Mercator Ocean: marine heatwave bulletin for 3rd July 2023

Mercator Ocean International oceanographers examine marine heatwaves across the global ocean for the month of June and forecasts for the first part of July 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: Surface temperature anomaly from the GLO12 analysis and forecasting system (Mercator Ocean) averaged over the 30 days preceding 27 June 2023. The reference climatology is made up of daily GLORYS12 maps averaged over the period 1993-2016. Source: Mercator Ocean International

Key take-aways for the month of June 2023

Over the month of June, the North-East Atlantic and Mediterranean saw temperatures at least 1°C to 2°C higher than the 1993-2016 average, and the area remained in marine heatwave conditions throughout the month.
In the last week of June, the entire Atlantic seaboard of Europe was subjected to a marine heat wave ranging from moderate intensity in the North Sea to more severe or extreme intensity in the Bay of Biscay. Compared with previous weeks, conditions were relatively more moderate near the European coasts, probably due to the weather conditions and the end of the atmospheric heat wave over the European continent.
The western Mediterranean saw a moderate to strong heatwave at the end of June.

Forecasts up to 8 July 2023

From the beginning of July, the forecasts show a moderation of the heatwave over most of the North-East Atlantic and the Mediterranean, with a transition from the extreme category to a more moderate heatwave category. (figure 2)
Tropical Atlantic: the models see a heatwave to the west in the Gulf of Mexico and to the east of the basin in the area off Morocco, Western Sahara and Mauritania.
The tropical Pacific is experiencing a moderate heatwave, reaching high intensity near the coast of Peru, which is consistent with current El Nino conditions.
A strong heatwave could appear in the Bay of Bengal

Figure 2: Characteristics of marine heatwaves over the European zone shown in Figure 1, calculated from surface temperatures from the GLO12 analysis and forecasting system (Mercator Ocean). Solid lines: analysis, dashed lines: forecast. Number of marine heatwaves in blue (note that when the surface area of the heatwave is large, there are fewer heatwaves), total surface area (km²) of heatwaves in the area in black, surface area (km²) of heatwaves of moderate intensity (yellow), strong intensity (orange), severe intensity (red) and extreme intensity (brown). The bottom plot is a zoom on intense heat waves, there are 2 scales in order to better see the fluctuations from day to day. Criterion used: Oliver et al (2021) https://doi.org/10.1146/annurev-marine-032720-095144. Source: Mercator Ocean International

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.[1]

[1] IPCC AR6 SYR 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf