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Researchers have established that the rise in oceanic heat drawdown in the equatorial Pacific, North Atlantic and Southern Ocean basins is one of the crucial reasons why there has been a global warming hiatus in recent years.

The new analysis was done by Professor Sybren Drijfhout from the University of Southampton and collaborators from the National Oceanography Centre (NOC) Dr Adam Blaker, Professor Simon Josey, Dr George Nurser and Dr Bablu Sinha, together with Dr Magdalena Balmaseda from the European Centre for Medium Range Weather Forecasting (ECMWF).  Professor Drijfhout said that this was important as current climate models have been unable to simulate the pause, but their study gave clues to where the heat was drawn down and by which processes, so this could serve as a benchmark for climate models on how to improve their projections of future global mean temperature. Previously, the drawdown of heat by the Equatorial Pacific Ocean over the hiatus period, due to cool sea-surface temperatures associated with a succession of cool-surface La Nina episodes, was thought to be sufficient to explain the hiatus. However, this new analysis reveals that the northern North Atlantic, the Southern Ocean and Equatorial Pacific Ocean are all important regions of ocean heat uptake. Each basin contributes a roughly equal amount to explaining the hiatus, but the mechanisms of heat drawdown are different and specific in each basin. In the North Atlantic, more heat has been retained at deep levels as a result of changes to both the ocean and atmospheric circulations, which have led to the winter atmosphere extracting less heat from the ocean. In the Southern Ocean, the extra drawdown of heat had gone unnoticed and is increasing on a much longer timescale (multi-decadal) than the other two regions (decadal). Here, gradual changes in the prevailing westerly winds have modified the ocean-atmosphere heat exchange, particularly in the Southern Indian Ocean. The team calculated the change in the amount of heat entering the ocean using a state-of-the-art high resolution ocean model developed and run by NOC scientists that are driven by surface observations. The study is published in Geophysical Research Letters.

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