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. 2), 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?

Summer 2019 Report (last Updated 9/12/2019)

We are now tracking a large MHW (Fig. 1) that has formed in the northeast Pacific Ocean, in an area reminiscent of the 2014–16 blob. This new marine heatwave is currently referred to as the Northeast Pacific Marine Heatwave of 2019, or NEP19. Like the blob, NEP19 emerged over the past few months as a ridge of high pressure dampened the winds that otherwise mix and cool the ocean’s surface.

The area covered by this feature stretches from Alaska to California and ranks as the second-largest MHW in the northern Pacific Ocean in the last 40 years (Fig. 3). Sea surface temperatures are as much as 3°C above average.

Coastal upwelling of deep, cold water has kept the warm expanse mostly offshore, although it appears to have come ashore in Washington and could do so in other areas as upwelling wanes in the fall. If atmospheric conditions persist and NEP19 moves into nearshore coastal waters, it is likely to have impacts on coastal ecosystems this fall.

We are continuing to monitor the area, duration, and coastal proximity of this MHW and are coordinating 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 (>1.65 times the standard deviation of the SSTa field), along with their areal extent, and duration. The original 2014-16 blob had contiguous patches which lasted more than six months and were >700,000 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 25% 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 preparation)

    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: The most recent standardized sea surface temperature anomalies (standardized SSTa = SSTa divided by SSTa STDEV) in the California Current ecosystem. The image shows the current extent of the Northeast Pacific Marine Heatwave of 2019, or NEP19, off the US West Coast and Gulf of Alaska.

Figure 2 heatmap

Figure 2: The large marine heatwave known as "the blob" at its near maximum areal extent in March 2015. 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 >1.65x the local SD and the small circle denotes the centroid of the feature.


Figure 3 heatmap

Figure 3: 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.


California Current