Hydrography and flow in the rift valley of the Mid-Atlantic Ridge

Slow-spreading mid-ocean ridges such as the Mid-Atlantic Ridge are characterized by deep axial rift valleys which are isolated from the water on the ridge flanks. Topographic effects therefore have a significant impact on the rift-valley hydrography and dynamics but little is known about the details. Known processes of global importance acting near the axes of midocean ridges include high rates of diapycnal mixing associated with the rough topography and high-temperature hydrothermal circulation, a major source for a number of chemical constituents of the ocean.Physical data sets from the rift valley of two connected segments of the Mid-Atlantic Ridge, which include the largest known hydrothermal vent field of the Atlantic, were analyzed to investigate the segment-scale hydrography, dynamics and geothermal fluxes. The data include two quasi-synoptic hydrographic and particle plume surveys (one year apart) and one-year-long records from an array of moored current meters.The hydrographic properties of the rift-valley water were similar during the two surveys, suggesting a stable state characterized by inflow from the eastern ridge flank, unidirectional along-segment flow (directly observed during an entire year), and monotonic along-valley hydrographic gradients consistent with high rates of diapycnal mixing. Geothermal processes do not appear to contribute significantly to these patterns. The data contain signaturesof a range of dynamical processes consistent with high rates of diapycnal mixing, including hydraulically controlled sill flows, topographic lee waves and high-energy tidal flows.The spatial distribution of the light-scattering anomalies associated with the dispersing hydrothermal particle plume are consistent with the dynamical observations. Close to the vent field the particle distribution is highly inhomogeneous but density-averaged profiles indicate that the mean plume is Gaussian in depth. To quantify the fluxes associated with the hydrothermal plume the corresponding hydrographic anomalies were determined. The complexity of the hydrography within the rift valley precludes the application of "standard" methods ho that a new method had to be developed resulting in the first quantitative hydrographic anomaly measurements of an Atlantic hydrothermal plume. The hydrographic and particle anomalies of this plume are linearly correlated, indicating that the particles behave conservatively in the near field.Estimates for the heat flux associated with the hydrothermal plume were derived using two established methods, one based on plume-rise modeling and the other on the advection of heat anomalies away from the vent field. Height-of-rise modeling yields values which are an order of magnitude too low because the plume model relies on a point-source assumption which is violated by the geometry of the vent field. The uncertainties associated with the advection method are particularly small at the site studied because of the uni-directionality of the flow field, the small uncertainties of the hydrographic anomaly measurements, and the Gaussian shape of the averaged near-field plume. The resulting estimate for the heat flux associated with the particle plume is 2.5 GW.Mass and heat budgets of the rift valley indicate that high diapycnal diffusivities are required to account for the hydrographic observations and suggest that a portion of the water flowing along the rift valley may be lost to the overlying water column. Inspired by the observations a simple analytical and numerical model for the flow within the rift valley was developed. The results indicate that the rift valley acts as an efficient low-pass filter with characteristic time scales of weeks to months, providing a plausible explanation for the persistence of the along-segment flow

Changes in bottom water physical properties above the mid-Atlantic ridge flank in the Brazil Basin

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 708–719, doi:10.1002/2017JC013375.Warming of abyssal waters in recent decades has been widely documented around the global ocean. Here repeat hydrographic data collected in 1997 and 2014 near a deep fracture zone canyon in the eastern Brazil Basin are used to quantify the long-term change. Significant changes are found in the Antarctic Bottom Water (AABW) within the canyon. The AABW in 2014 was warmer (0.08 ± 0.06 inline image), saltier (0.01 ± 0.005), and less dense (0.005 ± 0.004 inline image) than in 1997. In contrast, the change in the North Atlantic Deep Water has complicated spatial structure and is almost indistinguishable from zero at 95% confidence. The resulting divergence in vertical displacement of the isopycnals modifies the local density stratification. At its peak, the local squared buoyancy frequency ( inline image) near the canyon is reduced by about 20% from 1997 to 2014. Similar reduction is found in the basinwide averaged profiles over the Mid-Atlantic Ridge flank along 25 inline imageW in years 1989, 2005, and 2014. The observed changes in density stratification have important implications for internal tide generation and dissipation.NSF Grant Number: OCE-12350942018-07-3

