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Even as the heat content of the world’s oceans reached a record high in 2023, relatively small regions are cooling, the World Metrological Organization (WMO) highlighted in its State of the Global Climate 2023 report.
This cooling is occurring in a few places, including the subpolar North Atlantic Ocean extending from near the surface down to a depth of over 800 metres. ‘
Further, sea surface temperatures in the subpolar North Atlantic have decreased by about -17.3 degrees Celsius over the last century, according to a 2023 study published in the journal Climate Dynamics.
Other parts of the North Atlantic are recording the strongest warming, along with the Southern Ocean and the South Atlantic.
This contrasting pattern of heating and cooling in the North Atlantic Ocean is linked to the slowdown of the Atlantic Meridional Overturning Circulation (AMOC), a system of ocean currents that circulates water within the Atlantic Ocean, bringing warm water north and cold water south.
A recent 2024 paper published in the journal Science showed the AMOC system is en route to tipping. Researchers have warned that a future collapse could have severe impacts on the climate in the North Atlantic region.
Further, the cooling in the subpolar North Atlantic, known as the North Atlantic cold blob, could be driven by a host of mechanisms involving both the ocean and atmosphere, the Climate Dynamics paper reported. WMO explained that local interactions between the air and sea could be involved.
Around 90 per cent of the energy that has accumulated in the Earth system since 1971 is stored in the ocean. This has warmed the ocean and the heat content of the ocean has increased.
The ocean heat content in 2023 was the highest on record, exceeding the 2022 value by 13 ± 9 zetajoules (Zj). For comparison, the entire globe consumes around half a ZJ of energy to fuel economies. And 15 ZJ is enough energy to boil 2.3 billion Olympic-sized swimming pools of 50 m length, 25 m width and 2 m depth.
The rise is likely due to anthropogenic climate drivers such as greenhouse gas emissions, changes in land use that determines how much sunlight energy is reflected by land, and natural variability.
Further, global average sea-surface temperatures (SST) were also at a record high between the late Northern Hemisphere spring and the end of 2023.
The records for July, August and September were each broken by a large margin of around 0.21-0.27°C, WMO pointed out in its report.
The eastern North Atlantic, the Gulf of Mexico and the Caribbean, the North Pacific extending eastward from the Sea of Japan, the Arabian Sea and large areas of the Southern Ocean witnessed exceptional heating compared to the 1991-2020 baseline.
This trend continues in 2024. Since January 31, 2024, the daily SST for 60°South-60°North has reached new absolute records, according to Copernicus.
In January 2024, the average global SST for January over 60°S-60°N reached 20.97°C, a record for that month.