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.
10.1175/jpo-d-15-0161.1 AbstractThe 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.
Klymak, JM, Simmons HL, Braznikov D, Kelly S, MacKinnon JA, Alford MH, Pinkel R, Nash JD.
2016.
Reflection of linear internal tides from realistic topography: The Tasman continental slope. Journal of Physical Oceanography. 46:3321-3337.
10.1175/jpo-d-16-0061.1 AbstractThe reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of themode-1 internal tide asmode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget.
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.
10.1175/jpo-d-16-0063.1 AbstractThe 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.
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.
10.1175/JPO-D-15-0105.1 AbstractAbstractA 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.
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
10.1146/annurev-marine-010814-015746 AbstractWe 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.