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

Alford, MH, MacKinnon JA, Simmons HL, Nash JD.  2016.  Near-inertial internal gravity waves in the ocean. Annual Review of Marine Science, Vol 8. 8( Carlson CA, Giovannoni SJ, Eds.).:95-123., Palo Alto: Annual Reviews Abstract

We review the physics of near-inertial waves (NIWs) in the ocean and the observations, theory, and models that have provided our present knowledge. NIWs appear nearly everywhere in the ocean as a spectral peak at and just above the local inertial period f, and the longest vertical wavelengths can propagate at least hundreds of kilometers toward the equator from their source regions; shorter vertical wavelengths do not travel as far and do not contain as much energy, but lead to turbulent mixing owing to their high shear. NIWs are generated by a variety of mechanisms, including the wind, nonlinear interactions with waves of other frequencies, lee waves over bottom topography, and geostrophic adjustment; the partition among these is not known, although the wind is likely the most important. NIWs likely interact strongly with mesoscale and submesoscale motions, in ways that are just beginning to be understood.

Zhao, Z, Alford MH, Girton JB, Rainville L, Simmons HL.  2016.  Global observations of open-ocean mode-1 M2 internal tides. Journal of Physical Oceanography. 46:1657-1684. AbstractWebsite

AbstractA global map of open-ocean mode-1 M2 internal tides is constructed using sea surface height (SSH) measurements from multiple satellite altimeters during 1992–2012, representing a 20-yr coherent internal tide field. A two-dimensional plane wave fit method is employed to 1) suppress mesoscale contamination by extracting internal tides with both spatial and temporal coherence and 2) separately resolve multiple internal tidal waves. Global maps of amplitude, phase, energy, and flux of mode-1 M2 internal tides are presented. The M2 internal tides are mainly generated over topographic features, including continental slopes, midocean ridges, and seamounts. Internal tidal beams of 100–300 km width are observed to propagate hundreds to thousands of kilometers. Multiwave interference of some degree is widespread because of the M2 internal tide’s numerous generation sites and long-range propagation. The M2 internal tide propagates across the critical latitudes for parametric subharmonic instability (28.8°S/N) with little energy loss, consistent with the 2006 Internal Waves across the Pacific (IWAP) field measurements. In the eastern Pacific Ocean, the M2 internal tide loses significant energy in propagating across the equator; in contrast, little energy loss is observed in the equatorial zones of the Atlantic, Indian, and western Pacific Oceans. Global integration of the satellite observations yields a total energy of 36 PJ (1 PJ = 1015 J) for all the coherent mode-1 M2 internal tides. Finally, satellite observed M2 internal tides compare favorably with field mooring measurements and a global eddy-resolving numerical model.

MacKinnon, JA, Nash JD, Alford MH, Lucas AJ, Mickett JB, Shroyer EL, Waterhouse AF, Tandon A, Sengupta D, Mahadevan A, Ravichandran M, Pinkel R, Rudnick DL, Whalen CB, Alberty MS, Lekha JS, Fine EC, Chaudhuri D, Wagner GL.  2016.  A tale of two spicy seas. Oceanography. 29:50-61. AbstractWebsite

Upper-ocean turbulent heat fluxes in the Bay of Bengal and the Arctic Ocean drive regional monsoons and sea ice melt, respectively, important issues of societal interest. In both cases, accurate prediction of these heat transports depends on proper representation of the small-scale structure of vertical stratification, which in turn is created by a host of complex submesoscale processes. Though half a world apart and having dramatically different temperatures, there are surprising similarities between the two: both have (1) very fresh surface layers that are largely decoupled from the ocean below by a sharp halocline barrier, (2) evidence of interleaving lateral and vertical gradients that set upper-ocean stratification, and (3) vertical turbulent heat fluxes within the upper ocean that respond sensitively to these structures. However, there are clear differences in each ocean's horizontal scales of variability, suggesting that despite similar background states, the sharpening and evolution of mesoscale gradients at convergence zones plays out quite differently. Here, we conduct a qualitative and statistical comparison of these two seas, with the goal of bringing to light fundamental underlying dynamics that will hopefully improve the accuracy of forecast models in both parts of the world.

Voet, G, Alford MH, Girton JB, Carter GS, Mickett JB, Klymak JM.  2016.  Warming and weakening of the abyssal flow through Samoan Passage. Journal of Physical Oceanography. 46:2389-2401. AbstractWebsite

The abyssal flow of water through the Samoan Passage accounts for the majority of the bottom water renewal in the North Pacific, thereby making it an important element of the meridional overturning circulation. Here the authors report recent measurements of the flow of dense waters of Antarctic and North Atlantic origin through the Samoan Passage. A 15-month long moored time series of velocity and temperature of the abyssal flow was recorded between 2012 and 2013. This allows for an update of the only prior volume transport time series from the Samoan Passage from WOCE moored measurements between 1992 and 1994. While highly variable on multiple time scales, the overall pattern of the abyssal flow through the Samoan Passage was remarkably steady. The time-mean northward volume transport of about 5.4 Sv (1 Sv = 10(6) m(3) s(-1)) in 2012/13 was reduced compared to 6.0 Sv measured between 1992 and 1994. This volume transport reduction is significant within 68% confidence limits (60.4 Sv) but not at 95% confidence limits (+/-0.6 Sv). In agreement with recent studies of the abyssal Pacific, the bottom flow through the Samoan Passage warmed significantly on average by 1 x 10(-38)Cyr(-1) over the past two decades, as observed both in moored and shipboard hydrographic observations. While the warming reflects the recently observed increasing role of the deep oceans for heat uptake, decreasing flow through Samoan Passage may indicate a future weakening of this trend for the abyssal North Pacific.

Chinn, BS, Girton JB, Alford MH.  2016.  The impact of observed variations in the shear-to-strain ratio of internal waves on inferred turbulent diffusivities. Journal of Physical Oceanography. 46:3299-3320. AbstractWebsite

The most comprehensive studies of the spatial and temporal scales of diffusivity rely on internal wave parameterizations that require knowledge of finescale shear and strain. Studies lacking either shear or strain measurements have to assume a constant ratio between shear and strain (R-omega). Data from 14 moorings collected during five field programs are examined to determine the spatial and temporal patterns in R-omega and the influence of these patterns on parameterized diffusivity. Time-mean R-omega ranges from 1 to 10, with changes of order 10 observed over a broad range of scales. Temporal variability in R-omega is observed at daily, weekly, and monthly scales. Observed changes in R-omega could produce a 2-3 times change in parameterized diffusivity. Vertical profiles of R-omega, E-shear, and E-strain (shear or strain variance relative to Garret-Munk) reveal that both local topographic properties and wind variability impact the internal wave field. Time series of R-omega from each mooring have strong correlations to either shear or strain, often only at a specific range of vertical wave-numbers. Sites fall into two categories, in which R-omega variability is dominated by either shear or strain. Linear fits to the dominant property (i.e., shear or strain) can be used to estimate a time series of R-omega that has an RMS error that is 30% less than the RMS error from assuming R-omega = 3. Shear and strain level vary in concert, as predicted by the Garret-Munk model, at high E-shear values. However, at E-shear, 5, strain variations are 3 times weaker than shear.