Description: Five years of data from a line of dynamic height moorings (DHM), bottom-pressure recorders (BPR), and pressure-equipped inverted echo sounders (PIES) near the Atlantic Ocean western boundary at 26.5°N are used to evaluate the structure and variability of the Deep Western Boundary Current (DWBC) during 2004–2009. Comparisons made between transports estimated from the DHM+BPR and those made by the PIES demonstrate that the two systems are collecting equivalent volume transport information (correlation coefficient r=0.96, root-mean-square difference=6 Sv; 1 Sv=106 m3 s−1). Integrated to ∼450 km off from the continental shelf and between 800 and 4800 dbar, the DWBC has a mean transport of approximately 32 Sv and a standard deviation during these five years of 16 Sv. Both the barotropic (full-depth vertical mean) and baroclinic flows have significant variability (changes exceeding 10 Sv) on time scales ranging from a few days to months, with the barotropic variations being larger and more energetic at all time scales. The annual cycle of the deep transport is highly dependent on the horizontal integration distance; integrating ∼100 km offshore yields an annual cycle of roughly similar magnitude but shifted in phase relative to that found from current meter arrays in the 1980–1990s, while the annual cycle becomes quite weak when integrating ∼450 km offshore. Variations in the DWBC transport far exceed those of the total basin-wide Meridional Overturning Circulation (standard deviations of 16 Sv vs. 5 Sv). Transport integrated in the deep layer out to the west side of the Mid-Atlantic Ridge still demonstrates a surprisingly high variance, indicating that some compensation of the western basin deep variability must occur in the eastern basin.

Description: Global mean surface warming has stalled since the end of the twentieth century1, 2, but the net radiation imbalance at the top of the atmosphere continues to suggest an increasingly warming planet. This apparent contradiction has been reconciled by an anomalous heat flux into the ocean3, 4, 5, 6, 7, 8, induced by a shift towards a La Niña-like state with cold sea surface temperatures in the eastern tropical Pacific over the past decade or so. A significant portion of the heat missing from the atmosphere is therefore expected to be stored in the Pacific Ocean. However, in situ hydrographic records indicate that Pacific Ocean heat content has been decreasing9. Here, we analyse observations along with simulations from a global ocean–sea ice model to track the pathway of heat. We find that the enhanced heat uptake by the Pacific Ocean has been compensated by an increased heat transport from the Pacific Ocean to the Indian Ocean, carried by the Indonesian throughflow. As a result, Indian Ocean heat content has increased abruptly, which accounts for more than 70% of the global ocean heat gain in the upper 700 m during the past decade. We conclude that the Indian Ocean has become increasingly important in modulating global climate variability.

Description: As the upper layer of the world ocean warms gradually during the 20th century, the inter-ocean heat transport from the Indian to Atlantic basin should be enhanced, and the Atlantic Ocean should therefore gain extra heat due to the increased upper ocean temperature of the inflow via the Agulhas leakage. Consistent with this hypothesis, instrumental records indicate that the Atlantic Ocean has warmed substantially more than any other ocean basin since the mid-20th century. A surface-forced global ocean-ice coupled model is used to test this hypothesis and to find that the observed warming trend of the Atlantic Ocean since the 1950s is largely due to an increase in the inter-ocean heat transport from the Indian Ocean. Further analysis reveals that the increased inter-ocean heat transport is not only caused by the increased upper ocean temperature of the inflow but also, and more strongly, by the increased Agulhas Current leakage, which is augmented by the strengthening of the wind stress curl over the South Atlantic and Indian subtropical gyre.

Description: The rapid climate change programme (RAPID) has established a prototype system to continuously observe the strength and structure of the Atlantic meridional overturning circulation (MOC) at 26.5 degrees N. Here we provide a detailed description of the RAPID-MOC monitoring array and how it has evolved during the first four deployment years, as well as an overview of the main findings so far. The RAPID-MOC monitoring array measures: (1) Gulf Stream transport through Florida Strait by cable and repeat direct velocity measurements; (2) Ekman transports by satellite scatterometer measurements; (3) Deep Western Boundary Currents by direct velocity measurements; (4) the basin wide interior baroclinic circulation from moorings measuring vertical profiles of density at the boundaries and on either side of the Mid-Atlantic Ridge; and (5) barotropic fluctuations using bottom pressure recorders. The array became operational in late March 2004 and is expected to continue until at least 2014. The first 4 years of observations (April 2004-April 2008) have provided an unprecedented insight into the MOC structure and variability. We show that the zonally integrated meridional flow tends to conserve mass, with the fluctuations of the different transport components largely compensating at periods longer than 10 days. We take this as experimental confirmation of the monitoring strategy, which was initially tested in numerical models. The MOC at 26.5 degrees N is characterised by a large variability even on timescales as short as weeks to months. The mean maximum MOC transport for the first 4 years of observations is 18.7 Sv with a standard deviation of 4.8 Sv. The mechanisms causing the MOC variability are not yet fully understood. Part of the observed MOC variability consists of a seasonal cycle, which can be linked to the seasonal variability of the wind stress curl close to the African coast. Close to the western boundary, fluctuations in the Gulf Stream and in the North Atlantic Deep Water (NADW) coincide with bottom pressure variations at the western margin, thus suggesting a barotropic compensation. Ongoing and future research will put these local transport variations into a wider spatial and climatic context. (C) 2011 Elsevier Ltd. All rights reserved.

