Marine heatwave

Marine heatwaves are identified when sea temperatures remain above a threshold for at least five consecutive days, usually the 90th-percentile temperature for that place and time of year. The threshold is therefore relative rather than universal: a temperature that is extreme in a cool region may be ordinary in the tropics. Researchers commonly calculate anomalies from daily sea-surface-temperature records, although subsurface measurements are increasingly included.

Intensity describes how far temperatures exceed the threshold, duration measures persistence, and spatial extent records the affected area. Cumulative intensity combines magnitude and duration. Definitions can differ in baseline period, depth, and statistical treatment, so comparisons between studies require care. Marine heatwaves may occur as isolated episodes, recur seasonally, or form part of a broader ocean-warming trend.

Marine heatwaves arise when the ocean gains more heat than it loses, or when circulation transports unusually warm water into a region. Persistent atmospheric high-pressure systems can suppress clouds and winds, increasing solar heating and reducing evaporative cooling. Weak winds also limit vertical mixing, allowing warm water to accumulate near the surface.

Ocean currents, fronts, eddies, and changes in upwelling can concentrate heat. Large-scale climate variability, including El Niño–Southern Oscillation, can create favorable conditions across entire basins. Human-caused greenhouse-gas warming raises the background temperature, making many marine heatwaves more likely, more intense, and longer-lasting even when natural weather patterns remain the immediate trigger.¹ Some events are reinforced by feedbacks: warmer water increases stratification, which suppresses mixing and traps heat near the surface.

Marine heatwaves can reorganize ecosystems because many organisms tolerate only a limited temperature range. Corals may bleach when heat disrupts the partnership between coral animals and their symbiotic algae; prolonged stress can cause mortality.² Fish and mobile invertebrates may shift poleward or into deeper water, while plankton communities change in composition and timing.

Consequences extend to fisheries, aquaculture, tourism, and coastal protection. A heatwave can produce short-term increases in some warm-water species while reducing commercially valuable cold-water populations. It can also promote harmful algal blooms, marine disease, and mass mortality of seabirds or marine mammals, although outcomes depend on season, habitat, species, and food-web structure. The 2014–2016 northeast Pacific event, known as the Blob, was associated with broad ecological disruptions and fisheries impacts.³

Marine heatwaves are not confined to the sea surface: subsurface events can be hidden from satellite monitoring while affecting corals, kelp, and deep-dwelling organisms. Conversely, a surface heatwave may coexist with unusually cool water below because strong stratification prevents vertical exchange. Coastal geometry and bathymetry can make neighboring areas experience sharply different conditions.

Some events produce ecological surprises rather than simple declines. Species range expansions can create temporary new fisheries, predator–prey relationships may be rearranged, and heat-tolerant individuals can survive where others perish. Repeated heatwaves may leave lasting ecological memory through altered habitats, depleted breeding populations, or changed community composition. Forecasting systems now combine satellites, autonomous floats, ocean models, and climate predictions to provide early warning, but local prediction remains difficult because small-scale currents and habitats are poorly resolved.⁴

Anomaly

The difference between an observed temperature and the expected value for the same place and season.

Marine heatwave

A prolonged period of unusually warm ocean temperatures, commonly defined as at least five consecutive days above a location-specific extreme threshold.

Stratification

Layering in the ocean caused by differences in density, often strengthened by surface warming and associated with reduced vertical mixing.

Coral bleaching

The loss of symbiotic algae or their pigments by corals under stress, especially unusually high temperature.