Marine Heatwaves

Overview

Marine heatwaves (MHWs) are periods of extreme warm ocean temperature that persist for days to months, extend over thousands of kilometers, and penetrate multiple hundreds of meters into the deep ocean. Over the past two decades, MHWs have adversely affected marine organisms and ecosystems in all ocean basins, including critical foundation species like corals, seagrasses, and kelp. These events have led to widespread coral bleaching and mortality, loss of seagrass and kelp forests, shifts in species ranges, and the local—and potentially global—extinctions of coral species. MHWs have also been associated with mass mortality events among marine invertebrates and can force species to alter their behavior, increasing their vulnerability to harm. For instance, MHWs have been linked to a rise in whale entanglements in fishing gear. Additionally, the changing ocean conditions can facilitate the spread of invasive alien species, which can have devastating effects on marine foodwebs. A notable example is the MHW that developed in the northeast Pacific in 2014, which disrupted the entire foodweb and led to mass die-offs of seabirds and marine mammals. For more information on the impacts on ecosystems and biodiversity, please visit the Biodiversity Loss page on the CRL wiki.

MHWs also have significant socio-economic impacts, leading to substantial economic losses in sectors such as tourism, fisheries, and aquaculture (Figure 1). These events have been shown to reduce the productivity or cause mortality of economically important species, such as lobster and snow crab in the northwest Atlantic and scallops off Western Australia.

Marine heatwaves have become an urgent climate risk due to their increasing frequency, duration, intensity, and spatial extent. The number of MHW days doubled between 1982 and 2016, with these events becoming more prolonged, intense, and widespread—8 of the 10 most severe recorded MHWs have occurred in the past decade. Projections indicate that MHWs will continue to intensify with global warming. Climate models suggest that by 2081-2100, the frequency of MHWs could increase by approximately 50 times under the high-emission scenario (RCP8.5) and 20 times under the low-emission scenario (RCP2.6) relative to 1850–1900. The intensity of MHWs is projected to increase about tenfold under RCP8.5 by 2081–2100 compared to the 1850–1900 baseline.

Data

How to measure marine heatwaves

Hobday et al. (2016) developed a widely adopted definition of marine heatwaves (MHWs). According to this definition, an MHW is a period during which seawater temperatures exceed a seasonally varying threshold (typically the 90th percentile) for at least five consecutive days. Successive events with gaps of two days or less are considered part of the same MHW. Here are links to code implementations of the MHW definition by Hobday et al. (2016) in Python and R.

As a general guideline, an anomaly of 1 degree Celsius (roughly 2 degrees Fahrenheit) off the coast of California can indicate a marine heatwave, while anomalies of 2-3 degrees Celsius (approximately 4-6 degrees Fahrenheit) are indicative of more extreme marine heatwave events.

Relevant datasets to calculate marine heatwaves

• NOAA OI SST from 1982, a gridded high-resolution (0.25 degree) daily global sea surface temperature dataset from NOAA that continues to present.

• NOAA ERSST V5 from 1865, a gridded (2.0 degree) consistently analyzed monthly global sea surface temperature dataset from NOAA that continues to present.

Current observations:

• Marine heatwave tracker by Marine Heatwave International Working Group
• Marine heatwave observations by NOAA

Past California current marine heatwave events

• Detailed information (i.e., size, duration, distance from shore) of past marine heatwave events
• Animations of yearly sea surface temperature anoamlies

Forecasts:

• Forecasts up to 11.5 months ahead by Jacox et al. (2002)