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

Thorpe, SA, Malarkey J, Voet G, Alford MH, Girton JB, Carter GS.  2018.  Application of a model of internal hydraulic jumps. Journal of Fluid Mechanics. 834:125-148. AbstractWebsite

A model devised by Thorpe & Li (J. Fluid Mech., vol. 758, 2014, pp. 94-120) that predicts the conditions in which stationary turbulent hydraulic jumps can occur in the flow of a continuously stratified layer over a horizontal rigid bottom is applied to, and its results compared with, observations made at several locations in the ocean. The model identifies two positions in the Samoan Passage at which hydraulic jumps should occur and where changes in the structure of the flow are indeed observed. The model predicts the amplitude of changes and the observed mode 2 form of the transitions. The predicted dissipation of turbulent kinetic energy is also consistent with observations. One location provides a particularly well-defined example of a persistent hydraulic jump. It takes the form of a 390 m thick and 3.7 km long mixing layer with frequent density inversions separated from the seabed by some 200 m of relatively rapidly moving dense water, thus revealing the previously unknown structure of an internal hydraulic jump in the deep ocean. Predictions in the Red Sea Outflow in the Gulf of Aden are relatively uncertain. Available data, and the model predictions, do not provide strong support for the existence of hydraulic jumps. In the Mediterranean Outflow, however, both model and data indicate the presence of a hydraulic jump.

MacKinnon, JA, Alford MH, Ansong JK, Arbic BK, Barna A, Briegleb BP, Bryan FO, Buijsman MC, Chassignet EP, Danabasoglu G, Diggs S, Griffies SM, Hallberg RW, Jayne SR, Jochum M, Klymak JM, Kunze E, Large WG, Legg S, Mater B, Melet AV, Merchant LM, Musgrave R, Nash JD, Norton NJ, Pickering A, Pinkel R, Polzin K, Simmons HL, Laurent LSC, Sun OM, Trossman DS, Waterhouse AF, Whalen CB, Zhao Z.  2017.  Climate process team on internal-wave driven ocean mixing. Bulletin of the American Meteorological Society. Abstract

Recent advances in our understanding of internal-wave driven turbulent mixing in the ocean interior are summarized. New parameterizations for global climate ocean models, and their climate impacts, are introduced.Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

Savage, AC, Arbic BK, Richman JG, Shriver JF, Alford MH, Buijsman MC, Farrar JT, Sharma H, Voet G, Wallcraft AJ, Zamudio L.  2017.  Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies. Journal of Geophysical Research-Oceans. 122:2519-2538. AbstractWebsite

High horizontal-resolution (1/12: 5 degrees and 1/25 degrees) 41-layer global simulations of the HYbrid Coordinate Ocean Model (HYCOM), forced by both atmospheric fields and the astronomical tidal potential, are used to construct global maps of sea surface height (SSH) variability. The HYCOM output is separated into steric and nonsteric and into subtidal, diurnal, semidiurnal, and supertidal frequency bands. The model SSH output is compared to two data sets that offer some geographical coverage and that also cover a wide range of frequencies-a set of 351 tide gauges that measure full SSH and a set of 14 in situ vertical profilers from which steric SSH can be calculated. Three of the global maps are of interest in planning for the upcoming Surface Water and Ocean Topography (SWOT) two-dimensional swath altimeter mission: (1) maps of the total and (2) nonstationary internal tidal signal (the latter calculated after removing the stationary internal tidal signal via harmonic analysis), with an average variance of 1: 05 and 0: 43 cm(2), respectively, for the semidiurnal band, and (3) a map of the steric supertidal contributions, which are dominated by the internal gravity wave continuum, with an average variance of 0: 15 cm2. Stationary internal tides (which are predictable), nonstationary internal tides (which will be harder to predict), and nontidal internal gravity waves (which will be very difficult to predict) may all be important sources of high-frequency "noise" that could mask lower frequency phenomena in SSH measurements made by the SWOT mission.

Ansong, JK, Arbic BK, Alford MH, Buijsman MC, Shriver JF, Zhao ZX, Richman JG, Simmons HL, Timko PG, Wallcraft AJ, Zamudio L.  2017.  Semidiurnal internal tide energy fluxes and their variability in a Global Ocean Model and moored observations. Journal of Geophysical Research-Oceans. 122:1882-1900. AbstractWebsite

We examine the temporal means and variability of the semidiurnal internal tide energy fluxes in 1/25 degrees global simulations of the Hybrid Coordinate Ocean Model (HYCOM) and in a global archive of 79 historical moorings. Low-frequency flows, a major cause of internal tide variability, have comparable kinetic energies at the mooring sites in model and observations. The computed root-mean-square (RMS) variability of the energy flux is large in both model and observations and correlates positively with the time-averaged flux magnitude. Outside of strong generation regions, the normalized RMS variability (the RMS variability divided by the mean) is nearly independent of the flux magnitudes in the model, and of order 23% or more in both the model and observations. The spatially averaged flux magnitudes in observations and the simulation agree to within a factor of about 1.4 and 2.4 for vertical mode-1 and mode-2, respectively. The difference in energy flux computed from the full-depth model output versus model output subsampled at mooring instrument depths is small. The global historical archive is supplemented with six high-vertical resolution moorings from the Internal Waves Across the Pacific (IWAP) experiment. The model fluxes agree more closely with the high-resolution IWAP fluxes than with the historical mooring fluxes. The high variability in internal tide energy fluxes implies that internal tide fluxes computed from short observational records should be regarded as realizations of a highly variable field, not as "means" that are indicative of conditions at the measurement sites over all time.

Alberty, MS, Billheimer S, Hamann MM, Ou CY, Tamsitt V, Lucas AJ, Alford MH.  2017.  A reflecting, steepening, and breaking internal tide in a submarine canyon. Journal of Geophysical Research-Oceans. 122:6872-6882. AbstractWebsite

Submarine canyons are common features of the coastal ocean. Although they are known to be hotspots of turbulence that enhance diapycnal transport in their stratified waters, the dynamics of canyon mixing processes are poorly understood. Most studies of internal wave dynamics within canyons have focused on a handful of canyons with along-axis slopes less steep than semidiurnal (D-2) internal wave characteristics (subcritical). Here, we present the first tidally resolving observations within a canyon with a steeply sloping axis (supercritical). A process study consisting of two 24 h shipboard stations and a profiling mooring was conducted in the La Jolla Canyon off the coast of La Jolla, CA. Baroclinic energy flux is oriented up-canyon and decreases from 182 +/- 18 W m(-1) at the canyon mouth to 46 +/- 5 W m(-1) near the head. The ratio of horizontal kinetic energy to available potential energy and the observed group speed of each mode are lower than expected for freely propagating D-2 internal waves at each station, indicating partial reflection. Harmonic analysis reveals that variance is dominated by the D-2 tide. Moving up-canyon, the relative importance of D-2 decreases and its higher harmonics are needed to account for a majority of the observed variance, indicating steepening. Steep internal tides cause large isopycnal displacements (approximate to 50 m in 100 m water depth) and high strain events. These events coincide with enhanced O( 10-7-10-5 m(2) s(-3)) dissipation of turbulent kinetic energy at mid-depths.