Mercator Ocean: Marine heatwave bulletin for 25 August 2023

Mercator Ocean International oceanographers examine marine heatwaves across the global ocean for this month of August. 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 22 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 22 August 2023. Source: Mercator Ocean International.

Assessment for 25 August

  • Europe zone :
    o The heat wave in the west of the Mediterranean basin has evolved from moderate to strong.
    o The marine heatwave in the Atlantic west of the Iberian Peninsula remains strong. Weekly temperature anomalies reach +1.5°C
  • Global zone :
    o The tropical Pacific is still presents a moderate to strong heat wave condition, reaching high intensity near the coast of Peru – which is consistent with current El Niño conditions.
    o The marine heat waves in the North Pacific remain within moderate conditions.
    o The heat wave in the North Tropical Atlantic is still present, with a slight greater extent of the “strong” category.
    o The heat wave in the Indian Ocean is spreading across the western part of the basin, reaching strong categories in the Arabian Sea and north of Madagascar, with weekly temperature anomalies reaching +0.5°C to +1°C.
    o Weekly temperature anomalies reach +2°C, +1.5°C, +3°C for the North Atlantic (Labrador Sea), Tropical Atlantic and Eastern Tropical Pacific zones respectively.
    o The Tropical Pacific, Tropical Atlantic and North Pacific areas concerned have continued to experience extreme temperatures for more than 30 days.

Note: Forecasts for next week’s marine heatwaves are not available.

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