Mercator Ocean: Marine heatwave bulletin 4 September 2023

Mercator Ocean International oceanographers examine marine heatwaves across the global ocean. 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 29 August 2023 across the global Ocean. GLORYS 12 analysis. Source: Mercator Ocean International
Figure 2: GLO12 surface temperature anomaly averaged over the 7 days preceding 5 September 2023. Source: Mercator Ocean International.

Assessment for 29 August

  • Europe zone :
    • The heat wave has almost disappeared from the Mediterranean basin, but there is still a signal to the east of the basin, between Greece, Syria and Egypt. This formation corresponds to temperature anomalies of around 1°C.
    • The marine heatwave present in the North Atlantic to the west of the Iberian Peninsula continues to diminish in both extent and intensity, falling mainly into the Moderate category over the remaining areas. However, anomalies are still around 1.5°C in this region.
  • Global zone :
    • In the North Tropical Atlantic, the heatwave is continuing to increase in intensity, with the western part of the region moving from moderate to strong. The weekly temperature anomaly in this zone is in the region of +1 to +2°C.
    • The trend continues in the tropical Pacific: the heatwave corresponding to the formation of El Nino persists and has intensified in the eastern part of the basin, moving into the strong category. The weekly temperature anomaly in this area is around 3°C.
    • In the north-east Pacific, the heatwave is stable in extent and remains in the moderate category.
    • o The heatwave south of the Arabian Sea is stable and still locally in the strong category.
    • The affected areas of the tropical Pacific and tropical Atlantic, as well as the North Pacific, have continued to experience extreme temperatures for more than 30 days.

Forecasts up to 5 September

  • Europe zone :
    • By 5 September, Mercator Ocean International expects the heat waves in the Mediterranean and off the Iberian Peninsula to disappear almost completely.
  • Global zone :
    • Forecasts show a strengthening of heat waves in the tropical Atlantic associated with temperature anomalies of +1°C.
    • In the equatorial Pacific, heat waves are continuing and temperature anomalies are still around +3°C.
    • To the west of the Indian Ocean, the situation is stable: the heat wave persists with weekly temperature anomalies of around +1°C.

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