Mercator Ocean: Marine heatwave bulletin for 4 August 2023

Mercator Ocean International oceanographers examine marine heatwaves across the global ocean and provide forecasts for the second week of August 2023. They analyse a variety of datasets from observation analyses (satellite sea surface temperature maps) to model analyses (assimilating satellite and in situ observations) and model forecasts.^¹

*Figure 1: Map of marine heat wave categories for the 1st of August 2023 for the entire world ocean. GLO12 analysis. Source: Mercator Ocean International*

Figure 2: GLO12 surface temperature anomaly (Mercator Ocean) averaged over the 30 days preceding the 1st of August 2023. The reference climatology consists of daily GLORYS12 maps averaged over the period 1993-2016. Source: Mercator Ocean International.

Assessment of the first week of August

The heatwave in the Mediterranean basin is gradually disappearing, although it is still present in the southern part of the basin: to the west, in the Alboran Sea, the heatwave, which has been going on for over a month, remains strong to extreme locally; further to the centre, in eastern Libya, the level is severe. Models and observations are in agreement on the state of the basin.
In the tropical North Atlantic (15°N to 30°N), the heatwave is strong overall, with locally severe levels now concentrated further west.
The whole of the North Atlantic basin is experiencing a positive surface temperature anomaly, with weekly anomalies of more than 4 degrees persisting to the west of Newfoundland, where a strong to severe heatwave is still raging.
The tropical Pacific is in a moderate heatwave condition reaching high intensity near the coast of Peru, which is consistent with current El Niño conditions.
Marine heat waves in the North Pacific are developing into strong conditions, particularly off Japan and near the north-west coast of the United States.

Forecasts for the 2nd week of August

The forecasts confirm that the heatwave in the Mediterranean basin is easing, and will continue to reduce until it disappears completely. The Alboran Sea is still at a high to severe level, very locally on the Moroccan coast close to the Strait. The eastern coasts of the basin are at moderate levels, with the exception of strong intensities to the east of Greece and on the Egyptian coast.
The tropical zone of the North Atlantic remains in strong marine heatwave conditions, and local areas of severe levels are concentrated in the Gulf of Mexico.
The state of the North Atlantic basin continues in terms of positive surface temperature anomalies.

Figure 3: Characteristics of marine heat waves over the European zone shown in Figure 1, calculated from surface temperatures from the GLO12 analysis and forecasting system (Mercator Ocean). Solid lines: analysis, dashed lines: forecast. Number of marine heat waves in blue (note that when the surface area of the heat wave is large, there are fewer heat waves), total surface area (km2) of heat waves over the zone in black, surface areas (km2) of heat waves of moderate intensity (yellow), strong intensity (orange), severe intensity (red) and extreme intensity (brown).
*Criteria used: Oliver et al (2021) https://doi.org/10.1146/annurev-marine-032720-095144*

*Figure 4: GLO12 surface temperature anomaly (Mercator Ocean) averaged over the 7 days preceding 4 August 2023, over the North Atlantic. Source : Mercator Ocean International.*

What are marine heatwaves?

Marine heatwaves (MHW) are extreme rises in ocean temperature for an extended period of time. They can occur at different locations in the ocean, and their magnitude and frequency have increased over the last couple of decades, with harmful impacts on ecosystems, and human activities. According to the latest report released by the Intergovernmental Panel on Climate Change (IPCC AR6 SYR), it is found with high confidence that in the near-term at 1.5°C global warming, the increasing frequency of marine heatwaves will increase risks of biodiversity loss in the oceans, including from mass mortality events.²

How are marine heatwaves calculated?

A marine heatwave is a heat episode during which the temperature is significantly higher than a certain threshold for at least 5 consecutive days.

*Figure 4: Adapted from Hobday et al. (2018)*

The seasonally-varying threshold is defined on a daily basis according to a sufficiently long climatic period (in this case 1993-2016). So, for a given place and a given day, knowing all the surface temperatures observed over the last 30 years, a heatwave situation is defined as one where the temperature measured is within 10% of the maximum values observed (i.e. above the 90th quantile, see diagram), for at least 5 consecutive days.

The main characteristics of heatwaves are their duration and intensity. The intensity for a given day corresponds to the value in degrees above the 90th quantile (blue arrow), which can either be calculated as the cumulative intensity throughout the heatwave event, or the maximum intensity.

Heatwaves are categorised on the basis of their deviation from the mean temperature or anomaly (green arrow): a deviation of more than 2 times the difference between the 90th quantile and the mean corresponds to a heatwave in the strong category; a deviation of more than 3 times corresponds to a heatwave in the severe category; and a deviation of more than 4 times corresponds to a heatwave in the extreme category.

¹Analysis of datasets: OSTIA sea surface temperature observations analysis (Copernicus Marine Service), OISST sea surface temperature observations analysis (NOAA), GLO12 model (Copernicus Marine Service, Mercator Ocean International)

² IPCC AR6 SYR 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf