Mercator Ocean: Marine heatwave bulletin for 31 July 2023

Mercator Ocean International oceanographers examine marine heatwaves across the global ocean for the month of July and forecasts for the first 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: Marine heatwaves categories map for 25 July 2023 across the global Ocean. GLORYS 12 analysis. Source: Mercator Ocean International
Figure 2: Sea Surface temperature anomaly taken on 31 July, showing a 9-day forecast up to 08 August, 2023. There are heightened temperatures across the global ocean (red zones) as compared to the long-term average.  Source: Mercator Ocean International,

Key take-aways for the month of July 2023

  • A heatwave swept across the Mediterranean basin during July, starting in the western part and spreading to the whole basin in the second half of July. The Alboran Sea is the region most affected, with a heatwave present since the beginning of the month. Two peaks of extreme intensity were observed in the region between western Greece, Italy and Libya, as well as locally in the Middle East. Throughout the week, almost the entire western basin recorded anomalies 3°C above seasonal normals.
  • In the North Atlantic basin, the heatwave that swept across most of the North-East Atlantic in June gradually disappeared during the first fortnight of July. In the north-western part south of Greenland, in the Labrador Sea and west of Newfoundland, a strong to severe and locally extreme marine heatwave has been present since mid-July. Weekly anomalies of more than 4°C have been observed west of Newfoundland.
  • Throughout July, there was a heatwave in the tropical North Atlantic (from 15°N to 30°N), stretching across the entire width of the basin, with a category of strong to severe in the east and moderate to strong in the west.
  • The heatwave in the north-east Pacific remains moderate to strong
  • The tropical Pacific is experiencing a moderate heat wave, reaching high intensity near the coast of Peru, which is consistent with current El Niño conditions.
  • There are also heat waves in the southern hemisphere in the south-western Pacific east of New Zealand.

Forecasts for the 1st week of August

  • The heatwave present across the Mediterranean basin is gradually disappearing, starting in the northern part of the basin, but will remain extreme locally in the Alboran Sea and severe over Libya.
  • In the north-west Atlantic zone, the heatwave is easing off, going from being generally severe to extreme to moderate and locally severe.
  • The heatwave in the tropical North Atlantic, stretching across the entire tropical belt, remains stable in the models.
  • The heatwave in the tropical North Pacific is intensifying eastwards over Central America.
Figure 3: Characteristics of marine heat waves over the European region (30°N-70°N; 0°E-45°E) including the Baltic Sea, the Eastern Atlantic and the Mediterranean Sea, calculated from surface temperatures from Mercator Ocean’s GLO12 analysis and forecasting system. 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 in million square kilometres (M km2) of heat waves over the area in black. The surface areas of the different categories are shown in colour: moderate intensity (yellow), strong intensity (orange), severe intensity (red) and extreme intensity (brown).
The lower panel shows the curves for the severe (red) and extreme (brown) heatwaves in the middle panel, over a smaller range of areas (y-axis) to give a clearer picture of the trends (on the left, up to 1,000,000 square kilometres for the severe category; on the right, up to 250,000 square kilometres for the extreme category).

Criteria used: Oliver et al (2021) 

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