SUPER-MARINE HEATWAVES? A New Term For A Growing Concern... (2024)

2023 was a record-breaking year. According to theAnnual 2023 Global Climate Report, 2023 was the warmest year since global atmospheric temperature records began in 1850, at 1.18°C (2.12°F) above the 20th-century average of 13.9°C (57.0°F). Now, a new analysis led by NCEI scientists published inGeophysical Research Letters (GRL)shows that global mean sea surface temperatures also set a record high in 2023. The paper further indicates that these record-breaking temperatures were associated with extremely strong marine heatwaves that the authors dubbed super-marine heatwaves.

Decades of Data

Sea surface temperature (SST) is the temperature of the uppermost layer of seawater, which typically means the top few meters. SST data are collected by satellite observations or by in situ observations from ships, drifting buoys, moored buoys, and more recently, Surface Autonomous Vehicles such as Saildrones. Monitoring changes in SST is essential for understanding and mitigating the impacts of climate change and other environmental stressors.

NCEI’s Daily Optimum Interpolation Sea Surface Temperature (DOISST) is a global daily SST product with a resolution of 0.25° × 0.25° that includes data from September 1981 onwards and blends satellite SSTs with in situ SSTs from ships, buoys, and Argo floats. The SST analysis based on DOISST shows that globally (90°S–90°N) averaged daily SSTs started breaking historical seasonal record highs in March 2023 and continued to do so until early summer 2024. An all-time record high was set on April 4, 2023 (18.83°C), and was continually broken from July 16 until August 21, 2023 (19.00°C). The analysis validates that the daily global records shown by DOISST are consistent across different datasets as well as across analyses over different time periods. The SSTs in 2023 were record high over much of the global oceans.

The Daily Optimum Interpolation Sea Surface Temperature (DOISST) dataset has long been used by the climate community for monitoring, research, and assessment. The chart above is a snapshot of the real-time monitoring tool created by ClimateReanalyzer.org, showing the monthly SST lines from 1981–July 2024 for [60°S–60°N]. The orange line shows the record-breaking 2023 SSTs. The bold black line shows the temperatures measured in 2024. The center black dashed line is the historical climatological mean (1982–2011), enveloped with the two-times standard deviations. Credit: NOAA NCEI/Climate Reanalyzer.

The new GRL paper shows that the 2023 record-high SST anomaly is attributed to a long-term warming trend associated with increasing greenhouse gases, a shift to the warm phase of a multidecadal Pacific-Atlantic-Arctic (PAA) mode, and a warming associated with the transition from 2020–23 La Niña events to the 2023–24 El Niño event.

“The impact of the first two factors suggests high SSTs into the future, particularly in the Northern Hemisphere summer,” said Boyin Huang, NCEI Oceanographer and lead author of the paper.

Super-Marine Heatwaves

The paper also indicates that these record-breaking SSTs were extremely strong marine heatwaves (MHWs). The MHWs were so powerful that the authors came up with a new term for them: super-marine heatwaves.

MHWs usually describe any time when the ocean temperature is above the 90th percentile for five or more days in a row, with gaps of less than three days. This means that the temperatures are warmer than 90% of the previous observations for a given time of year. In 2023, MHWs were so extreme that the paper’s authors coined the term “super-MHW” to describe when the daily SST in 2023 exceeded the corresponding maximum daily SST from the period of 1982–2022. These super-MHWs occurred across the globe, including in the Arctic region.

Record-high SSTAs in August 2023 expressed in (a) average SSTAs (°C), (b) conventional MHW intensity index (degree-day), and (c) super-MHW intensity (°C). Credit:https://doi.org/10.1029/2024GL108369

Marine heatwaves can have significant impacts on ocean life includingfisheries collapseandcoral deaths. They can occur all over the world, including in the Arctic zone. The authors of the paper documented super-MHWs occurring in the central tropical Pacific near the dateline (0.1°−0.2°C), the eastern tropical Pacific east of 120°W (0.1°−0.5°C), the North Pacific north of 30°N (0.2°−1.0°C), the northern tropical Atlantic between the equator and 30°N (0.2°−0.5°C), the northwestern North Atlantic south of Greenland (0.5°C), the Arctic coastal zone (0.5°−2.0°), the western Indian Ocean west of 90°E (0.2°−1.0°C), and the Southern Ocean (0.2°−1.0°C). MHWs affect the biology and chemistry of an ecosystem.

Impacts of Record-Breaking Ocean Surface Temperatures

There are many impacts to the Earth’s climate systems associated with the record-breaking ocean surface temperature, one of them being the increased strength and rapid intensification of hurricanes, as warm waters provide fuel to the storms. Corresponding with the record-breaking ocean surface temperature in 2024 was the earliest Category 5 hurricane on record; Hurricane Beryl intensified in a very short time from a tropical storm to a Category 5 hurricane.

Hurricane Beryl (June 28–July 11, 2024) was the earliest-forming Category 5 hurricane on record. Beryl’s development was fueled by record-breaking ocean surface temperatures. Credit: NOAA NESDIS

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Reference: Huang, B., X. Yin, J. A. Carton, L. Chen, G. Graham, P. Hogan, T. Smith, and H.-M. Zhang, 2024: Record high sea surface temperatures in 2023. Geophysical Research Letters.https://doi.org/10.1029/2024GL108369