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 . 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”  (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 .
Large marine heatwaves have occurred each of the last four years (2019 - 2022), 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. These features were the 2nd, 3rd, 6th, and 4th largest heatwaves, respectively, on record for the eastern North Pacific since monitoring began in 1982 (calculated only within the analyzed region shown in Fig. 2; note some of these heatwaves were likely larger as they extended beyond the borders of this prescribed region at various times). Animations of recent years' heatwaves can be found here..
What are the latest conditions?
(last updated 11 September 2023)
We are currently tracking marine heatwave (MHW) NEP23A (Fig. 1 and 2), which formed in mid May 2023 and continues through this current update. During August 2023, NEP23A decreased somewhat in size and briefly receded from the coastline; however, it has now again increased in area (Fig. 4) and emerged along nearly the entire US west coast (Fig. 1, 2, and 5). NEP23A now covers at least 6 million km^2, and compared to past MHWs now stands as the 4th largest by area and in the top 20 by duration. Notably, NEP23A formed in the same region days after the previous MHW (NEP22A) dissipated below our area tracking threshold. Thus, although we denote NEP23A as a “new” heatwave, it is an extension of the previous event because it formed in a region where ocean surface temperatures were already elevated above normal, but just below our MHW thresholds.
The current heatwave forecast (https://psl.noaa.gov/marine-heatwaves/) suggests NEP2023A will continue in the offshore waters through the fall, with an increased risk of coastal warming in the spring of 2024. Additionally, we are still in an “El Niño advisory” status, with a 95% chance that El Niño will persist through the Northern Hemisphere winter (see https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml). El Niño combined with the current large offshore marine heatwave, may indicate the system is heading towards conditions similar to those encountered in 2014-2015, the last period during which we had a similar confluence of these two extremes. However, it is also possible that the heatwave may follow the previous pattern of the previous 4 years, which would instead see a weakening of the heatwave and recession further offshore during Oct-Nov. If that is the case, then we expect to see a return to cooler coastal temperatures until the El Niño brings back warmer coastal conditions in the early spring. 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.
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; e.g., 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).
Andrew Leising and Lynn DeWitt (SWFSC), Greg Williams (NWFSC)
 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
 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
 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
Figure 1: Daily sea surface temperature anomalies (SSTa) in the California Current ecosystem. Color represents SSTa, with the thick black line encircling regions which are in "heatwave status"; arrows represent wind speed and direction; thin lines represent atmospheric pressure at sea level. An animation of daily images through 2022 can be viewed here. SST data from Multi-scale Ultra-high Resolution (MUR) SST Analysis Anomaly (https://coastwatch.pfeg.noaa.gov/erddap/griddap/jplMURSST41anom1day.html); sea level pressure and wind data from NCEP/NCAR Reanalysis (https://coastwatch.pfeg.noaa.gov/erddap/esrlNcepRe.html).
Figure 2: Science-quality (delayed 3-weeks), daily interpolated standardized sea surface temperature anomalies (SSTa) in the California Current ecosystem available for analysis of MHW presence. Dark outline shows the current extent of MHW conditions, as delineated by values of the normalized SST + 1.29 SD from normal. Blue dashed line represents the US West Coast EEZ. Data from NOAA's Optimum interpolation Sea Surface Temperature analysis (OISST; https://www.ncdc.noaa.gov/oisst), with the SST anomaly calculated using climatology from NOAA's AVHRR-only OISST dataset.
Figure 3A.(left): The MHW known as "the blob" at its near maximum areal extent in September 2014; 3B.(right) The 2022 MHW (NEP22A) at its near maximum areal extent in August 2022. Figure details as in Figure 2.
Figure 4 (top): Retrospective analysis of sea surface temperature anomalies in the California Current region, 1982-present. Figure shows daily total surface area from the entire study region (Fig. 2) in heatwave status over time. Thin horizontal line indicates the area threshold cutoff (approx. 400,000 km2) used for tracking and analysis of LMHs. Color indicates the % of the US west coast EEZ (area within blue dashed line, Fig. 2) in heatwave status. 4 (bottom): Daily estimated area of SST anomalies in the California Current region over the previous 12 months, color coded (as above) by relative EEZ coverage.
Figure 5: Top graph shows the total % of the US west coast EEZ (not including waters off of Alaska) that is classified as in "heatwave" status over the past year. Left lower panel shows the region covered by the EEZ (blue dashed line) and then subregions: WA (1), OR (2), northern CA (3), central CA (4), and southern CA (5). Lower colored panel indicates the % coverage within each subregion that is in heatwave status. View detailed information about these individual regions, including interactive plots and a variety of LMH indicator outputs.