Turbulence observations in a buoyant hydrothermal plume on the East Pacific Rise

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 180–181, doi:10.5670/oceanog.2012.15.Hot vent fluid enters the ocean at high-temperature hydrothermal vents, also known as black smokers. Because of the large temperature difference between the vent fluid and oceanic near-bottom waters, the hydrothermal effluent initially rises as a buoyant plume through the water column. During its rise, the plume engulfs and mixes with background ocean water. This process, called entrainment, gradually reduces the density of the rising plume until it reaches its level of neutral buoyancy, where the plume density equals that of the background water, and it begins to spread along a surface of constant density.The data presented here were collected in the context of National Science Foundation grants OCE-0425361 and OCE-0728766

The Finescale Response of Lowered ADCP Velocity Measurements Processed with Different Methods

In a paper published in 2002 in this journal, K. Polzin et al. derive corrections for spectra of vertical shear calculated from lowered acoustic Doppler current profiler (LADCP) velocity data. To illustrate and validate the corrections, they use velocities derived with a specific implementation of the shear method for LADCP processing that is no longer supported or widely used. In several recent publications, spectral corrections specific to this old processing method have been applied without modification to LADCP data processed with the more modern and much more widely used velocity-inversion method, which is associated with significantly less damping at high vertical wavenumbers than the older method. The purpose of this work is to derive and validate spectral corrections appropriate for different LADCP processing methods

Deep circulation in the Lau Basin and Havre Trough of the western South Pacific Ocean from floats and hydrography

A system of meridional ridges in the western South Pacific Ocean frame the Lau Basin and Havre Trough, and form a barrier to direct communication between the far western South Pacific basins and the interior South Pacific Ocean. The eastern side of this system comprises the Tonga and Kermadec Ridges, the location of the main deep western boundary current entering the Pacific Ocean. Observations from floats released in the Lau Basin as part of the RIDGE2000 program suggested the presence of a western boundary current along the Lau Ridge exiting into the North Fiji Basin. Those observations, together with Argo sub-surface float data and repeat hydrographic sections, confirm and expand the boundary current observations along the Lau Ridge throughout the Lau Basin and into the Havre Trough, along the Colville Ridge. The observations also reveal two previously unrecognized westward flowing jets bisecting the Lau Basin and Havre Trough. Using an extension to the classic Stommel-Arons abyssal circulation model, the predicted strength and location of these boundary currents and their bifurcation is compared with the float observations. The model provides a simplified view of the dynamics controlling the boundary current structure in the deep basins. A comparison of transport within the western boundary current derived from float data, hydrographic sections, and the idealized analytical model indicates that roughly 4 Sv (below 1,000 db) is transported northward through the Lau Basin, exiting into the North Fiji Basin

Enhanced diapycnal diffusivity in intrusive regions of the Drake Passage

Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 1309-1321, doi:10.1175/JPO-D-15-0068.1.Direct measurements of oceanic turbulent parameters were taken upstream of and across Drake Passage, in the region of the Subantarctic and Polar Fronts. Values of turbulent kinetic energy dissipation rate ε estimated by microstructure are up to two orders of magnitude lower than previously published estimates in the upper 1000 m. Turbulence levels in Drake Passage are systematically higher than values upstream, regardless of season. The dissipation of thermal variance χ is enhanced at middepth throughout the surveys, with the highest values found in northern Drake Passage, where water mass variability is the most pronounced. Using the density ratio, evidence for double-diffusive instability is presented. Subject to double-diffusive physics, the estimates of diffusivity using the Osborn–Cox method are larger than ensemble statistics based on ε and the buoyancy frequency.This work was supported by grants from the U.S. National Science Foundation.2016-10-0

