Publications

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2014
Alford, MH, Klymak JM, Carter GS.  2014.  Breaking internal lee waves at Kaena Ridge, Hawaii. Geophysical Research Letters. 41:906-912.   10.1002/2013GL059070   AbstractWebsite

Shallow water oscillatory flows and deep ocean steady flows have both been observed to give rise to breaking internal lee waves downstream of steep seafloor obstacles. A recent theory also predicts the existence of high-mode oscillatory internal lee waves in deep water, but they have not previously been directly observed. Here we present repeated spatial transects of velocity, isopycnal displacement, and dissipation rate measured with towed instruments on the south flank of a supercritical ridge in Hawaii known as Kaena Ridge and compare them with predictions from a 3-D numerical model with realistic tidal forcing, bathymetry, and stratification. The measured and modeled flow and turbulence agree well in their spatial structure, time dependence, and magnitude, confirming the existence and predicted nature of high-mode internal lee waves. Turbulence estimated from Thorpe scales increases 2 orders of magnitude following downslope tidal flow, when the internal lee wave begins to propagate upslope and breaks.

Alford, MH, MacCready P.  2014.  Flow and mixing in Juan de Fuca Canyon, Washington. Geophysical Research Letters. 41:1608-1615.   10.1002/2013GL058967   AbstractWebsite

We report breaking internal lee waves, strong mixing, and hydraulic control associated with wind-driven up-canyon flow in Juan de Fuca Canyon, Washington. Unlike the flow above the canyon rim, which shows a tidal modulation typical on continental shelves, the flow within the canyon is persistently up-canyon during our observations, with isopycnals tilted consistent with a geostrophic cross-canyon momentum balance. As the flow encounters a sill near the canyon entrance at the shelf break, it accelerates significantly and undergoes elevated mixing on the upstream and downstream sides of the sill. On the downstream side, a strong lee wave response is seen, with displacements of O(100 m) and overturns tens of meters high. The resulting diffusivity is O(10−2 m2 s−1), sufficient to substantially modify coastal water masses as they transit the canyon and enter the Salish Sea estuarine system.

2013
Hosegood, PJ, Gregg MC, Alford MH.  2013.  Wind-driven submesoscale subduction at the north Pacific subtropical front. Journal of Geophysical Research: Oceans. 118:5333-5352.   10.1002/jgrc.20385   AbstractWebsite

Upper ocean observations from the north Pacific subtropical front during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss. Prior to generation, a sharp thermohaline front was intensified by confluent flow of 1–2 × 10−5 s−1. Relative vertical vorticity, ζ, across a surface-intensified, along-front jet on the warm side of a frontal trough was 0.5 f. During the storm, buoyancy loss arose due to cooling of ∼650 W m−2 and down-front wind stress <0.5 N m−2 that generated a southward, cross-front Ekman transport of dense water over light. The resulting wind-driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude. The intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline. They were approximately 20 m thick and horizontally elongated in the cross-frontal direction. The near-surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline, whereas the subsurface intrusions were composed of warm and saline water from the surface. The apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterized by O(1) Rossby and Richardson numbers, pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer, and sloping isopycnals. The intrusion properties, background environmental context, and forcing history are consistent with prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss, possibly resulting in symmetric instability.

Alford, MH, Girton JB, Voet G, Carter GS, Mickett JB, Klymak JM.  2013.  Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage. Geophysical Research Letters. 40:4668-4674.   10.1002/grl.50684   AbstractWebsite

We report the first direct turbulence observations in the Samoan Passage (SP), a 40 km wide notch in the South Pacific bathymetry through which flows most of the water supplying the North Pacific abyssal circulation. The observed turbulence is 1000 to 10,000 times typical abyssal levels —strong enough to completely mix away the densest water entering the passage—confirming inferences from previous coarser temperature and salinity sections. Accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence. Specifically, the flow accelerates substantially at the primary sill within the passage, reaching speeds as great as 0.55 m s−1. A strong hydraulic response is seen, with layers first rising to clear the sill and then plunging hundreds of meters downward. Turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill. In addition to the primary sill, other locations along the multiple interconnected channels through the Samoan Passage also have an effect on the mixing of the dense water. In fact, quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers, suggesting that abyssal mixing depends sensitively on bathymetric details on small scales.

2011
Gregg, MC, Hall RA, Carter GS, Alford MH, Lien RC, Winkel DP, Wain DJ.  2011.  Flow and mixing in Ascension, a steep, narrow canyon. Journal of Geophysical Research: Oceans. 116:C07016.   10.1029/2010JC006610   AbstractWebsite

A thin gash in the continental slope northwest of Monterey Bay, Ascension Canyon, is steep, with sides and axis both strongly supercritical to M2 internal tides. A hydrostatic model forced with eight tidal constituents shows no major sources feeding energy into the canyon, but significant energy is exchanged between barotropic and baroclinic flows along the tops of the sides, where slopes are critical. Average turbulent dissipation rates observed near spring tide during April are half as large as a two week average measured during August in Monterey Canyon. Owing to Ascension's weaker stratification, however, its average diapycnal diffusivity, 3.9 × 10−3 m2 s−1, exceeded the 2.5 × 10−3 m2 s−1 found in Monterey. Most of the dissipation occurred near the bottom, apparently associated with an internal bore, and just below the rim, where sustained cross-canyon flow may have been generating lee waves or rotors. The near-bottom mixing decreased sharply around Ascension's one bend, as did vertically integrated baroclinic energy fluxes. Dissipation had a minor effect on energetics, which were controlled by flux divergences and convergences and temporal changes in energy density. In Ascension, the observed dissipation rate near spring tide was 2.1 times that predicted from a simulation using eight tidal constituents averaged over a fortnightly period. The same observation was 1.5 times the average of an M2-only prediction. In Monterey, the previous observed average was 4.9 times the average of an M2-only prediction.

