A widely used definition identifies temperatures above the local 90th percentile for at least five consecutive days. Because the threshold is location- and season-specific, a heatwave is an anomaly—not one universal temperature.
Three things to know
- Common threshold
- >90th percentile
- Compared with the historical seasonal distribution at that location.
- Duration
- 5+ days
- Events can persist for weeks, months or longer.
- Drivers
- Air + currents + mixing
- Atmospheric blocking, weak winds and ocean transport can combine.
An ocean weather extreme
Marine heatwaves are identified relative to what is normally expected at the place and time of year. Scientists track their intensity, duration, extent and cumulative heat. Satellite data reveal surface patterns, while buoys and profilers show depth.
What the heat can change
Heat can trigger coral bleaching, kelp loss, disease, mass mortality and range shifts. Mobile species may move, while attached organisms cannot. Effects continue after temperatures fall if habitat, reproduction or food webs were disrupted.
Background warming loads the dice
Natural climate variability helps initiate many events, but a warmer baseline makes extreme temperatures more likely and can increase their severity. Attribution studies separate the role of long-term climate change from the weather and circulation that shape a specific event.
What people usually ask
Is a marine heatwave just warm water?
It is warmth that is statistically unusual for that location and season and that persists beyond a defined duration.
Do heatwaves reach the deep ocean?
Yes. Some are surface-confined, while others extend hundreds of meters or occur below a less dramatic surface signal.
Here’s where the answer came from
Open the original government science or research-institution source behind this page.