A Modified Field Infiltrometer Test for Clay Liners

Regulatory agencies are looking more frequently to in situ field hydraulic conductivity tests for the assessment of a liner\u27s compliance to a specified hydraulic conductivity. Most field tests have evaluated hydraulic conductivity by measuring the infiltration rate of the liner. The infiltration rate can be used to arrive at a hydraulic conductivity value if the hydraulic boundary conditions of the test can be identified or if the head loss at different depths can be measured. A test fill of a clay liner was evaluated for its saturated vertical hydraulic conductivity. This paper discusses the use of eight tensiometers to measure soil suction at four depths beneath a double ring infiltrometer. The hydraulic conductivity results using the tensiometer data displayed good consistency and agreed well with laboratory test results

Reconciling tracer and float observations of the export pathways of Labrador Sea Water

For more than fifty years, it has been generally accepted by oceanographers that the Deep Western Boundary Current (DWBC) is the principal conduit of recently-convected Labrador Sea Water (LSW) exported from the high-latitude North Atlantic to the equator. Supporting this supposition is observational evidence that the waters of the DWBC have consistently greater equatorward velocities, higher concentrations of passive tracers, and younger ages compared to ocean interior waters. However, recent observations and simulations of floats launched in the DWBC in the Labrador Sea show that most water parcels are quickly ejected from the DWBC and follow instead interior pathways to the subtropics. Here, we show that tracer observations from the last three decades are compatible with the existence of both DWBC and basin-interior export pathways. From analyses of observational data and model output, we find that equatorward transport in the basin interior is consistent with the large-scale vorticity balance at mid-depth. Furthermore, from the modeling analysis we show that despite higher, localized concentrations of tracer and particles in the DWBC, only 5% of particles released in the Labrador Sea are transported from the subpolar to subtropical gyre via a continuous DWBC pathway. Thus, the interior pathway is a significant contributor to LSW export. Highlights: - Lagrangian observations of Labrador Sea Water match Eulerian observations - There is deep equatorward flow in the basin interior - This interior pathway is significant compared to the pathway along the boundar

The present and future system for measuring the Atlantic meridional overturning circulation and heat transport

of the global combined atmosphere-ocean heat flux and so is important for the mean climate of the Atlantic sector of the Northern Hemisphere. This meridional heat flux is accomplished by both the Atlantic Meridional Overturning Circulation (AMOC) and by basin-wide horizontal gyre circulations. In the North Atlantic subtropical latitudes the AMOC dominates the meridional heat flux, while in subpolar latitudes and in the subtropical South Atlantic the gyre circulations are also important. Climate models suggest the AMOC will slow over the coming decades as the earth warms, causing widespread cooling in the Northern hemisphere and additional sea-level rise. Monitoring systems for selected components of the AMOC have been in place in some areas for decades, nevertheless the present observational network provides only a partial view of the AMOC, and does not unambiguously resolve the full variability of the circulation. Additional observations, building on existing measurements, are required to more completely quantify the Atlantic meridional heat transport. A basin-wide monitoring array along 26.5°N has been continuously measuring the strength and vertical structure of the AMOC and meridional heat transport since March 31, 2004. The array has demonstrated its ability to observe the AMOC variability at that latitude and also a variety of surprising variability that will require substantially longer time series to understand fully. Here we propose monitoring the Atlantic meridional heat transport throughout the Atlantic at selected critical latitudes that have already been identified as regions of interest for the study of deep water formation and the strength of the subpolar gyre, transport variability of the Deep Western Boundary Current (DWBC) as well as the upper limb of the AMOC, and inter-ocean and intrabasin exchanges with the ultimate goal of determining regional and global controls for the AMOC in the North and South Atlantic Oceans. These new arrays will continuously measure the full depth, basin-wide or choke-point circulation and heat transport at a number of latitudes, to establish the dynamics and variability at each latitude and then their meridional connectivity. Modeling studies indicate that adaptations of the 26.5°N type of array may provide successful AMOC monitoring at other latitudes. However, further analysis and the development of new technologies will be needed to optimize cost effective systems for providing long term monitoring and data recovery at climate time scales. These arrays will provide benchmark observations of the AMOC that are fundamental for assimilation, initialization, and the verification of coupled hindcast/forecast climate models

Redrawing the Iceland−Scotland overflow water pathways in the North Atlantic

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Zou, S., Bower, A., Furey, H., Susan Lozier, M., & Xu, X. Redrawing the Iceland−Scotland overflow water pathways in the North Atlantic. Nature Communications, 11(1), (2020): 890, doi:10.1038/s41467-020-15513-4.Iceland-Scotland Overflow Water (ISOW) is a primary deep water mass exported from the Norwegian Sea into the North Atlantic as part of the global Meridional Overturning Circulation. ISOW has historically been depicted as flowing counter-clockwise in a deep boundary current around the subpolar North Atlantic, but this single-boundary-following pathway is being challenged by new Lagrangian observations and model simulations. We show here that ISOW leaves the boundary and spreads into the interior towards the central Labrador and Irminger basins after flowing through the Charlie-Gibbs Fracture Zone. We also describe a newly observed southward pathway of ISOW along the western flank of the Mid-Atlantic Ridge. The partitioning of these pathways is shown to be influenced by deep-reaching eddies and meanders of the North Atlantic Current. Our results, in tandem with previous studies, call for a revision in the historical depiction of ISOW pathways throughout the North Atlantic.S.Z., A.B. and H.F. gratefully acknowledge the support from the Physical Oceanography Program of the U.S. National Science Foundation (grant number OCE-17-56361). S.Z. and M.S.L. also acknowledge support from NSF-OCE-17-56143. X.X. acknowledges support from NSF award 1537136. Gratitude is also extended to C. Böning and A. Biastoch for sharing FLAME output and trajectory calculation code

