Publications

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2016
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(8):2389-2401.   10.1175/JPO-D-16-0063.1   Abstract

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 circu- lation. 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 temper- ature 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 = 106 m3 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 (±0.4 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 × 10-3 °C yr-1 over the past two decades, as observed both in moored and shipboard hydrographic observations. While the warming reflects the recently observed in- creasing 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.

2015
Voet, G, Girton JB, Alford MH, Carter GS, Klymak JM, Mickett JB.  2015.  Pathways, Volume Transport and Mixing of Abyssal Water in the Samoan Passage. Journal of Physical Oceanography. 45(2):562-588.   10.1175/JPO-D-14-0096.1   Abstract

The flow of dense water through the Samoan Passage accounts for the major part of the bottom water renewal in the North Pacific and is thus an important element of the Pacific meridional overturning circulation. A recent set of highly resolved measurements used CTD/LADCP, a microstructure profiler, and moorings to constrain the complex pathways and variability of the abyssal flow. Volume transport estimates for the dense northward current at several sections across the passage, calculated using direct velocity measurements from LADCPs, range from 3.9 × 106 to 6.0 × 106 ± 1 × 106 m3 s-1. The deep channel to the east and shallower pathways to the west carried about equal amounts of this volume transport, with the densest water flowing along the main eastern channel. Turbulent dissipation rates estimated from Thorpe scales and direct microstructure agree to within a factor of 2 and provide a region-averaged value of O(10-8) W kg-1 for layers colder than 0.8°C. Associated diapycnal diffusivities and downward turbulent heat fluxes are about 5 × 10-3 m2 s-1 and O(10) W m-2, respectively. However, heat budgets suggest heat fluxes 2–6 times greater. In the vicinity of one of the major sills of the passage, highly resolved Thorpe-inferred diffusivity and heat flux were over 10 times larger than the region-averaged values, suggesting the mismatch is likely due to undersampled mixing hotspots.

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(17):4668–4674.   10.1002/grl.50684   Abstract

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.

2010
Voet, G, Quadfasel D.  2010.  Entrainment in the Denmark Strait overflow plume by meso-scale eddies. Ocean Science. 6:301–310.   10.5194/os-6-301-2010   Abstract

The entrainment of buoyant ambient water into the overflow plume of Denmark Strait and the associated downstream warming of the plume are estimated using time series of currents and temperature from moored instrumentation and classical hydrographic data. Warming rates are highest (0.4–0.5 K/100 km) within the first 200 km of the sill, and decrease to 0.05–0.1 K/100 km further downstream. Stirring by mesoscale eddies causes lateral heat fluxes that explain the 0.1 K/100 km warming, but in the first 200 km from the sill also vertical diapycnal fluxes, probably caused by breaking internal waves, must contribute to the entrainment.

Fer, I, Voet G, Seim KS, Rudels B, Latarius K.  2010.  Intense mixing of the Faroe Bank Channel overflow. Geophysical Research Letters. 37(2):L02604.   10.1029/2009GL041924   Abstract

The continuous, swift flow of cold water across the sill of the Faroe Bank Channel, the deepest passage from the Nordic Seas to the North Atlantic Ocean, forms a bottom-attached dense plume (overflow). The amount and distribution of entrainment and mixing that the overflow encounters during its descent influence the ventilation of the deep North Atlantic, however, remain poorly known due to lack of direct measurements. Using the first direct turbulence measurements, we describe the dynamic properties and mixing of the overflow plume as it descends toward the Iceland Basin. The vigorously turbulent plume is associated with intense mixing and enhanced turbulent dissipation near the bottom and at the plume-ambient interface, but with a quiescent core. Our measurements show a pronounced transverse circulation consistent with rotating plume dynamics, a strong lateral variability in entrainment velocity, and a vertical structure composed of order 100 m thick stratified interface and comparably thick well-mixed bottom boundary layer with significant transport and entrainment.

Voet, G, Quadfasel D, Mork KA, Søiland H.  2010.  The mid-depth circulation of the Nordic Seas derived from profiling float observations. Tellus A. 62(4):516–529.   10.1111/j.1600-0870.2010.00444.x   Abstract

The trajectories of 61 profiling Argo floats deployed at mid-depth in the Nordic Seas—the Greenland, Lofoten and Norwegian Basins and the Iceland Plateau—between 2001 and 2009 are analysed to determine the pattern, strength and variability of the regional circulation. The mid-depth circulation is strongly coupled with the structure of the bottom topography of the four major basins and of the Nordic Seas as a whole. It is cyclonic, both on the large-scale and on the basin scale, with weak flow (<1 cm s−1) in the interior of the basins and somewhat stronger flow (up to 5 cm s−1) at their rims. Only few floats moved from one basin to another, indicating that the internal recirculation within the basins is by far dominating the larger-scale exchanges. The seasonal variability of the mid-depth flow ranges from less than 1 cm s−1 over the Iceland Plateau to more than 4 cm s−1 in the Greenland Basin. These velocities translate into internal gyre transports of up to 15 ± 10 × 106 m3 s−1, several times the overall exchange between the Nordic Seas and the subpolar North Atlantic. The seasonal variability of the Greenland Basin and the Norwegian Basin can be adequately modelled using the barotropic vorticity equation, with the wind and bottom friction as the only forcing mechanisms. For the Lofoten Basin and the Iceland Plateau less than 50% of the variance can be explained by the wind.

2008
Dickson, RR, Dye S, Jónsson S, Köhl A, Macrander A, Marnela M, Meincke J, Olsen S, Rudels B, Valdimarsson H, Voet G.  2008.  The overflow flux west of Iceland: Variability, origins and forcing. Arctic–Subarctic Ocean Fluxes. ( Dickson RR, Meincke J, Rhines P, Eds.).:443–474.: Springer Abstract

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