The prediction, verification, and significance of flank jets at mid-ocean ridges

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 277–283, doi:10.5670/oceanog.2012.26.One aspect of ocean flow over mid-ocean ridges that has escaped much attention is the capacity of a ridge to convert oscillatory flows into unidirectional flows. Those unidirectional flows take the form of relatively narrow jets hugging the ridge's flanks. In the Northern Hemisphere, the jets move heat and dissolved and particulate matter poleward on the west and equatorward on the east of north-south trending ridges. Recent measurements and a model of flow at the East Pacific Rise at 9–10°N show that these ridge-parallel flows can extend 10–15 km horizontally away from the ridge axis, reach from the seafloor to several hundreds of meters above ridge crest depth, and have maximum speeds in their cores up to 10 cm s–1. Because of their along-ridge orientation and speed, the jets can significantly affect the transport of hydrothermal vent-associated larvae between vent oases along the ridge crest and, possibly, contribute to the mesoscale stirring of the abyssal ocean. Because jet-formation mechanisms involve oscillatory currents, ocean stratification, and topography, the jets are examples of "stratified topographic flow rectification." Ridge jets have parallels in rectified flows at seamounts and submarine banks.JWL is supported by the National Oceanic and Atmospheric Administration (NOAA) Pacific Marine Environmental Laboratory and by NOAA Vents Program. The work of other authors has been supported by National Science Foundation through grants OCE-0424953 and OCE-0425361, LADDER (LArval Dispersion along the Deep East pacific Rise)

Turbulent Mixing in a Deep Fracture Zone on the Mid-Atlantic Ridge

Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04–0.10-Sv (1 Sv ≡ 106 m3 s−1) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy ε increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides. During spring to neap tides, ε varies from O(10−9) to O(10−10) W kg−1 below 3500 m around the 2-layer interface. The detection of temperature overturns during tidal flow reversal, which almost fully opposes the deep up-canyon mean flow, confirms the canyon middepth enhancement of ε. The internal tide energy flux, particularly enhanced at the sill, compares with the lower-layer energy loss across the sill. Throughout the canyon away from the sill, near-inertial waves with downward-propagating energy dominate the internal wave field. The present study underlines the intricate pattern of the deep turbulent mixing affected by the mean flow, internal tides, and near-inertial waves

Indonesian Throughflow Partitioning Between Leeuwin and South Equatorial Currents

Indonesian Throughflow (ITF) waters move along multiple pathways within the Indian Ocean. The western route is within the thermocline of the South Equatorial Current (SEC), and the southern route is via injection into the Leeuwin Current (LC) along western Australia. We use gridded Argo data to examine heat content anomaly (HCa) within three boxes in the eastern Indian Ocean, one adjacent to the ITF outflow from the Indonesian Seas (ITF box), the second in the eastern portion of the SEC (SEC box), and the third in the LC (LC box). Although interannual HCa variability in the SEC and ITF boxes is well correlated, a large increase in HCa within the ITF box does not appear in the SEC box in 2011 but is evident in the LC box. The 2011 change in the SEC–LC partitioning is investigated using GODAS reanalysis by examining the strength of the SEC and LC during a 2009 HCa increase within the ITF box and the subsequent increase in 2011. During 2009, a strong SEC and weakened LC spread the increased ITF HCa into the central Indian Ocean, whereas a weak SEC and strengthened LC during 2011 transmit the HCa signal to the south. Near-surface winds and mean sea level pressure from NCEP–NCAR reanalysis reveal that Ningaloo Niño events led to shifts in ocean circulation during 2000 and 2011. LC and SEC exports show a high negative correlation at interannual time scales, indicating that a reduction of outflow from one pathway is partially compensated by an increase from the other

Vertical kinetic energy and turbulent dissipation in the ocean

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geophysical Research Letters 42 (2015): 7639–7647, doi:10.1002/2015GL065043.Oceanic internal waves are closely linked to turbulence. Here a relationship between vertical wave number (kz) spectra of fine-scale vertical kinetic energy (VKE) and turbulent dissipation ε is presented using more than 250 joint profiles from five diverse dynamic regimes, spanning latitudes between the equator and 60°. In the majority of the spectra VKE varies as inline image. Scaling VKE with inline image collapses the off-equatorial spectra to within inline image but underestimates the equatorial spectrum. The simple empirical relationship between VKE and ε fits the data better than a common shear-and-strain fine-scale parameterization, which significantly underestimates ε in the two data sets that are least consistent with the Garrett-Munk (GM) model. The new relationship between fine-scale VKE and dissipation rate can be interpreted as an alternative, single-parameter scaling for turbulent dissipation in terms of fine-scale internal wave vertical velocity that requires no reference to the GM model spectrum.National Science Foundation Grant Numbers: OCE-0728766, OCE-0425361, OCE-0424953, OCE-1029722, OCE-0622630, OCE-1030309, OCE-1232962, and Office of Naval Research Grant Number: N00014-10-1031