Published on: 2023/07/25
The world has experienced record-breaking sea-surface temperatures (SST) in June. Particularly in the northeastern Atlantic ocean, temperatures have been rising steadily from May onwards to an unprecedented 1.36° C above average for June. The climate science community, ourselves included, heavily discussed this large-scale marine heatwave. Which processes are responsible for the record breaking temperatures in the North Atlantic? Can we expect consequences for weather over Europe?
A combination of oceanographic as well as atmospheric factors are the likely cause of the marine heatwave. In this case, a high pressure field above the Azores (the well-known ‘Azore High’) as well as over the sea west of Ireland led to a lot of heat absorption by the sea surface. The upper layer of the ocean quickly warmed due to the high solar radiation in this area, while simultaneously less mixing with deeper (and thus cooler) ocean layers occurred due to low near-surface wind speeds. Clearly also anthropogenic warming is important which drives a slow but steady warming of the ocean’s surface. IPCC reports say that 90% of the excess heat in the climate system is absorbed by oceans already.
Under climate change, extended periods of higher ocean temperatures have doubled since 1982. Climate projections show a further increase in ocean marine heatwaves up to 50 times more frequent in 2100 compared to pre-industrial times if emissions continue to increase strongly (IPCC, 2019).
Image credit: C3S/ECWMF
Next to the impact of marine heatwaves on marine life, extreme weather can also cause serious impacts. Sea surface temperatures are the main driver of weather on the sub-seasonal to seasonal (S2S) timescales. Newest S2S studies show that also for Europe, global connections between SST and air temperature can be found (e.g. Van Straaten et al.,2022). Our data-driven S2S air temperature model for Europe systematically learns such long range teleconnections in a systematic manner.
In the figures below we can see that both for the Mediterranean area (Italy, Southern France and part of the Iberian Peninsula) and the UK (highlighted in green), a strong positive signal can be found between June SST from the Tropical Atlantic and Northern Atlantic respectively and air temperature in August over these areas. These SST regions line up very well with the regions where we have seen an ocean marine heatwave in June. We therefore expect an extension of the warm July temperatures in these areas into August.
Image credit: Beyond Weather
Image credit: Beyond Weather
The climate system is a delicate and intertwined system of many variables. Especially over Europe, we are seeing a faster increase in heatwave frequency than over the rest of the world, and climate models fail to capture this rapid increase (Rousi et al, 2022; Vautard et al, 2023). It looks like climate change is increasingly affecting the interplay between ocean flows and atmospheric circulations. Such dynamic changes might lead to more abrupt changes and hence surprises for society. Furthermore, we have only seen the initial build up of El Nino so far, and it is expected that El Nino will further strengthen over the coming year. This will lead to further warming of the global ocean.
Will 2023 continue to break records? Stay tuned…
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