What is a marine heatwave?
Marine heatwaves, or MHWs, occur when ocean temperatures are much warmer than usual for an extended period of time; they are specifically defined by the difference between the current temperature and the expected temperature for a specific location and time of year [1]. MHWs are a growing field of study worldwide because of their effects on ecosystem structure, biodiversity, and regional economies.
In 2014 a large MHW was identified as it began dominating the northeast Pacific Ocean. Eventually known as “The Blob” [2] (Fig. 3A.), this basin-scale MHW was unique in the history of monitoring in the California Current, and persisted until mid-2016. Researchers documented many ecological effects associated with “The Blob”, including unprecedented harmful algal blooms, shifting distributions of marine life, and changes in the marine food web [3].
Large marine heatwaves have occurred each of the last seven years (2019 - 2025), all typically beginning during the spring in the far offshore region of the open North Pacific, impacting the US West Coast during the fall, and finally terminating during late winter. Of the last seven years, six of these were among the largest heatwaves on record for the eastern North Pacific since monitoring began in 1982 (calculated only within the analyzed region shown in Fig. 2). However, it is worth noting some of these heatwaves were likely larger than our calculated area, as they extended beyond the borders of our prescribed analysis region at various times. Animations of recent years' heatwaves can be found here..
To further investigate past MHW events, access this table of detailed information (i.e., size, duration, distance from shore) or an archive of past yearly animations.
What are the latest conditions?
(last updated 30 September, 2026)
We continue to track two large marine heatwaves in the Northeast Pacific region (Fig. 1, Fig. 2): NEP25A located near the coast of California, and NEP26A in the far west and north of the region, within the Gulf of Alaska. NEP25A has been a contiguous feature we have tracked since May 2025 (hence its name, NEP25A, which denotes it as the first large MHW tracked starting in 2025); NEP26A developed in May 2026 in the far offshore region. Along the northern California coast through Washington, coastal temperatures have fluctuated over the past month from average to now slightly above average (Fig. 1) with small patches that occasionally qualify as marine heatwaves (Fig. 2), with the patterns related to local winds and the presence or absence of upwelling in this region.
NEP26A, in the offshore northwestern part of our study region, has been variable in its size and spatial coverage over the past month, but has generally increased in area and intensity. Similarly, NEP25A, off the southern portion of the US West Coast, has also generally expanded and intensify, particularly in the past few weeks. It is important to note that these temperature anomalies off the northern part of the West Coast for these past few months, and the warming off of California up until the end of September, are not due directly to El Niño, but rather other atmospheric and oceanic drivers. However, as of this narrative update, it is likely that some of the warming off southern California at the end of September may finally be the result of more direct warming due to El Niño drivers, such as oceanic Kelvin waves.
The current heatwave forecast (https://psl.noaa.gov/marine-heatwaves/) suggests a high likelihood that marine heatwaves will continue through the spring and summer in the offshore regions, while marine heatwaves in coastal regions progressively move from south to north as El Niño-driven warming develops over the next several months. The El Niño forecast (as of September 10, 2026) continues to report we are in an "ENSO (El Niño Southern Oscillation) advisory" status, with a high probability of El Niño strengthening to "very strong" during the fall and winter (see https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml).
Although the current MHW off California has somewhat “preconditioned” the region with above-average water temperatures before we encounter additional warming this fall due to El Niño, the two features may not necessarily be additive. Rather, we suspect that greater impacts may accrue from the prolonged duration of warmer than normal temperatures in the coastal region. Essentially the coastal zone may transition from anomalously warm waters associated with the current marine heatwave right into similarly warm waters due to the El Niño, somewhat matching the observations in 2015 when we also transitioned from “The Blob” to another moderately strong El Nino during winter 2015-16. We will continue to monitor the area, duration, and coastal proximity of surface water temperatures for these features in the northeast Pacific and communicate with other researchers and policy-makers to understand the array of possible West Coast impacts.
Animations and images of past years' developing heatwaves can be found here: NEP25A animation, NEP24A animation, NEP23A animation; previous archived animations.
View new and ongoing analyses highlighting regional conditions associated with individual west coast National Marine Sanctuaries and states (Washington, Oregon, and northern, central, and southern California).
What is the MHW Tracker (aka "Blobtracker")?
Developed by oceanographers from NOAA Fisheries’ Southwest Fisheries Science Center as an experimental tool for natural resource managers, the California Current MHW Tracker (aka “Blobtracker”) is a program designed to describe and thereby provide historical context for current and past large marine heatwaves. It also produces a range of indices that could help forecast or predict future MHWs expected to impact our coast.
Because “The Blob” dramatically affected natural resources, including economically valuable fisheries, predictive forecasts will help natural resource managers, businesses, and coastal communities anticipate changes and mitigate possible damages in the future.
The California Current MHW Tracker automatically analyzes sea surface temperature anomalies (SSTa) from 1982- present, with a particular focus on detecting the presence of significant ”Blob-Class” events. Sea surface temperature (SST) data were obtained from a variety of different platforms (satellites, ships, buoys) on a regular global grid at a resolution of 1/4°, as provided by NOAA’s OISST program.
