Marine heatwave bulletin - 27 October 2023

Marine heatwave bulletin – 27 October 2023

Mercator Ocean International (MOi) 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

Assessment for October 24

Europe zone

The marine heatwave along the Atlantic seaboard, from the Iberian peninsula to the English Channel, has clearly diminished in intensity, falling into the moderate category or returning to temperatures close to seasonal normals.
In the Mediterranean, the marine heatwave present in the north-western basin is continuing around Corsica and Sardinia, at a strong to locally severe level, with weekly temperature anomalies of the order of 2°C to 3°C.

Figure 1: Marine heatwave category map for the global ocean for October 24, 2023. PSY4 analysis. Source: Mercator Ocean International

Global zone

In the Tropical Atlantic, the geographical extent of the marine heatwave present from the Iberian Peninsula to the equator remained stable. Its intensity has decreased, moving from strong to moderate in places. The marine heatwave in the Caribbean Sea and the Gulf of Mexico remains at severe to extreme levels in places. The associated weekly temperature anomalies are of the order of 1°C to 2°C.
In the Tropical Pacific, the marine heatwave linked to the formation of El Niño remains stable, with moderate to strong levels of intensity overall. The weekly temperature anomaly in the eastern part of the basin is in the region of 1.5°C to 3°C.
The marine heatwave in the North Pacific has continued to ease, falling to moderate overall. Weekly temperature anomalies are around 1°C to 2°C.
In the western Indian Ocean, the marine heatwave to the west of the Arabian Sea has diminished in intensity from severe to strong. Further east, along the west Indian coast, the marine heatwave intensified, reaching severe levels. Weekly temperature anomalies are in the region of 1°C to 2°C.

*Figure 2: Weekly surface temperature anomalies for the global ocean for the week of October 24 to 31, 2023 PSY4 analysis. Source: Mercator Ocean International.*

Forecasts for October 31

Europe zone

For October 31st, the Mercator Ocean International (MOI) forecasting system predicts a stable situation on the Atlantic coast and around the Iberian Peninsula, where some areas remain in moderate category.
The marine heatwave present in the north-western Mediterranean is easing off, falling to moderate overall. A moderate marine heatwave is appearing over a large part of the eastern basin.

Global area

For October 31st, MOi forecasts the marine heatwave in the Tropical Atlantic will remain stable in extent and intensity, with moderate to severe categories. The marine heatwave in the Caribbean Sea and the Gulf of Mexico remains generally severe.
The situation in the Tropical Pacific remains stable.
The marine heatwave in the North Pacific remains stable, generally in the moderate to locally severe category.
The marine heatwave in the western Indian Ocean is intensifying, moving up to the strong category over most of the area concerned.
A marine heatwave appears to be developing around Indonesia.

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