Professor and Associate Director, Marine Physical Laboratory

Research Interests

  • Ocean dynamics (including internal waves, the mixed layer, abyssal overflows and turbulence) and their impact on the global circulation and coastal ecosystems

Education

  • B.A. Astrophysics, Swarthmore College, 1993
  • Ph.D. Oceanography, Scripps Institution of Oceanography, 1998

Recent Publications

MacKinnon, JA, Alford MH, Voet G, Zeiden K, Johnston STM, Siegelman M, Merrifield S, Merrifield M.  2019.  Eddy wake generation from broadband currents near Palau. Journal of Geophysical Research: Oceans. Abstract

Wake eddies are frequently created by flow separation where ocean currents encounter abrupt topography in the form of islands or headlands. Most previous work has concentrated on wake eddy generation by either purely oscillatory (usually tidal) currents, or quasi-steady mean flows. Here we report measurements near the point of flow separation at the northern end of the Palau island chain, where energetic tides and vertically sheared low-frequency flows are both present. Energetic turbulence measured near the very steeply sloping ocean floor varied cubically with the total flow speed (primarily tidal). The estimated turbulent viscosity suggests a regime of flow separation and eddying wake generation for flows that directly feel this drag. Small-scale (∼ 1 km), vertically sheared wake eddies of different vorticity signs were observed with a ship-board survey on both sides of the separation point, and significantly evolved over several tidal periods. The net production and export of vorticity into the wake, expected to sensitively depend on the interplay of tidal and low frequency currents, is explored here with a simple conceptual model. Application of the model to a 10-month mooring record suggests that inclusion of high frequency oscillatory currents may boost the net flux of vorticity into the ocean interior by a depth dependent factor of 2 to 25. Models that do not represent the effect of these high frequency currents may not accurately infer the net momentum or energy losses felt where strong flows encounter steep island or headland topography.

Pratt, LJ, Voet G, Pacini A, Tan S, Alford MH, Carter GS, Girton JB, Menemenlis D.  2019.  Pacific Abyssal Transport and Mixing: Through the Samoan Passage versus around the Manihiki Plateau. Journal of Physical Oceanography. 49:1577-1592. AbstractWebsite

AbstractThe main source feeding the abyssal circulation of the North Pacific is the deep, northward flow of 5–6 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) through the Samoan Passage. A recent field campaign has shown that this flow is hydraulically controlled and that it experiences hydraulic jumps accompanied by strong mixing and dissipation concentrated near several deep sills. By our estimates, the diapycnal density flux associated with this mixing is considerably larger than the diapycnal flux across a typical isopycnal surface extending over the abyssal North Pacific. According to historical hydrographic observations, a second source of abyssal water for the North Pacific is 2.3–2.8 Sv of the dense flow that is diverted around the Manihiki Plateau to the east, bypassing the Samoan Passage. This bypass flow is not confined to a channel and is therefore less likely to experience the strong mixing that is associated with hydraulic transitions. The partitioning of flux between the two branches of the deep flow could therefore be relevant to the distribution of Pacific abyssal mixing. To gain insight into the factors that control the partitioning between these two branches, we develop an abyssal and equator-proximal extension of the “island rule.” Novel features include provisions for the presence of hydraulic jumps as well as identification of an appropriate integration circuit for an abyssal layer to the east of the island. Evaluation of the corresponding circulation integral leads to a prediction of 0.4–2.4 Sv of bypass flow. The circulation integral clearly identifies dissipation and frictional drag effects within the Samoan Passage as crucial elements in partitioning the flow.

Wagner, GL, Flierl G, Ferrari R, Voet G, Carter GS, Alford MH, Girton JB.  2019.  Squeeze dispersion and the effective diapycnal diffusivity of oceanic tracers. Geophysical Research Letters. 46:5378-5386. Abstract

Abstract We describe a process called “squeeze dispersion” in which the squeezing of oceanic tracer gradients by waves, eddies, and bathymetric flow modulates diapycnal diffusion by centimeter to meter-scale turbulence. Due to squeeze dispersion, the effective diapycnal diffusivity of oceanic tracers is different and typically greater than the average “local” diffusivity, especially when local diffusivity correlates with squeezing. We develop a theory to quantify the effects of squeeze dispersion on diapycnal oceanic transport, finding formulas that connect density-averaged tracer flux, locally measured diffusivity, large-scale oceanic strain, the thickness-weighted average buoyancy gradient, and the effective diffusivity of oceanic tracers. We use this effective diffusivity to interpret observations of abyssal flow through the Samoan Passage reported by Alford et al. (2013, https://doi.org/10.1002/grl.50684) and find that squeezing modulates diapycnal tracer dispersion by factors between 0.5 and 3.

Alford, MH, Simmons HL, Marques OB, Girton JB.  2019.  Internal tide attenuation in the North Pacific. Geophysical Research Letters. 46:8205-8213. Abstract

Multisatellite altimetry and an eddy-resolving model with tides are used to quantify the attenuation of the mode-1 M2 internal tide as it propagates from three major sources in the North Pacific. The model is used to correct the altimetric fluxes for the nonstationary signal that altimeters cannot detect. Because internal tides in the North Pacific are highly stationary, these corrections do not materially impact the decay rate estimates. Fluxes are integrated in wedges extending from the sources to account for interference and radial spreading. Observed attenuation rates are consistent with e-folding scales between 750 and 3,000 km, suggesting weak dissipation rates (≤10−10 W/kg or 0.75×10−3 W/m2) compared to typical open-ocean turbulence levels, implicating near-inertial waves and higher-mode internal tides in providing the balance of the dissipation in the ocean interior.

Brizuela, N, Filonov A, Alford MH.  2019.  Internal tsunami waves transport sediment released by underwater landslides. Scientific Reports. 9 AbstractWebsite

Accelerated by gravity, submarine landslides transfer energy to the marine environment, most notably leading to catastrophic tsunamis. While tsunamis are thought to use less than 15% of the total energy released by landslides, little is known about subsurface processes comprising the rest of their energy budgets. Here, we analyze the first set of observations depicting a lake's interior response to underwater landslides and find that sediment transport is modulated by baroclinic waves that propagate along vertical gradients in temperature and sediment concentration. When traveling along a shallow thermocline, these waves can reach past topographic features that bound turbidity currents and thus expand the influence area of underwater landslides. With order of magnitude calculations, we estimate that observed thermocline internal waves received roughly 0.7% of available landslide energy and infer their contribution to homogenize the lake's thermodynamical properties by means of turbulent mixing. Lastly, we show that landslides in our data set modified the lake's intrinsic dynamical modes and thus had a permanent impact on its circulation. This suggests that measurements of subsurface wave propagation are sufficient to diagnose bathymetric transformations. Our experiment constitutes the first direct observation of both internal tsunami waves and turbidity current reflection. Moreover, it demonstrates that background density stratification has a significant effect on the transport of sediment after submarine landslides and provides a valuable reference for numerical models that simulate submarine mass failures.