Ocean Sciences Research Spotlight

The North Atlantic Ocean's Missing Heat Is Found in Its Depths

In the 2000s, the North Atlantic stopped absorbing as much atmospheric warmth. However, the ocean lost only a little heat—the rest was held deeper below the surface by altered circulation patterns.

Source: Geophysical Research Letters


Earth’s oceans quietly and effectively moderate the globe’s temperature. As greenhouse gases blanket Earth in an insulating haze, the oceans sequester much of the atmosphere’s heat in their depths. Ocean temperatures rise as warm air is sucked out of the atmosphere, but some oceanic regions absorb more heat than others, and these thermodynamics are in constant flux.

Since the mid-2000s, the North Atlantic Basin’s upper layers have stopped storing as much atmospheric-derived warmth—to the point where it lost its crown as the greatest-warming basin, in the mid-2000s, to the tropical waters of the Indian and Pacific Oceans. In the Atlantic, heat moved down from the upper layers of the ocean into the deeper sea, hiding much of the water’s warming.

The first part of the North Atlantic’s transformation occurred when the waters of the eastern North Atlantic mixed in the winter of 2005, Somavilla et al. explain in a new paper: Swirling ocean waters changed the overall characteristics toward a saltier, warmer, and denser environment. This newly dense water sinks down from the surface, carrying along the warmth it absorbed during its stint at the top. This transfer of heat down to the depths wasn’t a one-time occurrence during the winter of 2005 either; it continued on by altering the flow of the ocean itself.

Mixing the waters and increasing densities have altered the ocean circulation patterns, the authors suggest. The local currents actually flipped around from a southward flow to a northward flow in the eastern North Atlantic. This reversal in direction brings warmer salty water up from the tropics only to sink down to the North Atlantic’s deeper waters because of the salt’s higher density. The North Atlantic Subpolar Gyre also contracted, which allows even more warm and salty water from the tropics to make its way north.

This continuous supply of salty southern waters continues the process of oceanic convection in which the warmer water on the surface cycles down to deeper layers of the ocean. However, the oceans can’t store or hide away the heat forever; eventually, they will reach capacity and be unable to buffer the globe’s warming as effectively. (Geophysical Research Letters, doi:10.1002/2015GL067254, 2016)

—Cody Sullivan, Writer Intern

Citation: Sullivan, C. (2016), The North Atlantic Ocean’s missing heat is found in its depths, Eos, 97, doi:10.1029/2016EO047009. Published on 3 March 2016.

© 2016. The authors. CC BY-NC 3.0
  • davidlaing

    The slowdown in heating of the North Atlantic is more likely due to partial recovery of Earth’s ozone layer since enactment of the Montreal Protocol on Substances that Deplete the Ozone Layer. That stopped the increased irradiance of Earth’s surface by high-energy solar ultraviolet-B radiation that caused the dramatic global warming incident from 1975 to 1998.

    • Raquel Somavilla

      Thanks for you comment.
      It could have some effect. On the other hand, the injection of the heat accumulated during decades to the deep ocean through strong winter mixing events would contribute to a slowdown in heat uptake in the North Atlantic. How? Because strong convective mixing releases heat to the atmosphere. One thing does not prevent the other.

      • davidlaing

        Interesting hypothesis, although I fail to understand the mechanism whereby heat delivery from the ocean depths would cause a slowdown of heat uptake at the surface. Is this just a hypothesis, or do you have actual evidence to back it up?

        • Raquel Somavilla

          Perhaps I’m wrong, but i think you have in mind slow processes such diffusive processes or heave (deepening of isotherms) as responsinble for heat transfer to the deep ocean concurrent to ocean heat uptake at the surface. This is not what happens in this case through strong mixing-induced heat injection events. I assume that you understand that for having convective mixing deeper than normal you need abnormally high heat losses from the ocean to the atmosphere. That is how the heat uptake is reduced (because the heat loss is increased). So, the next question is how deep convective mixing can injected heat from the upper layers to the deep ocean. For that, you necessarily need an excess of salinity at the surface. Then, you can generate at the surface waters dense enough to mix with deep waters with higher temperature and salinity values than the deep waters below. When they mixed vertically because they have similar density, it results in a sudden temperature and salinity increase of the deep waters (sudden heat and salt content increase at depth). I hope this answer your question. You can find more information in the paper and in the supplementary information (e.g. Section 3 of the paper and comments to Table S1 of the Suppl. Inform).

          • davidlaing

            OK, but again, is this just theoretical speculation, or is there hard data to back it up?