Collaborative Research: Representing Internal-Wave Driven Mixing

in Global Ocean Models

Project Outcomes Report

This award was made to a Climate Process Team (CPT) comprised of about 25 scientists from multiple U.S. universities and from two national climate modeling centers. CPTs bring together observationalists, theoreticians, process modelers, and climate modelers to synthesize existing observations and develop parameterizations that capture observational behaviors for implementation in climate models. The tools used in our CPT included existing observations collected in field campaigns and by arrays of autonomous floating sensors; theory; idealized process models used to understand detailed ocean physics; global high-resolution ocean models run for relatively short periods of time for operational purposes; and global coarse-resolution ocean and climate models run over longer periods of time for climate prediction.     

Our CPT focused on the turbulent mixing associated with internal gravity wave breaking.  These waves lie on the interfaces of oceanic layers with different densities. Just as waves on the surface of the ocean break, internal waves also break. When they do, they mix colder, denser waters below with warmer, lighter waters above. Hence, internal wave breaking exerts an important control on the density stratification of the ocean, impacting the large-scale oceanic circulation. The latter stores and transports vast amounts of heat and carbon and is a crucial component of the global climate system. Because internal wave breaking impacts the large-scale circulation, but at the same time takes place on spatial scales that are too small to be resolved in current climate models, the breaking must be parameterized in such models. We therefore need to better understand the three-dimensional spatial geography of internal wave generation and breaking. 

The oceanic internal gravity wave spectrum, which covers a wide range of frequencies, arises from interactions of waves that are forced by three mechanisms: the action of rapidly changing winds creating so-called “near-inertial” flows in the upper ocean, tidal flow over rough topographic features on the ocean bottom creating “internal tides” (internal waves of tidal frequency), and slowly varying flows of currents and turbulent eddies over rough topography creating so-called “lee waves”. All three of these processes have been examined in recent field studies. Near-inertial waves and internal tides are primarily generated in the middle of the ocean, and lose some of their energy locally, but also propagate long distances before losing the remainder of their energy. This dual local/non-local nature of internal waves makes parameterizing their behavior quite challenging. Lee waves have only recently been seen as an important component of the global oceanic energy budget.

Our team made several advances in understanding the three-dimensional geography of ocean mixing. On the observational front, we made the first global maps of mixing, by synthesizing archived measurements of highly sensitive instruments known as microstructure profilers. We also made global maps of mixing from more abundant but less direct measurements by global arrays of profiling floats. Our global mixing maps can be used to test a variety of ocean models. On the modeling front, we showed that near-inertial wave breaking deepens the oceanic mixed layer (the thin upper-ocean layer having nearly uniform water properties) by up to 30%, thus changing the sea surface temperature and precipitation in climatically important ways. Building upon prior efforts in parameterizing internal tide mixing, we demonstrated that the large-scale oceanic circulation simulated by global climate models is impacted by the spatial distribution of mixing by breaking lee waves, as well as by breaking internal tides. We found that parameterizations of lee wave breaking also strongly impact the flows and energy budgets in higher-resolution operational ocean models. We continued to refine process models of mixing occurring on small spatial scales, to better understand the physics of ocean mixing. We found that the continental shelves are a significant “sink” of internal wave energy radiating away from deep-ocean sources. Through comparison of high-resolution models with satellite observations, we demonstrated that internal tides lose substantial energy in the open-ocean, via mechanisms that are still not fully understood. We showed that high-resolution ocean models forced by both atmospheric fields and the astronomical tides are beginning to develop a credible internal wave spectrum. Finally, we created a repository of ocean vertical mixing parameterizations, in a package called CVMix (Community ocean Vertical Mixing Project), that can be used by ocean modelers throughout the world. 

Aside from the improvements to our knowledge of ocean mixing, this CPT also yielded some intangible benefits. The new collaborations fostered between observationalists, theoreticians, and modelers by the CPT are ongoing and will continue to yield results well beyond the lifetime of this project. In addition, because much of the CPT research was performed by five postdoctoral fellows, the project contributed greatly to the training of young scientists, who have moved on or will shortly move on to permanent positions in academia and industry.

Last Modified: 08/29/2016
Modified by: Brian Arbic