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Harper Simmons Senior Principal Oceanographer hsimmons@apl.washington.edu |
Education
B.A. Physics, University of Alaska, 1995
M.S. Coastal Hydrology, University of Alaska, 1996
Ph.D. Physical Oceanography, Florida State University, 2000
Publications |
2000-present and while at APL-UW |
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Near-inertial energy variability in a strong mesoscale eddy field in the Iceland Basin Voet, G., and 13 others including H.L. Simmons, C.B. Whalen, R.-C. Lien, and J.B. Girton, "Near-inertial energy variability in a strong mesoscale eddy field in the Iceland Basin," Oceanography, 37, 34-47, doi:10.5670/oceanog.2024.302, 2024. |
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1 Dec 2024 ![]() |
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An 18-month deployment of moored sensors in Iceland Basin allows characterization of near-inertial (frequencies near the Coriolis frequency f with periods of ~14 h) internal gravity wave generation and propagation in a region with an active mesoscale eddy field and strong seasonal wind and heat forcing. The seasonal cycle in surface forcing deepens the mixed layer in winter and controls excitation of near-inertial energy. The mesoscale eddy field modulates near-inertial wave temporal, horizontal, and vertical scales, as well as propagation out of the surface layer into the deep permanent pycnocline. Wind-forced near-inertial energy has the most active downward propagation within anticyclonic eddies. As oceanic surface and bottom boundaries act to naturally confine the propagation of internal waves, the vertical distribution of these waves can be decomposed into a set of "standing" vertical modes that each propagate horizontally at different speeds. The lowest modes, which propagate quickly away from their generation sites, are most enhanced when the mixed layer is deep and are generally directed southward. |
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Why near-inertial waves are less affected by vorticity in the Northeast Pacific than in the North Atlantic Thomas, L.N., S.M. Kelly, T. Klenz, W.R. Young, L. Rainville, H.L. Simmons, V. Hormann, and I. Stokes, "Why near-inertial waves are less affected by vorticity in the Northeast Pacific than in the North Atlantic," Oceanography, 37, 10-21, doi:10.5670/oceanog.2024.301, 2025. |
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1 Dec 2024 ![]() |
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Over 35 years ago, the influential Ocean Storms Experiment (OSE) in the Northeast Pacific documented, for the first time, the generation of near-inertial waves (NIWs) by a storm and the subsequent radiation of the waves away from the forcing. The NIWs were observed to radiate equatorward and downward, consistent with the theory of β-refraction, which attributes such NIW propagation to the gradient in Earth’s planetary vorticity, β. Surprisingly, there was no evidence that gradients in the vorticity of mesoscale eddies in the region affected the NIWs, despite the fact that these gradients were nearly 10 times larger than β. In contrast, NIWs observed in the recent Near-Inertial Shear and Kinetic Energy in the North Atlantic Experiment (NISKINe) were strongly affected by the mesoscale eddy field in the region. In this article we explain the distinct behavior of the NIWs observed in the two experiments through a careful reanalysis of the observations, which are then interpreted using simulations and NIW-mean flow interaction theory. The observed differences can be partially attributed to how NIWs were measured in the two experiments. But more interestingly, we find that wind energy was injected primarily into low vertical modes during OSE and more broadly into higher modes during NISKINe. This, combined with the stronger stratification in the Northeast Pacific, implies that NIWs are more dispersive and hence less susceptible to being modified by vorticity there than they are in the North Atlantic. |
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Characterization of mixing at the edge of a Kuroshio intrusion into the South China Sea: Analysis of thermal variance diffusivity measurements Sanchez-Rios, A., R.K. Shearman, C.M. Lee, H.L. Simmons, L. St. Laurent, A.J. Lucas, T. Ijichi, and S. Jan, "Characterization of mixing at the edge of a Kuroshio intrusion into the South China Sea: Analysis of thermal variance diffusivity measurements," J. Phys. Oceanogr., 54, 1121-1142, doi:10.1175/JPO-D-23-0007.1, 2024. |
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15 Jan 2024 ![]() |
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The Kuroshio occasionally carries warm and salty North Pacific Water into fresher waters of the South China Sea, forming a front with a complex temperature-salinity (T-S) structure to the west of the Luzon Strait. In this study, we examine the T-S interleavings formed by alternating layers of North Pacific water with South China Sea water in a front formed during the winter monsoon season of 2014. Using observations from a glider array following a free-floating wave-powered vertical profiling float to calculate the fine-scale parameters Turner angle, Tu, and Richardson number, Ri, we identified areas favorable to double diffusion convection and shear instability observed in a T-S interleaving. We evaluated the contribution of double diffusion convection and shear instabilities to the thermal variance diffusivity, X, using microstructure data and compared it with previous parameterization schemes based on fine-scale properties. We discover that turbulent mixing is not accurately parameterized when both Tu and Ri are within critical ranges (Tu > 60, Ri < 1/4). In particular, X associated with salt finger processes was an order of magnitude higher (6.7 x 10-7 K2 s-1) than in regions where only velocity shear was likely to drive mixing (8.7 x 10-8 K2 s-1). |