Research Interests

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


  • 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.

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.

Alford, MH, MacKinnon JA, Pinkel R, Klymak JM.  2017.  Space-time scales of shear in the North Pacific. Journal of Physical Oceanography. 47:2455-2478. AbstractWebsite

The spatial, temporal, and directional characteristics of shear are examined in the upper 1400m of the North Pacific during late spring with an array of five profiling moorings deployed from 25 degrees to 37 degrees N (1330 km) and simultaneous shipboard transects past them. The array extended from a regime of moderate wind generation at the north to south of the critical latitude 28.8 degrees N, where parametric subharmonic instability (PSI) can transfer energy from semidiurnal tides to near-inertial motions. Analyses are done in an isopycnal-following frame to minimize contamination by Doppler shifting. Approximately 60% of RMS shear at vertical scales >20m (and 80% for vertical scales >80 m) is contained in near-inertial motions. An inertial back-rotation technique is used to index shipboard observations to a common time and to compute integral time scales of the shear layers. Persistence times are O(7) days at most moorings but O(25) days at the critical latitude. Simultaneous shipboard transects show that these shear layers can have lateral scales >= 100 km. Layers tend to slope downward toward the equator north of the critical latitude and are more flat to its south. Phase between shear and strain is used to infer lateral propagation direction. Upgoing waves are everywhere laterally isotropic. Downgoing waves propagate predominantly equatorward north and south of the critical latitude but are isotropic near it. Broadly, results are consistent with wind generation north of the critical latitude and PSI near it-and suggest a more persistent and laterally coherent near-inertial wave field than previously thought.

Savage, AC, Arbic BK, Alford MH, Ansong JK, Farrar JT, Menemenlis D, O'Rourke AK, Richman JG, Shriver JF, Voet G, Wallcraft AJ, Zamudio L.  2017.  Spectral decomposition of internal gravity wave sea surface height in global models. Journal of Geophysical Research-Oceans. 122:7803-7821. AbstractWebsite

Two global ocean models ranging in horizontal resolution from 1/12 degrees to 1/48 degrees are used to study the space and time scales of sea surface height (SSH) signals associated with internal gravity waves (IGWs). Frequency-horizontal wavenumber SSH spectral densities are computed over seven regions of the world ocean from two simulations of the HYbrid Coordinate Ocean Model (HYCOM) and three simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). High wavenumber, high-frequency SSH variance follows the predicted IGW linear dispersion curves. The realism of high-frequency motions (> 0.87cpd) in the models is tested through comparison of the frequency spectral density of dynamic height variance computed from the highest-resolution runs of each model (1/25 degrees HYCOM and 1/48 degrees MITgcm) with dynamic height variance frequency spectral density computed from nine in situ profiling instruments. These high-frequency motions are of particular interest because of their contributions to the small-scale SSH variability that will be observed on a global scale in the upcoming Surface Water and Ocean Topography (SWOT) satellite altimetry mission. The variance at supertidal frequencies can be comparable to the tidal and low-frequency variance for high wavenumbers (length scales smaller than approximate to 50 km), especially in the higher-resolution simulations. In the highest-resolution simulations, the high-frequency variance can be greater than the low-frequency variance at these scales.

Luecke, CA, Arbic BK, Bassette SL, Richman JG, Shriver JF, Alford MH, Smedstad OM, Timko PG, Trossman DS, Wallcraft AJ.  2017.  The global mesoscale eddy available potential energy field in models and observations. Journal of Geophysical Research-Oceans. 122:9126-9143. AbstractWebsite

Global maps of the mesoscale eddy available potential energy (EAPE) field at a depth of 500 m are created using potential density anomalies in a high-resolution 1/12.5 degrees global ocean model. Maps made from both a free-running simulation and a data-assimilative reanalysis of the HYbrid Coordinate Ocean Model (HYCOM) are compared with maps made by other researchers from density anomalies in Argo profiles. The HYCOM and Argo maps display similar features, especially in the dominance of western boundary currents. The reanalysis maps match the Argo maps more closely, demonstrating the added value of data assimilation. Global averages of the simulation, reanalysis, and Argo EAPE all agree to within about 10%. The model and Argo EAPE fields are compared to EAPE computed from temperature anomalies in a data set of moored historical observations (MHO) in conjunction with buoyancy frequencies computed from a global climatology. The MHO data set allows for an estimate of the EAPE in high-frequency motions that is aliased into the Argo EAPE values. At MHO locations, 15-32% of the EAPE in the Argo estimates is due to aliased motions having periods of 10 days or less. Spatial averages of EAPE in HYCOM, Argo, and MHO data agree to within 50% at MHO locations, with both model estimates lying within error bars observations. Analysis of the EAPE field in an idealized model, in conjunction with published theory, suggests that much of the scatter seen in comparisons of different EAPE estimates is to be expected given the chaotic, unpredictable nature of mesoscale eddies.