Mercator Ocean International oceanographers examine marine heatwaves across the global ocean for the first weeks of August and develop forecasts for the third 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

Assessment for 15 August 2023

• Europe zone :
  o The heat wave in the Mediterranean basin has practically disappeared.
  o The marine heatwave in the Atlantic to the west of the Iberian Peninsula has been upgraded from moderate to strong. Weekly temperature anomalies reach +1.5°C.
• Global zone:
  o The tropical Pacific is still in moderate to strong heatwave conditions, reaching high intensity near the coast of Peru, which is consistent with current El Niño conditions.
  o Marine heatwaves in the North Pacific have diminished in extent and intensity (currently in moderate marine heatwave condition).
  o The North Tropical Atlantic heat wave is still present, with moderate to strong intensity.
  o A heat wave to the south of the Arabian Sea is developing and reaching the strong category locally.
  o Weekly temperature anomalies reach +2°C, +1.5°C, +3°C for the North Atlantic (Labrador Sea), Tropical Atlantic and Eastern Tropical Pacific areas respectively.
  o 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 for 22 August 2023

• Europe zone :
  o For August 22, it is expected a strengthening of the marine heatwave around the Iberian Peninsula, on the Atlantic and Mediterranean sides, with strong to severe intensities.
  o Weekly temperature anomalies will be close to +2°C around the Iberian Peninsula.
• Global zone:
  o For 22 August, Mercator Ocean International forecasts a continuation of extreme temperatures in the equatorial Pacific and the North Atlantic (Labrador Sea).
  o In the tropical North Atlantic, an increase in the intensity of the marine heatwave is expected, with a strong level, even reaching the severe category locally.
  o Weekly temperature anomalies are in the order of +2°C to 3°C, +1°C, +2 to +3°C, +1°C for the North Atlantic (Labrador Sea), Tropical Atlantic, Eastern Tropical Pacific and Arabian Sea zones respectively.

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.

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