Propagation and transformation of upper North Atlantic deep water from the subpolar gyre to 26.5°N

Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re‐examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy‐rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid‐Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid‐Atlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer

Estimating the predictability of an oceanic time series using linear and nonlinear methods

Author Posting. © American Geophysical Union, 2004. 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 109 (2004): C08002, doi:10.1029/2003JC002148.This study establishes a series of tests to examine the relative utility of nonlinear time series analysis for oceanic data. The performance of linear autoregressive models and nonlinear delay coordinate embedding methods are compared for three numerical and two observational data sets. The two observational data sets are (1) an hourly near-bottom pressure time series from the South Atlantic Bight and (2) an hourly current-meter time series from the Middle Atlantic Bight (MAB). The nonlinear methods give significantly better predictions than the linear methods when the underlying dynamics have low dimensionality. When the dimensionality is high, the utility of nonlinear methods is limited by the length and quality of the time series. On the application side we mainly focus on the MAB data set. We find that the slope velocities are much less predictable than shelf velocities. Predictability on the slope after several hours is no better than the statistical mean. On the other hand, significant predictability of shelf velocities can be obtained for up to at least 12 hours.This research was supported by Office of Naval Research grants N00014-01-1-0260, N00014-92-J-1481, and N10014-99-1-0258

On the origin and pathway of the saline inflow to the Nordic Seas: insights from models

The behaviours of three high-resolution ocean circulation models of the North Atlantic, differing chiefly in their description of the vertical coordinate, are investigated in order to elucidate the routes and mechanisms by which saline water masses of southern origin provide inflows to the Nordic Seas. An existing hypothesis is that Mediterranean Overflow Water (MOW) is carried polewards in an eastern boundary undercurrent, and provides a deep source for these inflows. This study, however, provides an alternative view that the inflows are derived from shallow sources, and are comprised of water masses of western origin, carried by branches of the North Atlantic Current (NAC), and also more saline Eastern North Atlantic Water (ENAW), transported northwards from the Bay of Biscay region via a ‘Shelf Edge Current’ (SEC) flowing around the continental margins. In two of the models, the MOW flows northwards, but reaches only as far as the Porcupine Bank (53°N). In third model, the MOW also invades the Rockall Trough (extending to 60°N). However, none of the models allows the MOW to flow northwards into the Nordic Seas. Instead, they all support the hypothesis of there being shallow pathways, and that the saline inflows to the Nordic Seas result from NAC-derived and ENAW water masses, which meet and partially mix in the Rockall Trough. Volume and salinity transports into the southern Rockall Trough via the SEC are, in the various models, between 25 and 100% of those imported by the NAC, and are also a similarly significant proportion (20–75%) of the transports into the Nordic Seas. Moreover, the highest salinities are carried northwards by the SEC (these being between 0.13 and 0.19 psu more saline at the southern entrance to the Trough than those in the NAC-derived waters). This reveals for the first time the importance of the SEC in carrying saline water masses through the RockallTrough and into the Nordic Seas. Furthermore, the high salinities found on density surfaces appropriate to the MOW in the Nordic Seas are shown to result from the wintertime mixing of the saline near-surface waters advected northwards by the SEC/NAC system. Throughout, we have attempted to demonstrate the extent to which the models agree or disagree with interpretations derived from observations, so that the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models

Interior pathways of the North Atlantic meridional overturning circulation

To understand how our global climate will change in response to natural and anthropogenic forcing, it is essential to determine how quickly and by what pathways climate change signals are transported throughout the global ocean, a vast reservoir for heat and carbon dioxide. Labrador Sea Water (LSW), formed by open ocean convection in the subpolar North Atlantic, is a particularly sensitive indicator of climate change on interannual to decadal timescales. Hydrographic observations made anywhere along the western boundary of the North Atlantic reveal a core of LSW at intermediate depths advected southward within the Deep Western Boundary Current (DWBC). These observations have led to the widely held view that the DWBC is the dominant pathway for the export of LSW from its formation site in the northern North Atlantic towards the Equator. Here we show that most of the recently ventilated LSW entering the subtropics follows interior, not DWBC, pathways. The interior pathways are revealed by trajectories of subsurface RAFOS floats released during the period 2003-2005 that recorded once-daily temperature, pressure and acoustically determined position for two years, and by model-simulated 'e-floats' released in the subpolar DWBC. The evidence points to a few specific locations around the Grand Banks where LSW is most often injected into the interior. These results have implications for deep ocean ventilation and suggest that the interior subtropical gyre should not be ignored when considering the Atlantic meridional overturning circulation.Dissertatio

Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation

The Lagrangian method-where current location and intensity are determined by tracking the movement of flow along its path-is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid-20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice-free oceans and begin to construct basin-scale, and eventually global-scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC

Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region