Description: Ocean boundary current systems are key components of the climate system, are home to highly productive ecosystems, and have numerous societal impacts. Establishment of a global network of boundary current observing systems is a critical part of ongoing development of the Global Ocean Observing System. The characteristics of boundary current systems are reviewed, focusing on scientific and societal motivations for sustained observing. Techniques currently used to observe boundary current systems are reviewed, followed by a census of the current state of boundary current observing systems globally. The next steps in the development of boundary current observing systems are considered, leading to several specific recommendations.

Description: Author Posting. © American Geophysical Union, 2016. 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 121 (2016): 3159–3170, doi:10.1002/2015JC011165.

Description: The Tongue of the Ocean (TOTO) region located within the Bahamas archipelago is a relatively understudied region in terms of both its biological and physical oceanographic characteristics. A prey-field mapping cruise took place in the fall between 15 September 2008 and 1 October 2008, consisting of a series of transects and “clovers” to study the spatial and temporal variability. The region is characterized by a deep scattering layer (DSL), which is preyed on by nekton that serves as the food for beaked whale and other whale species. This study marks the first of its kind where concurrent measurements of acoustic backscatter and turbulence have been conducted for a nekton scattering layer well below the euphotic zone. Turbulence data collected from a Deep Microstructure Profiler are compared to biological and shear data collected by a 38 kHz Simrad EK 60 echo sounder and a hydrographic Doppler sonar system, respectively. From these measurements, the primary processes responsible for the turbulent production in the TOTO region are assessed. The DSL around 500 m and a surface scattering layer (SSL) are investigated for raised ε values. Strong correlation between turbulence levels and scattering intensity of prey is generally found in the SSL with dissipation levels as large as ∼10−7 W kg−1, 3 orders of magnitude above background levels. In the DSL and during the diel vertical migration, dissipation levels ∼10−8 W kg−1 were observed.

