Internal Wave Turbulence Near a Texel Beach

A summer bather entering a calm sea from the beach may sense alternating warm and cold water. This can be felt when moving forward into the sea (‘vertically homogeneous’ and ‘horizontally different’), but also when standing still between one’s feet and body (‘vertically different’). On a calm summer-day, an array of high-precision sensors has measured fast temperature-changes up to 1°C near a Texel-island (NL) beach. The measurements show that sensed variations are in fact internal waves, fronts and turbulence, supported in part by vertical stable stratification in density (temperature). Such motions are common in the deep ocean, but generally not in shallow seas where turbulent mixing is expected strong enough to homogenize. The internal beach-waves have amplitudes ten-times larger than those of the small surface wind waves. Quantifying their turbulent mixing gives diffusivity estimates of 10−4–10−3 m2 s−1, which are larger than found in open-ocean but smaller than wave breaking above deep sloping topography

S1 Constrains S4 in the Voltage Sensor Domain of Kv7.1 K+ Channels

Voltage-gated K+ channels comprise a central pore enclosed by four voltage-sensing domains (VSDs). While movement of the S4 helix is known to couple to channel gate opening and closing, the nature of S4 motion is unclear. Here, we substituted S4 residues of Kv7.1 channels by cysteine and recorded whole-cell mutant channel currents in Xenopus oocytes using the two-electrode voltage-clamp technique. In the closed state, disulfide and metal bridges constrain residue S225 (S4) nearby C136 (S1) within the same VSD. In the open state, two neighboring I227 (S4) are constrained at proximity while residue R228 (S4) is confined close to C136 (S1) of an adjacent VSD. Structural modeling predicts that in the closed to open transition, an axial rotation (∼190°) and outward translation of S4 (∼12 Å) is accompanied by VSD rocking. This large sensor motion changes the intra-VSD S1–S4 interaction to an inter-VSD S1–S4 interaction. These constraints provide a ground for cooperative subunit interactions and suggest a key role of the S1 segment in steering S4 motion during Kv7.1 gating

Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation1, 2, 3. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters4. Some of this deficit is made up by intense near-bottom mixing occurring in restricted ‘hot-spots’ associated with rough ocean-floor topography5, 6, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows7. Here we show that diapycnal mixing of mid-depth (~1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1Sv = 106 m3 s-1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography

Circulation in the Deep Brazil Basin

The Deep Basin Experiment (DBE), a part of the World Ocean Circulation Experiment (WOCE), is presently underway in the Brazil Basin of the South Atlantic. The program objectives and design philosophy are reviewed and early results are presented. Observations from a moored array along the southern boundary and neutrally buoyant float trajectories in the North Atlantic Deep Water and Antarctic Bottom Water are described with emphasis on their relationship to the recent flow schemes offered by Reid (1989). Also discussed are the process of cross isotherm mixing within the intense flow regime of the Verna Channel and observations of long period warming of the bottom water

Seasonal and spatial variations of Southern Ocean diapycnal mixing from Argo profiling floats

The Southern Ocean is thought to be one of the most energetic regions in the world’s oceans. As a result, it is a location of vigorous diapycnal mixing of heat, salt and biogeochemical properties1, 2, 3. At the same time, the Southern Ocean is poorly sampled, not least because of its harsh climate and remote location. Yet the spatial and temporal variation of diapycnal diffusivity in this region plays an important part in the large-scale ocean circulation and climate4, 5, 6. Here we use high-resolution hydrographic profiles from Argo floats in combination with the Iridium communications system to investigate diapycnal mixing in the Southern Ocean. We find that the spatial distribution of turbulent diapycnal mixing in the Southern Ocean at depths between 300 and 1,800 m is controlled by the topography, by means of its interaction with the Antarctic Circumpolar Current. The seasonal variation of this mixing can largely be attributed to the seasonal cycle of surface wind stress and is more pronounced in the upper ocean over flat topography. We suggest that additional high-resolution profiles from Argo floats will serve to advance our understanding of mixing processes in the global ocean interior