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
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 40km 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 passageconfirming inferences from previous coarser temperature and salinity sections. Accompanying towed measurements of velocity and temperature with horizontal resolution of about 250m 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 0.55m 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.

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.