Ocean Mixing Studies -- and -An Introduction to the UMass Experimental Center for Environmental Lidar (EXCEL) Abstract: Dye release experiments conducted in the seasonal pycnocline of the Sargasso Sea in June 2011 are used to estimate diapycnal and isopycnal diffusivities on time scales of many days, and spatial scales of 0.1 to 10 km. A shear-strain-diffusion model suggests vertical shear dispersion by low frequency shears and internal waves explain a fraction of the observed lateral spreading. A series of shorter (order 6 hrs) experiments surveyed using airborne Light Detection and Ranging (LIDAR) reveal details about the short-term evolution of the dye patches, including multiple instances of sinuous meanders of the patches early in the evolution, and/or evidence of filamentation along their periphery. This suggests weak smallscale (<1 km) differential lateral advection acting on the patches, possibly contributing to enhanced dispersion at later times in the longer experiments. Simple scaling implies an upper bound on the effective lateral diffusivity at the <1 km scale that is nearly an order of magnitude smaller than that at 1-5 km scales. To further advance the use of lidar for environmental science applications such as these, we have established a new UMass Experimental Center for Environmental Lidar (EXCEL). In brief, EXCEL is a multi-campus center (UMass Lowell, Dartmouth, Boston, and Amherst) that pools expertise among the four campuses in terrestrial, oceanographic, and wind lidar. As part of the Center's oceanographic related activities in particular, we are reaching out to potential collaborators within UMass and in the region that have interest and/or possible applications related to bathymetric lidar, spatially resolved wave data, water clarity data (e.g. tubidity, phytoplankton concentrations), marsh/wetland/beach/dune/channel topography/bathymetric data, and/or other marine/coastal related applications. We welcome any opportunities to collaborate with interested groups on these and related problems. About Miles A. Sundermeyer: Dr. Sundermeyer is an Associate Professor at the University of Massachusetts Dartmouth School for Marine Science and Technology. He earned his Ph.D. from the MIT/WHOI Joint Program in Physical Oceanography, and has been studying problems relating to ocean mixing for more than 20 years. His primary research interests include vertical and horizontal mixing processes, dye release and Lagrangian drifter studies, numerical modeling of two- and three-dimensional turbulent flows, numerical modeling of physical and biological interactions. (b ) (a ) 3.1 h 3.7 h 4.4 h 5.3 h 6.3 h 1 k (c ) Fig. 1: Dye dispersal at ~35 m depth in the seasonal pycnocline during the 2011 ONR LatMix field experiment. (a) Advection and evolution of rhodamine (pink), plus trajectories of drifter centroid and Lagrangian. (b) Rhodamine concentration (red circles) along the advected ship track (bold black) 135 h after release, revealing 510 km scale straining and dispersal. A fluorescein injection is also shown (green circles) 3-6 h after release, overlaid on airborne lidar track lines (bold blue lines). (c) Peak lidar fluorescence of the fluorescein patch reveals the short-time evolution at ~1-km scales by submesoscale stirring/straining. 1 0.25 1.6 1.9 2.5 Fig. 2: Evolution of a near-surface (3 m) fluoresceine dye patch as viewed by airborne lidar during the 2011 LatMix field experiment. Top row shows peak lidar fluorescence intensity during successive surveys, with time (hrs, indicated under each patch) increasing from left to right. Lower panels are the depth at which the peak return was observed. Wind was from the north at ~13 kts. There was also a mean density gradient within the ML oriented roughly NW-SE. Lower right inset shows a depth cross-section of the lidar return (marked by red line in upper panel), revealing deep tails (i.e., at the ML base) streaming behind (relative to the wind direction) the surface portion of the patch.