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 (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 09/07/2021)

We are currently tracking large marine heatwave NEP21A (Fig. 2) which began in late April 2021 in the same region where NEP20b declined (see description, next paragraph below). As of June 4, 2021, NEP21A had reached an area of ~2,400,000 km2 (Fig. 2, 4b), nearly tripling in area over the three prior weeks, and advancing ~100 km closer to the US west coast. During mid-June, this warm water feature began to break into smaller fragments, then re-formed during late June 2021, continued to expand, and reached the Canadian coastline. A lack of strong upwelling winds also led to anomalously warm temperatures along much of the US west coast during June, conditions which are separate from the effects of the MHW. Upwelling winds, leading to cooler coastal temperatures, then resumed by mid-July 2021, during which NEP21A receded from the coast. However, NEP21A has remained fairly strong in offshore waters (Fig. 2) and reached an area of approximately 4,100,00 km2 by late August, placing it within the top 10 marine heatwaves in terms of area since monitoring began in 1982. Waters in the southern California Bight remain warmer than normal but are considered a separate feature from the main NEP21A MHW. If the course of this current MHW follows those of the past two years, we might expect to see further incursions of NEP21A into the coastal regions of Oregon and Washington sometime within the next month as the seasonal cycle of upwelling transitions to a more downwelling mode. 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.

Marine heatwave NEP20b, which began in May 2020 and reached a maximum size of ~9.1 million km2 in late September 2020, steadily decreased in size during the winter and spring, continued a displacement further offshore, and on April 4, 2021 fell below the area threshold (400,000 km2) for classification as a large marine heatwave (Fig. 4b). At its maximum size, NEP20b was the 2nd largest MHW (by a slight margin) seen in this region since satellite monitoring and analysis began in 1982 (Fig. 4). It is also now one of the longest lasting heatwaves on record, having lasted 309 days. View an animation of marine heatwave development during 2020.

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?

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. 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. 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.

Maximum MHW extent, 2014 vs 2020

Figure 3A.(left): The MHW known as "the blob" at its near maximum areal extent in September 2014; 3B.(right) The 2020 MHW (NEP20b) at its near maximum areal extent in September 2020. Figure details as in Figure 2.


Figure 4 area vs time, 1982 to present

Figure 4 area vs time, last 12 months

Figure 4A: Retrospective analysis of sea surface temperature anomalies in the California Current region, 1982-present. Figure shows the 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 MHWs. Color indicates the % of the US west coast EEZ (area within blue dashed line, Fig. 2) in heatwave status. 4B: 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 one year area and intensity

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 MHW indicator outputs.

California Current