Does an Unusually Warm Reading Mean a Marine Heatwave?

The research definition combines a threshold with persistence

Hobday and colleagues proposed a widely used quantitative definition in 2016: temperatures above a seasonally varying 90th-percentile threshold for at least five days, referenced to a historical climatology, with 30 years recommended for the baseline. The threshold follows the time of year. A winter event can therefore qualify at an absolute temperature that would be ordinary in summer. The question is how exceptional the temperature is locally and seasonally, rather than whether it exceeds one national cutoff. The original marine heatwave definition explains that framework.

Persistence makes this an event measure rather than a label for a hot instant. A brief afternoon rise and a sustained warm episode can share a peak temperature while having different durations. A complete analysis must identify the daily series, threshold calculation, and event rules before comparing events. A chart containing many observations is not sufficient unless those observations have actually been evaluated using that method.

What this site's seasonal flag actually tests

Water Temperature Live targets the five most recent complete calendar years of hourly observations from the same station. For publication, each station-month-year must contain at least 70% of its expected hourly values, and a calendar month needs at least three qualifying years. The displayed 10th–90th percentile band pools qualifying hourly readings for that UTC calendar month. Months without enough coverage remain marked as insufficient data. These are the rules in our seasonal baseline methodology.

A fresh reading above that month's upper percentile receives an above-usual-range notice. That notice does not test a five-day run, does not use a day-of-year threshold, and does not use a 30-year climatology. It also does not reject the sensor reading as invalid. A temperature can be unusual compared with recent history and still pass the source's quality checks. Treat the flag as a prompt to examine the record, not a declaration that an event has been classified.

Operational products can implement different climatologies and event rules. Read the method attached to the particular map or dataset.

Changing the baseline can change the answer

A percentile is a rank within a specified reference distribution. Moving to different reference years, smoothing the seasonal cycle differently, or changing which days are available can shift the threshold. Research by Schlegel and colleagues tested how record length, missing observations, and trends affect heatwave detection. Their results support reporting these choices explicitly instead of treating all datasets as interchangeable. Their primary study on sub-optimal data evaluates those sources of uncertainty.

Operational products also deserve their own method check. NOAA Coral Reef Watch's Marine Heatwave Watch describes a daily threshold built from a 1985–2012 reference period and an 11-day window around each calendar day. The watch assigns a category to each day above its threshold; that daily classification does not itself establish the five-day persistence required by Hobday’s event definition. Its published implementation illustrates why a product's name alone is not enough to reconstruct its calculation. Comparing that map with our monthly station flag requires acknowledging both the reference-period difference and the difference in spatial sampling. NOAA publishes the product's climatology and algorithm.

A station and a regional event answer different questions

NOAA's National Marine Ecosystem Status tracks both the spatial coverage of heatwaves and an intensity measure accumulated through time. Those indicators are calculated from a gridded sea-surface dataset. They address the extent and persistence of warming across marine ecosystems, while a harbor or pier station describes a particular instrument location. Regional and local evidence can complement one another without being identical. NOAA explains its ecosystem heatwave indicators.

For a warm reading on this site, begin with its timestamp and quality status, then inspect the recent history and available seasonal coverage. For a regional heatwave question, consult a documented daily monitoring product and retain its date, baseline, and area definition. An absent local seasonal flag is not evidence that a regional event cannot exist; the local baseline might simply be unavailable. Equally, one flagged instrument cannot establish the dimensions or biological consequences of a regional event.