The influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water-mass transformation, stratification maintenance, and the overturning circulation. However, the distribution of diapycnal (near vertical) mixing is not well described by sparse ship-based observations since this mixing is both spatially patchy and temporally intermittent. Recently, techniques have been developed to infer diapycnal mixing rates from finescale (tens to hundreds of meter vertical scales) shear or strain, using assumptions about the underlying internal wave dynamics that often drive turbulent mixing. The project?s investigators have begun to apply these techniques to strain measured by the global Argo array. They have used strain information from Argo float profiles in the upper 2,000 m of the ocean to generate over 400,000 estimates of the turbulent energy dissipation rate, indicative of ocean mixing. While these estimates rely on numerous assumptions, and do not take the place of direct measurement methods, they have been shown to be a good proxy for the distribution of diapycnal mixing. Temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans. They corroborate previous observations linking elevated dissipation rates to regions of rough topography. Heightened estimated dissipation rates have been observed in areas of high eddy kinetic energy, along the equatorial band, as well as heightened diffusivity in high latitudes where stratification is weak. The seasonal dependence of mixing is observed the Northwest Pacific, suggesting a wind-forced response in the upper ocean.
In this project, graduate student, Caitlin Whalen, will continue this work for her doctoral thesis. She will tackle two of the major open questions that these initial results pose. First, the correlation between the estimated diapycnal mixing and the eddy kinetic energy field will be investigated by considering the distribution of the mixing estimates across a composite eddy derived from Argo-float and satellite sea level anomaly data. This distribution will be compared with an eddy modeled by the MIT ocean circulation model inspired by the characteristics of the composite eddy. Second, the locally elevated diapycnal mixing along the equator will be compared to equatorial microstructure measurements to corroborate the pattern, and the observed oscillations along the equator will be placed in the context of equatorial waves and ENSO, inspired by previous results that link near-surface equatorial mixing with these phenomena.
Broader Impacts: The project will train a graduate student, Ms. Whalen, who has been and will continue to be involved in a variety of outreach activities, including volunteering at the Birch Aquarium, assisting teachers and social media outreach efforts during the CalEchoes student-run research cruise, participating in workshops to educate local teachers about oceanography, and discussing her work with the local TV news media. Scientifically, the proposed work contributes to our understanding of global patterns of diapycnal mixing in the ocean, a major unresolved process in global ocean models. The principal investigator is leading a NSF-funded Climate Process Team (CPT) tasked with improving representations of diapycnal mixing in global climate models. To the extent that both the magnitude and distribution of diapycnal mixing is likely to change in a future climate (as, for example, wind stress and associated mesoscale patterns evolve), accurate prediction of future or past climate requires development of parameterizations of turbulent mixing that are based on appropriate physics. The results of the project will be incorporated into ongoing CPT efforts.
