13 December 2018

Disentangling the Ocean-Atmosphere Interactions in the North Atlantic

Powerful new methodology illuminates large-scale influences on sea surface temperature that drive Atlantic Multidecadal Variability.

Science

The North Atlantic ocean plays an important role in modulating Earth’s climate on decadal to multidecadal timescales by absorbing, releasing and transporting heat over the planet. These slowly fluctuating sea surface temperatures ( SSTs) are often referred to by the term “Atlantic Multidecadal Variability” (AMV). Scientists remain unsure of how the ocean, atmosphere, and their interactions generate this variability. Because the interactions are so complex, teasing out the relative roles of ocean, atmosphere and interactions between them has been difficult. In a study aimed at isolating the processes driving the AMV, scientists from the U.S. Department of Energy’s Pacific Northwest National Laboratory led the development of a powerful climate model methodology to disentangle the coupled ocean-atmosphere interactions in model simulations with full ocean and atmosphere circulations—a fully-coupled system. They found that the ocean drove stronger multidecadal variability (greater than 30 years), while the atmosphere drove weaker variability up to interdecadal timescales (10–30 years).

Impact

The study identified some of the most important atmospheric and ocean processes that produce and damp variations in North Atlantic SSTs. Surface interactions between the atmosphere and ocean decreased the ocean-driven variability because surface heat fluxes acted to damp the ocean driven SST variability. These results imply that the strength of surface coupling in global climate models might be one of the reasons why many Earth system models simulate weaker AMV compared to observations. This is an important consideration for improving this essential feature of the Earth system in models. 

Summary

Modeling studies show that both the ocean and atmosphere play a part in AMV-like variability, but scientists continue to debate the specific mechanisms driving this variability. In this study, researchers introduced a methodology to help identify the processes that contribute to SST variations in the North Atlantic within the fully-coupled system. Their decomposition showed that internal variability in the ocean caused very large SST variation in the North Atlantic on multidecadal timescales, while the atmosphere produced much weaker SST variations with shorter (up to interdecadal) timescales.

When the ocean and atmosphere interact at the surface, the strong ocean driven North Atlantic SST variations are dissipated through surface heat release to the atmosphere. The resulting fully coupled AMV is only slightly stronger than the North Atlantic SST variations driven by the atmosphere features alone, but the variability in the fully coupled system occurred at longer (multi-decadal) timescales. The ways that the North Atlantic SST and surface heat fluxes vary—by increasing and decreasing in-sync with each other, or ahead or behind each other—provides crucial hints about the relative importance of internal ocean processes in driving the AMV and the role of heat fluxes in its dissipation.

This analysis provides insight into why some coupled models simulate weaker-than-observed AMV, and why traditional slab (or simple mixed-layer) ocean models can also appear to drive an AMV-like variability, even though they do not simulate the full ocean circulation. 

Contact
Phil Rasch
Pacific Northwest National Laboratory
Publications
Garuba, O, J Lu, H Singh, F Liu, and P Rasch.  2018.  "On the Relative Roles of the Atmosphere and Ocean in the Atlantic Multidecadal Variability."  Geophysical Research Letters 45(17): 9186-9196.  https://doi.org/10.1029/2018gl078882.