light to dark blue, dropping below negative 1.5 degrees Celsius) is centered in the North Pacific, roughly between Hawaii and the Aleutian Islands. Western Pacific: The western tropical Pacific shows neutral to slightly cooler than average temperatures.

This spring we watched a North Pacific meridional mode-like pattern unfold, with ocean warming extending from California toward the central tropics and reaching marine heatwave conditions. Historically, this is a classic early-warning signal for El Niño development. Our CS-LIM initialized from Spring 2026 conditions also points toward El Niño development in Winter 2026/27, with characteristics resembling a Central Pacific event. If that happens, we might expect some warming in the Gulf of Alaska.

However, because Winter 2025/26 did not produce substantial warm anomalies in that region, there is little subsurface heat storage to ‘reemerge’, without which the surface warming may not be too extreme. Alternatively, in a rarer situation that an Eastern Pacific El Niño develops, we might not see open ocean heatwaves in the broader Northeast Pacific at all.

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