This award was made to a Climate Process Team (CPT) comprised of about 25 scientists from multiple institutions, including Scripps Institution of Oceanography at University of California San Diego, Woods Hole Oceanographic Institution, University of Michigan, the Geophysical Fluid Dynamics Laboratory (GFDL) of the National Oceanic and Atmospheric Administration, the National Center for Atmospheric Research (NCAR), the University of Washington, the University of Alaska Fairbanks, Florida State University, and University of Victoria. It also involved collaborations with other institutions such as University of Copenhagen and Oregon State University. 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 primary CPT work was funded by a previous grant – the main outcomes of the original research are described below.  The present award supported two things.  First, it allowed us to hold a meeting in October 2015 for not only the scientists directly involved in the original work, but a broader segment of our scientific community. Second, it supported us in writing up the results from our original work,  combined with insights from the 2015 conference in the form of a comprehensive paper, which has just been published in the Bulletin of the American Meteorological Society.

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 focused upon in recent physical oceanographic field studies. 

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.  We demonstrated that inserting parameterizations of mixing driven by breaking internal tides and breaking lee waves into global climate models strongly impacts the large-scale oceanic circulation.  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.  Finally, 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, a development that we are just starting to exploit.  Although the CPT grant is expired, the collaborations it fostered between observationalists, theoreticians, and modelers are ongoing and will continue to yield results for years to come.

Much of the CPT research was performed by five postdoctoral fellows.  The CPT grant thus 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: 12/07/2017
Modified by: Jennifer A Mackinnon