Tracking Ocean Heat Uptake during the Surface Warming Hiatus
Recent research has observed a slowing of the global surface temperature increase over the past 16 years. While this “hiatus” is regarded a result of natural variability within the centennial warming trend (even though radiative forcing changes also contribute), two theories have emerged to further explain the change. One theory ties the hiatus to the negative phase of the Interdecadal Pacific Oscillation, manifesting as a La Niña-like cooling, with intensified easterly winds over the equatorial Pacific. The other theory centers on a decadal timescale, vertical ocean energy redistribution, which occurs through excessive global, deep-ocean heat uptake concurrent with a positive radiative imbalance at the top of the atmosphere.
The Pacific and Indian Oceans are the key regions to track ocean heat uptake during the surface warming hiatus.
A research team, including a DOE scientist at Pacific Northwest National Laboratory, proposes that the two theories are not mutually exclusive. They showed that the deep heat penetration in the Atlantic and Southern Ocean basins is not unique to the hiatus but is characteristic of human-caused global warming. Further, this change merely reflects the depth of the ocean’s mean meridional overturning circulation in the basin. However, they also found that the heat redistribution in the upper 350m between the Pacific and Indian Oceans is closely tied to the surface warming hiatus. During hiatus events, the Indian Ocean shows an atypical warming below 50m because of the increased heat transport by the Indonesian through-flow (ITF), an ocean current providing a low-latitude passage for warm, fresh water from the Pacific to the Indian Ocean. This heat transport happens in response to the intensified trade winds over the equatorial Pacific.