Here, we define “Blob-Class” MHWs based on their strength (>1.29 times the standard deviation of the SSTa field; i.e., the top 90% of the data), along with their areal extent (area > 400,000 km², which includes the top 20% of all heatwaves recorded in this region since 1982). The “Blobtracker” program groups all contiguous pixels satisfying the strength threshold, and then tracks those contiguous regions over time, as long as they continue to satisfy the area threshold and spatially overlap at least part of a similar feature from the day before. This allows the tracking and classification of these large marine heatwaves as they evolve and move around the north Pacific, similar to how one would track large storms or hurricanes. We have adopted a naming convention for these tracked heatwaves of giving them a number based on the year of their first crossing the area threshold, and a letter corresponding to the order in which they arise each year (e.g. the second “Blob-Class” heatwave that arises in 2010, would be called NEP10b, with the NEP standing for Northeast Pacific). Besides tracking these large “Blob-Class” features, we also provide indices related to how much of the EEZ (Exclusive economic zone) of the US west coast is in heatwave status, by calculating the % total area within the EEZ exceeding the strength threshold, regardless of the area-tracking threshold (Fig. 4).
What controls the presence of heatwaves near the coastline?
For fisheries management purposes, the most important aspect of these heatwaves are whether they come into close contact with our coasts and within the EEZ, as these are the regions where the majority of our fisheries resources are based. Although local forces are important, the dominant feature which controls coastal water temperatures in the California Current system (CCS) is alongshore wind patterns. Because of the orientation of the US west coast coastline (predominantly N-S) and the general flow of the CCS (from north to south along the coast), and the effects of coriolis, our coast is heavily impacted by a process known as upwelling. Essentially, when wind blows from north towards the south along this coast, it causes an “upwelling” of deeper, colder, nutrient rich waters to rise to the surface, displacing warmer surface waters offshore. Vice versa, when winds are weak, or blow from south to north, this may shut off (termed upwelling “relaxation”), or even reverse upwelling (termed “downwelling”), which allows surface waters to warm, and/or offshore waters to advect towards the coast.
Therefore, our working hypothesis is that much of the timing and occurrence of large marine heatwaves within the US west coast EEZ is related to changes in the winds driving upwelling. When upwelling winds weaken or reverse, coastal waters tend to warm. Further, when there are already large marine heatwaves in the offshore region, changes in wind direction and strength can lead to those features advecting into, or becoming contiguous with, warmed coastal waters during these upwelling “relaxation” or “downwelling” events if those events last for a significant period of time. Because of the presumed importance of these basin-scale winds driving upwelling patterns, which are in turn driven by basin-scale atmospheric pressure patterns (further described here), we have begun to closely monitor the wind and pressure patterns across this region, along with SSTa (Fig. 1).
Have marine heatwaves broken any records so far this year?
2026 has already seen record-breaking temperatures in some areas of the California Current due to marine heatwaves. Within the California Current Exclusive Economic Zone (EEZ), we analyzed sea surface temperatures using satellite-based data extending back to 1982 to look for times and locations when new records were set during 2026. We define a “record breaker” as any location where the temperature on a specific day is higher than the previously recorded temperature at that location for that specific day of the year. We found a substantial number of times and locations when daily sea surface temperature records have already been broken during 2026; very few areas have been without record breaking temperatures (grey areas in Fig. 7), whereas some locations in southern California have already had over 150 days of record-breaking temperatures (red areas in Fig. 7). To the north off Oregon and Washington (Fig. 8) the number of days of record breaking temperatures has so far been relatively small – around only 5-20 days – with the majority of these records occurring around May (Fig. 8). Off central California, similarly, there have 5-30 record breaking days so far this year, with most records being broken from Jan - April (Fig. 9). In southern California (Fig. 10), where temperature records have been broken at some locations from 50 to 150 days, most of these records fell January and May, 2026 (Fig. 10). Note that in most cases, previous temperature records were set either during the 1997/1998 El Niño or the unprecedented 2014/2015 marine heatwave, known as “The Blob”.
Project leads
Andrew Leising and Lynn DeWitt (SWFSC), Greg Williams (NWFSC)
References
[1] Hobday, A. J. et al. (2016), A hierarchical approach to defining marine heatwaves, Prog. Ocean., 141, pp. 227-238, 10.1016/j.pocean.2015.12.014
[2] Bond, N. A., Cronin, M. F., Freeland, H., & Mantua, N. (2015). Causes and impacts of the 2014 warm anomaly in the NE Pacific. Geophysical Research Letters, 42, 3414–3420. https://doi.org/10.1002/ 2015GL063306
[3] Cavole LM, Demko AM, Diner RE, Giddings A, Koester I, et al. (2016). Biological impacts of the 2013–2015 warm-water anomaly in the northeast Pacific: winners, losers, and the future. Oceanography 29(2):273–85
