Why do we care?
Marine heatwaves are episodes when the surface ocean is anomalously warm for days to months. In recent years, the Northeast Pacific, spanning the open ocean area from California up to Alaska, is one of the ocean regions that has experienced several of the largest scale, most persistent, and extremely intense marine heatwaves on record. These events have caused severe ecosystem impacts and damaged the blue economy to a great extent, including low productivity of red abalone, mass mortality events for mammals and seabirds, and harmful algal blooms that led to prolonged closures of razor clam, rock crab, and Dungeness crab fisheries. It is therefore a top priority to understand how these Northeast Pacific marine heatwaves have occurred historically and translate these mechanistic understandings into predictive skill.
Ever since I was a graduate student, I have had a strong interest in understanding Northeast Pacific marine heatwaves. Despite having done several studies on the subject, I still feel that there are missing pieces. One of them is understanding how these events vary seasonally and what ocean conditions contribute to their seasonal variations. This curiosity is rooted in the fact that the tropical Pacific El Niño-Southern Oscillation (ENSO), the dominant contributor of marine heatwaves globally, tends to follow a predictable seasonal timeline. Typically, ENSO initiates in the boreal spring and grows into full-blown events in the winter.
With this in mind, my collaborators and I hypothesized that ENSO’s influence on the Northeast Pacific must also be seasonally dependent, shaping the characteristics of marine heatwaves that emerge there. But things quickly became complicated. ENSO isn’t a single, uniform phenomenon; it swings between warm and cold phases, termed El Niño and La Niña, respectively. To make matters more complex, these events have different centers of action from year to year, although generally they can be grouped into two distinct “flavors”, either Central Pacific or Eastern Pacific events.
Furthermore, the Northeast Pacific has local ocean memory processes that also vary seasonally. One key process is called the “reemergence” mechanism. In winter, the ocean mixes more deeply, allowing temperature anomalies to extend below the surface. As summer arrives, the surface layer becomes shallower, leaving some of that anomalous heat hidden beneath it. When winter returns and the ocean begins to mix deeply again, that stored heat can be brought back to the surface, allowing past ocean conditions to help shape surface ocean temperature almost a year later.
Therefore, the question naturally becomes: how much of the seasonal characteristics of Northeast Pacific marine heatwaves is due to the remote tropical Pacific influence versus local North Pacific processes?
The approach
To answer this question, we needed far more marine heatwaves than nature has provided. Because these events rarely occur, it is difficult to draw robust conclusions from the limited observational record. We therefore turned to a multivariate empirical dynamical model called the “Cyclostationary Linear Inverse Model (CS-LIM)”. After training the model with observations, we generated thousands of “synthetic” marine heatwaves. This massive boost in sample size allows us to analyze how events starting in one season evolve differently than those starting in another.
What did we find?
Marine heatwave flavors linked to ENSO diversity
Perhaps the most interesting result is how the “flavor” of a marine heatwave depends on the “flavor” of ENSO. We found that different regions of the Northeast Pacific tend to be associated with different ENSO types and phases. The Gulf of Alaska tends to experience marine heatwaves when the tropical Pacific is in a Central Pacific El Niño phase. In contrast, the Central North Pacific tends to see events triggered during an Eastern Pacific La Niña. The regions in between appear to be less influenced by the tropical Pacific.
Reemergence as part of the ocean memory processes
While ENSO diversity explains much of the “flavor” of these marine heatwaves, it isn’t the only driver. Across the Northeast Pacific, we found clear signs of seasonal ocean memory indicative of the reemergence mechanism. This is more so in regions where tropical Pacific’s influence is weaker. In these areas, reemergence plays a larger role in pushing conditions over the threshold of marine heatwaves. Of course, other ocean memory processes, like temperature anomalies carried eastward by ocean currents from near Japan, also play a role.
Seasonality of Northeast Pacific marine heatwaves
The combined influence from these seasonal dynamics results in Northeast Pacific marine heatwaves also varying seasonally. Unlike ENSO, which peaks in the winter, marine heatwaves aren’t as strictly tied to the calendar. They can and do occur year-round, partly because different seasonal drivers compensate for one another across the year. Even so, their characteristics change with season: winter events tend to be more persistent, when the tropical Pacific is in a favorable ENSO state and substantial thermal anomalies from the previous year “reemerge” to strengthen the surface warming. In contrast, summer events are often more intense, but they don’t last nearly as long.
Implications for predicting future marine heatwaves
So how do our findings apply to our current state? As of Spring 2026, we are watching 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 point 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 marine heatwaves across the broad Northeast Pacific open ocean at all.