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.
Large marine heatwaves have occurred each of the last three years (2019, 2020, and 2021), 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 heatwaves were the 3rd, 2nd, and 7th 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). The most recent event, marine heatwave NEP21A, which began in April 2021 and reached a maximum size of approximately 4.5 million km2 in early September 2021, fell below the area threshold (400,000 km2) for large marine heatwave classification on Dec 22, 2021 (Fig. 4B). NEP21A lasted 236 days, qualifying it as the 6th longest heatwave on record. View an animation of marine heatwave development during 2020.
What are the latest conditions?
(last updated 10/17/2022)
We continue to track large marine heatwave NEP22A that formed in late January 2022. During recent weeks, this heatwave has expanded to encompass nearly 8 million km^2 (Fig. 1 and Fig 2) and has lasted over 250 days, making this the 4th largest and 4th longest marine heatwave in this region since monitoring began in 1982. After only a brief period of upwelling in late July, this heatwave or its offshoots have resided fairly close to shore for most of August-October 2022 and encompassed much of the coastline, although small patches of cool water remain in various locations and times due to local upwelling. For the past several weeks, more than 50% of the US west coast EEZ has been in heatwave status (Figure. 4). Also notable is the increase in heatwave intensity (how much warmer the water is compared to normal) over the past several weeks in the far offshore region of the northern Northeast Pacific (Fig. 1 and 2). This indicates that NEP22A may still be gathering strength, rather than dissipating, as most heatwaves do in the fall.
A brief summary of the history of 2022 heatwaves: NEP22A developed on Jan 18th, 2022 when it first exceeded our size threshold (>400,000 km^2 in area). By mid-February 2022, NEP22A had already reached an area of ~3 million km^2 (*Note: actual sizes may be larger than reported here as we are only considering the region bounded in Figure 2 for all presented analyses). Similar to previous years, this heatwave arose in the general offshore region where the previous year’s heatwave (NEP21A) terminated, and within 30 days of the end of that previous event. Thus, it is likely that the region may have had some residual warm waters which preconditioned the region for this new heatwave development.
NEP22A remained mostly outside of the US EEZ (blue dashed line in Fig. 2) until Mid-May 2022, when there were major reversals in the typical southern pattern of winds which drive upwelling along the coast (view these winds here), most notably during May 5-19, May 22-28, and June 3-11. During these periods, NEP22A reached large portions of the central US west coast (regions 3 and 4 in Fig. 5), and up to 50% of some regions were in heatwave status during parts of May and June. These conditions are similar to, but of a larger scale than, what happened in 2021 when an "upwelling relaxation" event allowed an offshore heatwave (NEP21A) to impact the coast for a week in mid-June 2021. During late June and July, NEP22A also split into two fairly large, but slightly separated fragments, one to the north and one to the south. Since that time, these features have merged and split several times, likely dependent on the large-scale wind patterns bisecting this area from west to east as driven by the large-scale atmospheric pressure fields.
We have also been tracking substantial warming off the coast of southern California, within the Southern California Bight (SCB). Conditions within the SCB and to the south along the coast remain warmer than normal, and have exceeded the heatwave threshold for most of the summer and now fall. Several times since the beginning of August, warming in the SCB has been contiguous with NEP22B, although the warming began much earlier than the large-scale intrusions of either MHW into the coastal region. 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?
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.
Andrew Leising and Steven Bograd (SWFSC)
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 (https://coastwatch.pfeg.noaa.gov/erddap/griddap/jplMURSST41anom1day.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. SST 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 2021 MHW (NEP21) at its near maximum areal extent in September 2021. Figure details as in Figure 2.
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: 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.