Quasi-Lagrangian structure and variability of the subtropical western North Atlantic circulation

A large body of quasi-Lagrangian trajectory (SOFAR float) data collected from 1976-1979 from 700 m and 2000 m in the western North Atlantic is examined, and it is shown that the character of the trajectories varies markedly over regions as small as a few degrees of latitude and longitude. Kinetic energy increases to the north and west in the basin at both levels. At 700 m a large northwest energy gradient is present between 30N and 31N at 70W. At 2000 m kinetic energy increases to the north and west with the largest gradients very near to the Blake Escarpment at the western boundary of the basin. At both levels the region in the vicinity of 28N, 70W appears to be locally a minimum of kinetic energy. At very low temporal frequencies the trajectories indicate that zonal motions are more energetic than meridional away from the western boundary in the thermocline. At the deeper level the trajectories appear to be influenced by the local bottom topography, though at 2000 m over very flat areas such as the Nares Abyssal Plain zonal motions appear to dominate over meridional at low frequencies as in the thermocline. From the data it is possible to examine three regions of the subtropical western North Atlantic. North of approximately 32N and west of 60W, there is evidence of a westward recirculation in the thermocline, based on one very long trajectory. To the south of the recirculation regime in the greater MODE region (25-30N, 67-75W) there is evidence that individual fluid parcels in the thermocline undergo large rms displacements but small net displacements from their initial positions over times of the order of a year and may remain in this region-for a period of several years. To the south and east of the MODE region there is repeated evidence of the presence of a well-defined eastward flow in the thermocline of approximately 4 cm sec—1, extending zonally at least to the eastern edge of the sampled region. At 2000 m, a southerly mean flow is present west of 67W and south of 32N, and there is indication that very near to the western boundary in this region the southerly deep transport can be sizeable. In many respects the 2000 m trajectories examined here from west of 67W are both qualitatively and quantitatively similar to the much larger body of trajectory data collected during MODE-I and discussed by Freeland et al. (1975). Trajectories from east of 67W at 2000 m show much lower kinetic energy levels than their more western counterparts, and the trajectories of three instruments set in the Nares Abyssal Plain region also indicate the possibility of larger horizontal length scales for the energy containing eddies and weaker horizontal dispersion than in the greater MODE region

The North Atlantic current system : a scientific report, 19-20 April 1993, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

Conference name: North Atlantic Current (NAC) System; 19-20 April 1993, Woods Hole Oceanographic Institution, Woods Hole, MAOn April 19-20, 1993 a two-day workshop was held at the Woods Hole Oceanographic Institution on "The North Atlantic Current (NAC) System". The workshop, which was sponsored by NSF/NOAA/ONR reflected a growing sense of excitement and interest in the oceanographic community in the NAC system and its role in the large scale circulation of the North Atlantic Ocean and Climate of the adjoining landmasses. The presence of the North Atlantic Current with its warm waters at such high latitudes, and its role in both the wind-driven and thermohaline circulations makes it unique amongst the Western Boundary Currents of the oceans. Being on the one hand part of the wind-driven circulation and on the other hand the upper branch of the "Global Conveyor Belt", the North Atlantic current is indeed an enigma, suggesting fundamental issues about the nature of the coupling between the two 'roles' of the current that will need to be addressed. But it was also clear from the workshop discussions that there remain considerable uncertainty about the basic structure of the NAC. A high level of interest in these questions was evident at the workshop. The lectures, presentations, and the discussion sessions where observational and modelling issues were debated, brought out many ideas for the development and focus of future research of the NAC and surrounding waters. This report is intended to provide not only a synopsis of the lectures, papers, and ideas that were discussed, but also a scientific statement from the workshop reflecting a growing consensus for initiating a coordinated research effort in the region.NSF/NOAA/ON

Observations of a barotropic planetary wave in the western North Atlantic

SOFAR float observations from 1300 m depth are used to describe a major feature of the large-scale, subthermocline velocity field observed in the western North Atlantic (31 N, 70W), during the 1978 POLYMODE Local Dynamics Experiment (LDE). The two-month-long intensive phase of the LDE was dominated by a highly polarized, oscillatory flow which had many of the characteristics of a barotropic planetary wave...

