What Is the Hottest Seawater Ever Recorded? Kuwait Bay

The hottest seawater ever reliably measured hit 37.6°C (99.7°F) in Kuwait Bay in the Persian Gulf in July 2020, logged by a Kuwait Institute for Scientific Research station during a heatwave. A Florida buoy reached 38.4°C (101.1°F) in July 2023, but scientists question that reading, so the Gulf record stands.

• In July 2020, the offshore KISR01 station in the middle of Kuwait Bay recorded 37.6°C (99.7°F), the highest sea surface temperature verified by in situ measurement, published in a 2020 peer-reviewed study. (Marine Pollution Bulletin (ScienceDirect))
• The event combined a heatwave, neap tides, and an extended spell of humid Kous winds, and it coincided with large-scale fish kills along the full length of Kuwait's coastline and at Iraq's Shatt Al Arab. (Marine Pollution Bulletin (ScienceDirect))
• The Arabian/Persian Gulf is recognized as one of the warmest estuaries on the planet, and the UK's National Oceanography Centre calls it the hottest sea on Earth. (National Oceanography Centre (via Offshore Energy))
• The Gulf averages only about 35 m (115 ft) deep; high salinity traps heat near the surface, and limited exchange through the Strait of Hormuz keeps the heat in. (Curious Expeditions)
• In July 2023 a buoy at Manatee Bay, Florida, logged 38.4°C (101.1°F), but scientists question the reading because the water was shallow and murky and may have been warmed by nearby land. (IFLScience)

Where was the hottest seawater ever recorded?

The verified record belongs to Kuwait Bay, at the northwestern corner of the Persian Gulf. From 2016 to 2020, researchers ran detailed in situ sea surface temperature measurements across the bay, and the data peaked in July 2020 at the offshore KISR01 station, moored in the middle of the bay. The reading, 37.6°C (99.7°F), was higher than anything previously recorded in the Gulf, and the authors reported it in Marine Pollution Bulletin as a world record extreme, verified by direct measurement rather than satellite estimates.

The conditions behind it were a pile-up of extremes. A heatwave sat over the region, tides were at their weakest (neap tides, which reduce mixing), and an extended period of Kous winds brought punishing humidity. Together they trapped heat in the bay's shallow water until instruments registered the record. The ecological signal was immediate: large-scale fish kills struck intermittently along the entire Kuwaiti coastline, and dead fish from many different species also washed up along Iraq's Shatt Al Arab waterway at the Gulf's northern edge. The paper's authors framed the event as possible evidence of global warming's effects and a call for urgent environmental policy.

Why does the Persian Gulf get so hot?

The Gulf is built to hold heat. It averages only about 35 m (115 ft) deep, so the sun warms a thin slab of water instead of a deep column. It is semi-enclosed, trading water with the Indian Ocean only through the narrow Strait of Hormuz, which slows turnover and lets heat accumulate through the summer. The surrounding land is arid desert with minimal cloud cover, so solar radiation pours in, and high evaporation leaves the water very salty. Saltier water is denser and mixes less with the cooler layers below, which pins the warmth at the surface. The result is routine extremes. Surface temperatures regularly pass 32°C (90°F) in summer across the Gulf, and the National Oceanography Centre notes that its corals endure summers above 34°C (93°F) before cooling to about 16°C (61°F) in winter, a 20°C (36°F) seasonal swing. Most coral reefs on Earth live within a 6°C (11°F) annual range, which shows how far outside the norm the Gulf runs. For comparison on the heat side of the planet, the Lut Desert holds the hottest land-surface measurement ever made, and Death Valley is the hottest place on Earth by air temperature.

What about the 101°F reading in Florida?

In July 2023, a buoy at Manatee Bay, just north of the Florida Keys, logged 37.9°C (100.2°F) on a Sunday and 38.4°C (101.1°F) on Monday evening, two triple-digit days in a row. Had the reading held, it would have been the hottest sea surface temperature ever seen, topping the Kuwait Bay record set three years earlier. But scientists immediately flagged problems. The buoy sat in the shallow, murky waters of the Everglades, not the open ocean, where warm land, a thin water column, and dense seagrass can all push a sensor's reading above the true sea temperature. The U.S. National Weather Service meteorologist who reported the numbers and outside experts alike said the record status was unclear, and the readings were still awaiting verification when IFLScience covered the story. Until that validation happens, the Kuwait Bay measurement, taken offshore in open bay water and published with full methodology, remains the record that experts cite.

What does water this hot do to marine life?

The Gulf's own wildlife shows the cost. The July 2020 heatwave did not just warm the water; it killed fish on a coastline-wide scale, with species dying that rarely appear in the Gulf's routine summer fish kills. Corals bleach when heat expels the symbiotic algae living in their tissues, and the 2020 event added to a pattern of bleaching across the region's reefs. Florida's 2023 heat showed the same damage at even higher temperatures. Teams from the Coral Restoration Foundation visiting Sombrero Reef, a restoration site they had worked for over a decade, found 100 percent coral mortality, and the Looe Key nursery lost nearly all its corals as well. Warm water also feeds hurricanes: tropical systems are more likely to form and intensify over hot upper-ocean water, so record sea temperatures raise the stakes for storm season as well. The Gulf's corals, which already live in warmer water than most reefs are predicted to face by the end of this century, are now studied worldwide as a preview of what extreme heat does to reef ecosystems. Dallol's extreme landscape and the Dead Sea's harsh chemistry show how life adapts at the edges, and Gulf corals belong in that company.

What do people get wrong about ocean heat records?

The most common mistake is treating the Florida reading as the new record. It is hotter on paper, 38.4°C against 37.6°C, but a record needs a validated measurement, and the Manatee Bay buoy's shallow, murky setting leaves its number in doubt. The Kuwait Bay figure was measured offshore, repeated across five years of monitoring, and published in a peer-reviewed journal, which is why experts still point to it. Another mistake is assuming the hottest seawater must come from the tropics' open ocean. It comes from a shallow, semi-enclosed gulf where geography concentrates the sun's energy. And a third is thinking that because Gulf corals survive 34°C water, reefs everywhere can take the heat. Gulf corals are the exception: they endure a 20°C annual swing that would kill most reef systems, and even they are showing stress as the Gulf heats up faster than the world average.

Reading	Temperature	When	Status
Kuwait Bay, Persian Gulf (KISR01 station)	37.6°C (99.7°F)	July 2020	Verified record, peer-reviewed
Manatee Bay, Florida (buoy)	38.4°C (101.1°F)	July 2023	Unverified, disputed
Typical Persian Gulf summer surface	above 32°C (90°F)	every summer	Routine
Persian Gulf winter surface	about 16°C (61°F)	every winter	Routine
The hottest seawater ever reliably recorded is 37.6°C, measured in Kuwait Bay during a brutal July 2020 heatwave. The Persian Gulf's shallow, salty, sun-baked basin makes it the hottest sea on the planet, and its record stands until a challenger can prove its reading the hard way: offshore, calibrated, and verified.
How we made this article: FactSpire researched and drafted it with AI assistance and checked every fact against the sources listed above.