# Marine heatwaves can run much hotter by day than standard ocean monitoring shows

> A Communications Earth & Environment study led by researchers at Shanghai Jiao Tong University found that marine heatwaves often produce stronger daytime surface extremes than standard monitoring captures. The team reports that the daytime subskin sea surface temperature peak is typically 20...

Canonical URL: https://www.argo.net/marine-heatwaves-can-run-much-hotter-by-day-than-standard-ocean-monitoring-shows/
Byline: Shanghai Jiao Tong University
Published: 2026-08-01T11:40:02+00:00
Categories: News, Oceans

![Earth and its oceans viewed from space](https://www.argo.net/wp-content/uploads/2026/08/ocean_temperature_satellite.jpg)

A [Communications Earth & Environment study](https://www.nature.com/articles/s43247-026-03841-0) led by researchers at **Shanghai Jiao Tong University** found that **marine heatwaves** often produce stronger daytime surface extremes than standard monitoring captures. The team reports that the daytime **subskin sea surface temperature** peak is typically 20 to 30% higher during these events, which can push the warmest part of the day another 0.5 to 1.0 degrees Celsius above estimates built from foundation temperatures.

Many organisms and many exchanges between the ocean and the atmosphere happen in the upper few meters, where the water can warm fast under calm, sunny conditions. The study argues that routine tracking based on **foundation temperature**, a night or pre-dawn reference designed to minimize the daily cycle, can miss some of the heat near-surface ecosystems actually experience. Coral reefs are one obvious example and plankton in the top layer also live inside that warmer band. The same blind spot affects evaporation and gas exchange. It also carries into evening atmospheric feedbacks that respond to the water people and ecosystems actually touch at the surface.

## What the new study found

The researchers combined hourly subskin temperatures from the [OSTIA](https://ghrsst-pp.metoffice.gov.uk/ostia-website/index.html) system with day and night satellite snapshots. They then compared those observations with atmospheric reanalysis and with an ocean model that resolves sub-daily warming. Across most tropical and subtropical oceans, the daily temperature swing became larger during marine heatwaves instead of simply shifting upward as a block. That means the warmest hours accelerated faster than the daily mean did.

On average, the paper says the global diurnal cycle strengthened by about 20% and roughly 69% of the ocean area showed amplified daytime warming during marine heatwave conditions. The clearest hot spots appeared in the Pacific warm pool and the equatorial Indian Ocean. The pattern also stood out in semi-enclosed seas and in some coastal regions, where strong sunlight and weak winds often favor shallow daytime warm layers. In the tropics and subtropics, the mean diurnal amplitude under heatwave conditions reached about 0.17 degrees Celsius, with maxima above 0.3 degrees Celsius in the strongest regions.

The extra heat was not spread evenly through the water column. In the model results, warming above a foundation temperature near 10 meters was amplified by about 20% at 1 meter, 15% at 3 meters and 10% at 6 meters. That pattern fits the paper's core point: the strongest hidden heat sits near the surface, then weakens with depth. The authors say the signal still reaches into the upper **5 to 10 meters**, which is shallow on an ocean scale but still overlaps the habitat used by many surface-dwelling organisms.

## Why the hottest water stays near the top

The mechanism is fairly direct. Under marine heatwave conditions, winds often weaken while sunlight reaching the sea surface increases. A shallower **mixed layer** then traps more solar heat near the top of the ocean instead of stirring it downward quickly. Once that upper layer becomes more stable, the same weather pattern can keep reinforcing the daytime warming from one afternoon into the next.

The authors describe that as a thermodynamic feedback. Weak winds reduce turbulent mixing and also reduce latent heat loss, while clearer skies increase shortwave heating. In their analysis, those conditions let heat accumulate in the upper 5 to 10 meters and linger into the evening, which delays the normal timing of the daily temperature peak. The paper links that delayed phase to stronger afternoon and early evening extremes rather than only a brighter midday spike.

Standard climate records usually rely on a foundation temperature because it is more stable over time and better suited for long baselines. The paper does not argue that foundation records are wrong. It shows that they answer a different question than a daytime ecological exposure estimate does, especially when the top layer is warming faster than the water below.

## How the team measured the hidden heat

The study followed the widely used [Hobday marine heatwave definition](https://www.marineheatwaves.org/mhw-overview.html), which classifies an event when daily sea surface temperature stays above a seasonally varying 90th percentile threshold for at least five consecutive days. To keep that threshold stable, the authors built their baseline from NOAA's [OISST](https://www.ncei.noaa.gov/products/optimum-interpolation-sst) record over 1982 to 2011, then compared higher-frequency temperatures against that reference. They used an 11-day moving window around each calendar day and smoothed the result with a 31-day running mean, which is a standard way to avoid noisy threshold jumps.

Several data streams were used for different jobs. OSTIA supplied hourly subskin and skin temperatures from 2015 to 2024. [ERA5](https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5) supplied hourly atmospheric context from 2004 to 2024. MODIS offered four daily overpasses and the **MOM6 ocean model** provided 3-hourly simulations. Because MODIS has data gaps, the team did not detect marine heatwaves directly from MODIS. Instead, they identified heatwave dates from OISST and used those dates to composite the MODIS diurnal signal. The paper also reports strong large-scale agreement between the daily OSTIA and OISST fields and somewhat looser but still good agreement between MODIS and OISST.

The paper also checked whether its threshold choice changed the answer. Using a newer OSTIA-only climatology produced similar spatial patterns but lower marine heatwave intensity estimates by about 0.2 degrees Celsius, because the more recent baseline already includes warmer background conditions. The authors therefore kept the older OISST climatology so the comparison would use a cooler and more stable long-term reference.

The statistical testing was also explicit. Regional differences are reported with 95% confidence intervals. The authors tested grid-point contrasts between heatwave and non-heatwave conditions with a two-sided Student's t-test. They checked slope changes in the relationship between subskin and foundation anomalies with an analysis of covariance model. Those steps do not remove every uncertainty in a global synthesis, but they do show that the paper treated the amplification pattern as a measurable signal rather than a visual impression from a few maps.

## Where standard monitoring misses the most heat

The biggest practical mismatch showed up in low latitudes. Across tropical regions, 20 to 40% of foundation-defined marine heatwave days had subskin temperature anomalies that rose above the event's peak foundation intensity, with some tropical Pacific and Indian Ocean areas topping 40%. In other words, a large share of already hot days became even hotter near the surface than the standard event peak suggested. Regions at higher latitude and many upwelling zones usually stayed below 10%, so the hidden daytime signal is not distributed evenly around the world.

The intensity boost was strongest for daytime peaks. The authors report a global mean increase of about 28% in maximum intensity when subskin temperatures are used, while the global increase in cumulative intensity was much smaller, about 2.2%. That smaller average does not erase the biological risk, though, because short-lived peaks can still cross thermal limits even when the whole event does not become much longer. The abstract also notes that the revised subskin view can add tens of degree-days to cumulative exposure during events, which helps explain why brief daytime spikes can still raise ecological heat exposure.

The study also notes that some regions behave differently. The eastern equatorial Pacific showed a reduced diurnal cycle during marine heatwave conditions, likely because many of those events coincide with **El Niño** patterns that change cloud cover, alter winds and modify mixing. For monitoring agencies, the practical implication is straightforward: foundation temperature remains useful for stable climate tracking, but it should be paired with better-resolved daytime diagnostics when managers want to estimate surface ecological stress or air-sea heat exchange. The paper is also still listed by Nature as an unedited article in press, so small wording changes may appear before the final version. Even so, the central result is clear: monitoring only the stable reference layer can understate the daytime stress living near the ocean surface.