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. 3), that 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.

What are the latest conditions?

(last updated 12/01/2019)

We continue to track the marine heatwave known as the Northeast Pacific Marine Heatwave of 2019, or NEP19. Our latest images (Fig. 1,2) show the marine heatwave has decreased in slightly in size over the past few weeks, especially as compared to its maximum size in August 2019 (archived images can be found here). The feature has moved farther offshore, however, and surface temperatures have cooled across much of the nearshore area for the entire US west coast (Fig. 1).

Even with this decrease in size and intensity, the current marine heatwave, which formed in May 2019, is still among the most significant events seen in the northern Pacific Ocean during the last 40 years (Fig. 4,5). According to the latest imagery, NEP19 is approximately 195 days in age and 5 million km2 in size, with continued sea surface temperatures as much as 3°C above average.

Conditions will become increasingly dynamic as winter storms form in the Northeast Pacific. We continue to monitor the area, duration, and coastal proximity of this marine heatwave and coordinate 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: Provisional daily interpolated standardized sea surface temperature anomalies (SSTa) in the California Current ecosystem. Dark outline shows the current extent of the Northeast Pacific Marine Heatwave of 2019, or NEP19, off the US West Coast and Gulf of Alaska, as delineated by values of the normalized SSTa > 1.29 standard deviations from normal. SST data from NOAA's Coral Reef Watch program (, with the SST anomaly calculated using climatology from NOAA's OISST dataset. Note: this image is considered provisional and is subject to change, as it uses a different SST data source than the complete MHW reanalysis (see Figure 2).

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.

Figure 3 heatmap

Figure 3: The large marine heatwave known as "the blob" at its near maximum areal extent in September 2014. Color represents the SSTa standardized by the standard deviation (SD) at that location calculated at each pixel over 1984-2016. Dark outlines denote the area where the standardized SSTa is >1.29 times the local SD and the small circle denotes the centroid of the feature.


Figure 4 area vs time

Figure 4: Retrospective analysis of sea surface temperature anomalies in the California Current region, 1982-2019. 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 5a: Retrospective analysis of sea surface temperature anomalies in the California Current region during the last 6 months. Line colors as in Figure 4; however, dotted lines indicate provisional data calculated from the Coral Reef Watch SST product, rather than the OISST data (only available 3-weeks post real-time). Figure 5b. Intensity of the largest MHW over the past 6 months (dotted line based on provisional data, as noted above). Intensity is calculated as the average standardized SSTa over the entire area of a feature; units are multiples of the SSTa SD).


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