2007
Nash, JD, Alford MH, Kunze E, Martini K, Kelly S.  2007.  Hotspots of deep ocean mixing on the Oregon continental slope. Geophysical Research Letters. 34:L01605.   10.1029/2006GL028170   AbstractWebsite

Two deep ocean hotspots of turbulent mixing were found over the Oregon continental slope. Thorpe-scale analyses indicate time-averaged turbulent energy dissipation rates of ε > 10−7 W/kg and eddy diffusivities of Kρ ∼ 10−2 m2/s at both hotspots. However, the structure of turbulence and its generation mechanism at each site appear to be different. At the 2200-m isobath, sustained >100-m high turbulent overturns occur in stratified fluid several hundred meters above the bottom. Turbulence shows a clear 12.4-h periodicity proposed to be driven by flow over a nearby 100-m tall ridge. At the 1300-m isobath, tidally-modulated turbulence of similar intensity is confined within a stratified bottom boundary layer. Along-slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed. Such topography is common to most continental slopes, providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along-isobath.

2003
Alford, MH.  2003.  Improved global maps and 54-year history of wind-work on ocean inertial motions. Geophysical Research Letters. 30:1424.   10.1029/2002GL016614   AbstractWebsite

The global distribution and 54-year time dependence of the energy-flux from the wind to near-inertial motions is computed by driving a slab mixed-layer model with NCEP/NCAR Reanalysis winds, improving upon previous estimates [Alford, 2001; Watanabe and Hibiya, 2002]. The slab model is solved spectrally with frequency-dependent damping. The resulting solutions are more physically sensible than the previous, and more skillful at high latitudes, where the inertial frequency approaches the 4×-daily sampling of the Reanalysis winds. This enables Alford's calculation, whose domain was limited to ±50°, to be extended to the poles. The high-latitude reliability is demonstrated by direct comparison with a high-resolution regional model (REMO) in the NE Atlantic. The total power input, 0.47 TW, has increased by 25% since 1948, paralleling observed increases in extratropical cyclone frequency and intensity. If believable, the trend may have important consequences for modulation of the meridional overturning circulation.

2001
Alford, MH, Gregg MC.  2001.  Near-inertial mixing: Modulation of shear, strain and microstructure at low latitude. Journal of Geophysical Research: Oceans. 106:16947-16968.   10.1029/2000JC000370   AbstractWebsite

We report direct, quantitative measurements of mixing associated with three cycles of a single, energetic, downward-propagating near-inertial wave in the Banda Sea at 6.5°S, 128°E during October 1998. The wave dominates the shear, containing 70% of the total variance. Simultaneous depth/time series of shear, strain, Froude number (Fr), and microstructure allow direct computation of their coherence and phase from 50–120 m, for 14 days. In this depth range, 72% of diapycnal diffusivity (68% of dissipation) occurs in three distinct pulses, spaced at the inertial period of 4.4 days. These are collocated with maxima of transverse shear, strain and Fr. Inertial-band log diapycnal diffusivity, log10 Kρ, is coherent at the 95% confidence level with both components of shear and Froude number. In this data set, strain is more important than shear in modulating Fr. Owing to the low latitude, the inertial frequency (fo = 1/4.4 cycles per day) is much smaller than the diurnal and tidal frequencies. Consequently, near-inertial motions may be studied separately from tides and other motions via time-domain filtering. Semiempirical WKB plane-wave solutions with observed frequency ωo = 1.02fo and vertical scale 100 m explain 66% and 42% of inertial-band shear and strain variance, respectively. On the basis of the observed phase relationship between shear and strain, the wave is propagating equatorward, toward 295° true. Ratios of shear to strain and of parallel to transverse shear suggest that the wave's intrinsic frequency ωI ≈ 1.18feff. This indicates that background vorticity ζ has lowered the effective Coriolis frequency, feff = fo + ζ/2, relative to its planetary value, fo [Kunze, 1985]. Ray tracing suggests that the wave was generated near 6.9°S, 130.6°E, ∼20 days prior to the cruise, coincident with the end of high winds associated with the SE monsoon. A slab mixed layer model [Pollard and Millard, 1970], forced with National Center for Environmental Prediction (NCEP) model surface winds, confirms that fluxes from the wind to the ocean at this time were sufficient to generate the wave. A very simple model shows that mixing by monsoon-generated inertial waves may add an important and strongly time-dependent aspect to some regions' energy budgets.