Description: U.S. Office of Naval Research Grant Number: N00014-08-1-1162-01

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution October 1993

Description: Hydrographic and expendable current profiler (XCP) data taken during the Gulf of Cadiz Expedition in September 1988 are analyzed to diagnose the mixing and dynamics of the Mediterranean outflow. The overall structure of the outflow is consistent with that described in the historical literature (Heezen and Johnson, 1969). This data shows that the overflow transport doubles from .85 Sv to 1.9 Sv, and that the velocity weighted salinity decreases from 37.8 pss to 36.7 pss in the first 60 km of the path. The core salinity of the neutrally buoyant outflow near Cape St. Vincent is 36.6 pss, which indicates that most of the mixing has taken place close to the Strait in the initial descent of the outflow. Cross stream variations in the overflow T/S properties increase as the flow spreads from 10 km to 90 km wide. The outflow begins with less than a 0.5°C across-stream variation in temperature in the Strait with the saltiest, coldest water to the south and slightly fresher and warmer outflow to the north. As the outflow spreads, the northern near-shelf flow follows a path higher in the water column and mixes with warmer North Atlantic water than does the deeper offshore flow. Within the first 100 km, the cross stream variation in temperature on an isopycnal becomes more than a 2°C. The flow eventually settles along two preferred isopycnals: 27.5 and 27.8 (Zenk 1975b). The spreading of the flow contains both a barotropic and baroclinic character. The average change in angle above and below the maximum velocity of the outflow is 8°while at the edges of the flow the average direction of the outflow diverges by as much as 50°. Gradient Richardson numbers less than 1/4 are found in the interface (up to 50 m thick) between westward flowing Mediterranean water and eastward flowing North Atlantic water, even though there is a strong stabilizing stratification present. Bulk Froude numbers greater than 1 are found near the Strait coincident with the vigorous mixing noted above. Lower bulk Froude numbers were observed in regions where less entrainment was taking place. The momentum balances are diagnosed using hydrographic and XCP data. Evaluation of the cross stream momentum balance shows the importance of advection as the flow makes a 90 degree inertial turn upon entering the Gulf of Cadiz. A form of the Bernoulli function can be evaluated to infer the total stress (entrainment and bottom drag) acting on the outflow. This stress is as large as 5 Pa within 20 km of the Strait, while further downstream the stress decreases to about 1/2 Pa. The entrainment stress estimated from the property fluxes reaches a maximum of about 0.8 Pa near section C, indicating that bottom stress is dominant. Near the Strait, advection, bottom drag and the Coriolis force are all critical to the dynamics of the outflow. Further downstream, the outflow becomes a damped geostrophic current. A simple geostrophic adjustment model is used to show that in the absence of frictional stresses, the outflow would very quickly become geostrophically balanced and descend only about 10 m down the continental slope. Thus, friction is critical for the outflow to cross isobaths. A simple numerical model that uses a Froude number dependent entrainment and a quadratic bottom friction law is used to simulate the outflow (Price and Baringer, 1993). Some of the properties of the outflow including localized entrainment, large stresses and high Rossby number of the flow (initially as high as 0.6), are simulated rather well, though the model overestimates the magnitude of the outflow current. We suspect that this is a consequence of assuming a passive ocean. Two different methods for specifying the broadening of the flow are compared: one using the highly parameterized concept of Ekman spreading, the other using the conservation of potential vorticity. The potential vorticity broadening more accurately reproduces the observed width of the flow near Cape St. Vincent where the width varies inversely with the bottom slope. However, both methods produce essentially the same equilibrium temperature, salinity and transport of the outflow which is a testament to the robustness of the model solution. with the formation process of NADW.

Description: Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Annual Review of Marine Science 8 (2016): 185-215, doi:10.1146/annurev-marine-052915-100829.

Description: The ocean, a central component of Earth’s climate system, is changing. Given the global scope of these changes, highly accurate measurements of physical and biogeochemical properties need to be conducted over the full water column, spanning the ocean basins from coast to coast, and repeated every decade at a minimum, with a ship-based observing system. Since the late 1970s, when the Geochemical Ocean Sections Study (GEOSECS) conducted the first global survey of this kind, the World Ocean Circulation Experiment (WOCE) and Joint Global Ocean Flux Study (JGOFS), and now the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) have collected these “reference standard” data that allow quantification of ocean heat and carbon uptake, and variations in salinity, oxygen, nutrients, and acidity on basin scales. The evolving GO-SHIP measurement suite also provides new global information about dissolved organic carbon, a large bioactive reservoir of carbon.

Description: Climate Observations Division of the U.S. NOAA Climate Program Office and NOAA Research; Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA10OAR4320148; U.S. National Science Foundation [OCE- 0223869; OCE-0752970; OCE-0825163; OCE-1434000; OCE 0752972; OCE-0752980; OCE-1232962; OCE-1155983; OCE-1436748]; U.S. CLIVAR Project Office; Global Environment and Marine Department, Japan Meteorological Agency; Australian Climate Change Science Program (Australian Department of Environment and CSIRO); U.K. Natural Environment Research Council; European Union’s FP7 grant agreement 264879 (CarboChange); Horizon 2020 grant agreement No 633211; ETH Zurich Switzerland.

Description: Global mean surface warming has stalled since the end of the twentieth century1,2, but the net radiation imbalance at the top of the atmosphere continues to suggest an increasingly warming planet. This apparent contradiction has been reconciled by an anomalous heat flux into the ocean3–8, induced by a shift towards a La Niña-like state with cold sea surface temperatures in the eastern tropical Pacific over the past decade or so. A significant portion of the heat missing fromthe atmosphere is therefore expected to be stored in the Pacific Ocean. However, in situ hydrographic records indicate that Pacific Ocean heat content has been decreasing9. Here, we analyse observations along with simulations from a global ocean–sea ice model to track the pathway of heat. We find that the enhanced heat uptake by the Pacific Ocean has been compensated by an increased heat transport from the Pacific Ocean to the Indian Ocean, carried by the Indonesian throughflow. As a result, Indian Ocean heat content has increased abruptly, which accounts for more than 70% of the global ocean heat gain in the upper 700m during the past decade. We conclude that the Indian Ocean has become increasingly important in modulating global climate variability.

Description: PDF is Advanced Online copy 18 May 2015

Description: Published