The California Current Marine Heatwave Tracker – An experimental tool for tracking marine heatwaves

California Current Project


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 differences in expected temperatures for the 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” (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.

For recent comparison, the Northeast Pacific Marine Heatwave of 2019, also known as NEP19 (Fig. 3B) was the third largest and longest event recorded in the northern Pacific Ocean since 1982, when satellite-based remote sensing of sea surface temperatures began in a consistent fashion. NEP19 lasted 239 days and covered approximately 8.5 million km2 at its peak (Fig. 4, 5; archived images can be found here); it officially fell below our heat wave classification thresholds, and ended in terms of its surface expression, on January 17, 2020.

To further investigate past MHW events, access this table of detailed information (i.e., size, duration, distance from shore) or yearly animations. 

What are the latest conditions?

(last updated 2/10/2021)

Marine heatwave NEP20b, which reached a maximum size of ~9.1 million km2 in late September 2020, has steadily decreased in size over the past two months (present area of ~700,000 km2) and moved further offshore (Fig. 1, 2). Given its distance from shore, the majority of the US west coast EEZ (blue dashed outline, Fig. 2, 5) currently has no waters in heatwave status. At its maximum size, NEP20b was the 2nd largest MHW (by a very slight margin) seen in this region since satellite monitoring and analysis began in 1982 (Fig. 4).

NEP20b directly followed NEP20a, which was mainly centered in the northwest of the study region and had temperatures exceeding 2.0 SD above normal. NEP20a reached a maximum size of ~4 million km2 on April 26, 2020, making it the 9th largest MHW over the past 38 years (Fig. 5). After April, however, NEP20a lost strength and eventually split into smaller fragments. NEP20b arose from one of these fragments in early June, growing steadily and re-encompassing the area once occupied by NEP20a in early August, 2020. As discussed above, NEP20b then receded further offshore during Nov-December, 2020. This recession is similar to that seen with NEP19 during October and November, 2019; however, this year's recession of the heatwave is approximately 1 month later than in 2019. An animation of marine heatwave development during 2020 can be viewed here.

The smaller yet intensely warm area we had been tracking within the southern California Bight has now completely dissipated, with waters off southern California returning to near normal temperatures for this time of year (Fig. 1) and leaving mostly cool or climatological water temperatures in this region. We 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.

What is the MHW Tracker?

Developed by oceanographers from NOAA Fisheries’ Southwest Fisheries Science Center as an experimental tool for natural resource managers, the California Current MHW Tracker is a program designed to understand, describe, and provide a historical context for the 2014-16 blob.2 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 1984- 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°.

We found that blob-class MHWs can be classified 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, and duration. The original 2014-16 blob had contiguous patches which lasted more than six months and were >4,500,000 km2 in area. Based on our thresholding criteria, we suggest that the MHWs most likely to cause impacts to the west coast will be roughly 3 x the area of Alaska, come within 250 km of the coast, and last at least three months.

Project leads

Andrew Leising and Steven Bograd (SWFSC)

    1. 1. Hobday, A. J., Alexander, L. V., Perkins, S. E., Smale, D. A., Straub, S. C., Oliver, E. C., ... & Holbrook, N. J. (2016). A hierarchical approach to defining marine heatwaves. Progress in Oceanography, 141, 227-238.

    2. 2. Leising, A., et al. (in review)

    3. 3. Jacox, M. G., Hazen, E. L., Zaba, K. D., Rudnick, D. L., Edwards, C. A., Moore, A. M., & Bograd, S. J. (2016). Impacts of the 2015–2016 El Niño on the California Current System: Early assessment and comparison to past events. Geophysical Research Letters, 43(13), 7072-7080.

    4. 4. Jacox, M. G., Alexander, M. A., Mantua, N. J., Scott, J. D., Hervieux, G., Webb, R. S., & Werner, F. E. (2018). Forcing of multiyear extreme ocean temperatures that impacted California current living marine resources in 2016. Bulletin of the American Meteorological Society, 99(1), S27-S33.

Figure 1

Figure 1: Daily sea surface temperature anomalies (SSTa) in the California Current ecosystem. An animation of daily images through 2020 can be viewed here. SST data from Multi-scale Ultra-high Resolution (MUR) SST Analysis Anomaly (

Figure 2

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. Figure details as in Figure 1 (above). SST data from NOAA's Optimum interpolation Sea Surface Temperature analysis (OISST;, with the SST anomaly calculated using climatology from NOAA's AVHRR-only OISST dataset. NOTE: These data are currently not available.

Figure on Marine heatsave

Figure 3A.(left): The MHW known as "the blob" at its near maximum areal extent in September 2014; 3B.(right) The 2019 MHW (NEP19) at its near maximum areal extent in August 2019. Figure details as in Figure 1.


Figure 4 area vs time

Figure 4: Retrospective analysis of sea surface temperature anomalies in the California Current region, 1982-2020. Figure shows the relatively higher strength, size, and duration of MHW events during 2014-2016 time period, as well as the most recent 2019 MHW. Horizontal line represents the area threshold for finding features likely to impact the coastal region; colors indicate relative size of simultaneously tracked MHWs, with Black = largest, Magenta = 2nd largest, and Green = 3rd largest.

Figure 5 six month area and intensity

Figure 5 (NEW version, February 2021): 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 6 months. 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.


California Current