Record-breaking ocean temperatures contribute to extreme marine heatwaves

Global average sea surface temperatures last month reached unprecedented levels for the north Atlantic Ocean, which recorded exceptionally warm sea surface temperatures, with several extreme marine heatwaves that caused widespread coral bleaching.

Global average sea surface temperatures last month reached unprecedented levels for the north Atlantic Ocean, which recorded exceptionally warm sea surface temperatures, with several extreme marine heatwaves.

These were related to short-term changes in atmospheric circulation and longer-term changes in the ocean, according to analysis from the Copernicus Climate Change Service (C3S).

So far this year, discussions of our oceans and climate have largely focused on the onset of El Niño, recently declared by the World Meteorological Organization, and its potential for pushing global temperatures into “uncharted territory” by the end of 2023 and into 2024.

In fact, we have already entered uncharted territory due to the exceptionally warm conditions in the north Atlantic Ocean.

Already, sea surface temperatures globally were higher than in any previous May on record, and this continued into June with even larger anomalies compared to the average.

The next month was the warmest June globally, at just over 0.5°C above the 1991-2020 average exceeding June 2019 – the previous record – by a substantial margin. By mid-July, a marine heatwave spread across the Gulf of Mexico and the Caribbean with temperatures ranging between one and three degrees Celsius (~2-4.5˚F) above average.

Scientists recently discovered a reef off the Florida Keys has been completely bleached by a marine heat wave, leading to concerns about the ecosystem’s long-term stability.

The growing concerns of coral bleaching due to the ongoing marine heat wave across South Florida, the Gulf of Mexico, and the greater Caribbean led scientists at NOAA’s Atlantic Oceanographic & Meteorological Lab (AOML) to return to Cheeca Rocks on July 31st and August 1st, 2023.

As an inshore reef within the Florida Keys National Marine Sanctuary and a vital long-term climate monitoring site for the National Coral Reef Monitoring Program, Cheeca Rocks demonstrated persistence in the wake of the 2014 and 2015 bleaching event and is known to have some of the highest coral coverage compared to surrounding offshore reefs, but what NOAA scientists found was bleak.

It is essential to note that bleaching does not necessarily mean mortality.

Corals are able to recover from a bleaching event as long as the physical stress, in this case sustained, above average sea surface temperatures, is reduced to a level that allows the algal symbiont population to reestablish itself.

Record June temperatures were experienced across northwest Europe. Record-high sea surface temperature occurred over large swaths of the North Atlantic, leading to marine heatwave conditions around Ireland and the UK.

Parts of Canada, the United States, Mexico, Asia, and eastern Australia were significantly warmer than normal. It was cooler than normal over western Australia, the western United States, and western Russia.

Sea surface temperature anomaly (°C) for the month of June 2023, relative to the 1991-2020 reference period. Orange and red colors in the map above show the areas where the sea surface temperatures were above the 1991-2020 climatological average last month.

June 2023 saw temperatures above the 1991-2020 climatological average over most of Europe.

The highest anomalies were found in northwest Europe, where Ireland, the United KingdomBelgium, and the Netherlands had their warmest Junes on record. France and Scandinavia were also notably warmer than average. In contrast, the southern Balkans, Greece, Türkiye, and western Russia were colder than average. Mixed conditions occurred across Italy and Spain.

Most of Canada experienced above-average temperatures, with northern Ontario and northern Quebec particularly affected. Mexico and parts of the southern United States had well above-average temperatures which led to over 100 heat-related fatalities. Southern South America, southeast Africa, Iran, most of India, northeast Russia, and northern and eastern Australia saw above-average temperatures; there were nearly 100 fatalities reported in India due to the heat too.

A band of warmer-than-average anomalies also extended from central to eastern Asia, where Beijing had its hottest June day on record.

Conversely, colder-than-average temperatures occurred in the southwestern and eastern United States, Baja California, central South America, parts of northern Africa and south Asia, and across western Australia. 

Mixed, although pronounced anomalies, were found over Antarctica.Above-average air temperatures were largely found over the oceans. Most notably, exceptionally high sea surface temperatures were observed in large sectors of the North Atlantic, including marine heatwave conditions around Ireland and the UK. El Niño conditions continued to develop over the equatorial eastern Pacific. Air temperatures were unusually high over the Weddell Sea, with regions of relatively mild temperature extending northward over much of the southern Atlantic, Indian and Pacific Oceans. Air temperatures were higher than the 1991-2020 average throughout the tropics and over most of the North Pacific. In contrast, temperatures were lower than average from southeastern South America to the Amundsen Sea, off southeastern Australia, southern Africa, and western North America.

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