Directly Measured Currents and Estimated Transport Pathways of Atlantic Water between 59.5°N and the Iceland–Faroes–Scotland Ridge

Using vessel-mounted acoustic Doppler current profiler data from four different routes between Scotland, Iceland and Greenland, we map out the mean flow of water in the top 400 m of the northeastern North Atlantic. The poleward transport east of the Reykjanes Ridge (RR) decreases from ~8.5 to 10 Sv (1 Sverdrup=106 m3 s−1) at 59.5°N to 61°N to 6 Sv crossing the Iceland–Faroes–Scotland Ridge. The two longest ~1200 km transport integrals have 1.4–0.94 Sv uncertainty, respectively. The overall decrease in transport can in large measure be accounted for by a ~1.5 Sv flow across the RR into the Irminger Sea north of 59.5°N and by a ~0.5 Sv overflow of dense water along the Iceland–Faroes Ridge. A remaining 0.5 Sv flux divergence is at the edge of detectability, but if real could be accounted for through wintertime convection to \u3e400 m and densification of upper ocean water. The topography of the Iceland Basin and the banks west of Scotland play a fundamental role in controlling flow pathways towards and past Iceland, the Faroes and Scotland. Most water flows north unimpeded through the Iceland Basin, some in the centre of the basin along the Maury Channel, and some along Hatton Bank, turning east along the northern slopes of George Bligh Bank, Lousy Bank and Bill Bailey’s Bank, whereupon the flow splits with ~3 Sv turning northwest towards the Iceland–Faroes Ridge and the remainder continuing east towards and north of the Wyville-Thomson Ridge (WTR) to the Scotland slope thereby increasing the Slope Current transport from ~1.5 Sv south of the WTR to 3.5 Sv in the Faroes–Shetland Channel

AXIS—an Autonomous Expendable Instrument System

Author Posting. © American Meteorological Society, 2017. 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 Atmospheric and Oceanic Technology 34 (2017): 2673-2682, doi:10.1175/JTECH-D-17-0054.1.Expendable bathythermographs (XBT) to profile upper-ocean temperatures from vessels in motion have been in use for some 50 years now. Developed originally for navy use, they were soon adapted by oceanographers to map out upper-ocean thermal structure and its space–-time variability from both research vessels and merchant marine vessels in regular traffic. These activities continue today. This paper describes a new technology—the Autonomous Expendable Instrument System (AXIS)—that has been developed to provide the capability to deploy XBT probes on a predefined schedule, or adaptively in response to specific events without the presence of an observer on board. AXIS is a completely self-contained system that can hold up to 12 expendable probes [XBTs, XCTDs, expendable sound velocimeter (XSV)] in any combination. A single-board Linux computer keeps track of what probes are available, takes commands from ashore via Iridium satellite on what deployment schedule to follow, and records and forwards the probe data immediately with a time stamp and the GPS position. This paper provides a brief overview of its operation, capabilities, and some examples of how it is improving coverage along two lines in the Atlantic.Initial development of AXIS mechanical design elements wasmade possible by awards from the Cecil H. and Ida M. Green Technology Innovation Fund and the Sealark Foundation to the team of Dave Fratantoni, Keith von der Heydt (WHOI), and Terry Hammar (WHOI). Construction of the first full AXIS prototype was supported by a technology grant from the National Science Foundation (OCE-0926853) and the second one through an NSF-funded (OCE-1061185) subcontract from the University of Rhode Island.2018-06-2

Inverted Echo Sounder Telemetry System Report

From August 1989 until August 1990, a simple acoustic telemetry system was used for obtaining real-time data from 5 Inverted Echo Sounders (IESs) deployed in the SYNOP inlet array in the Gulf Stream east of Cape Hatteras. Every 24 hours, each IES calculated a representative travel time from a set of 48 measurements (τ), and telemetered that value to a listening station on Bermuda. From the received data, a daily time series of the depth of the 12oC isotherm (our proxy for main thermocline depth) over each IES was calculated. The position of the Gulf Stream North Wall through the IES array was calculated on a daily basis from the thermocline depth information at each IES site. The telemetry system is based on encoding data as a time delayed broadcast acoustic signal: the delay of the time of broadcast of the signal, with with respect to a reference time, is proportional to the data value. The changes in delay time, from one broadcast signal to the next, are recorded at a remote receiving station. The IESs were recovered in August 1990, with the exception of the one at site B2. The telemetered data from the IES at site B2 was, however received at Bermuda. The RMS agreement between thermocline depths, as calculated from the data on tape from the recovered IESs and as calculated from the received telemetry data, is 20 m. This compares favorably with the 19 m uncertainty in calibrating the τs as a measure of the thermocline depth. The RMS agreement between the position of the Gulf Stream path through the IESs as calculated from the tape data and the telemetry data is 5 km. This telemetry system is not IES specific. It could be used with other appropriately modified oceanographic instruments, such as current meters and pressure sensors

Discovery of an unrecognized pathway carrying overflow waters toward the Faroe Bank Channel

The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC). The pathways feeding the overflow remain, however, poorly resolved. Here we use multiple observational platforms and an eddy-resolving ocean model to identify an unrecognized deep flow toward the Faroe Bank Channel. We demonstrate that anticyclonic wind forcing in the Nordic Seas via its regulation of the basin circulation plays a key role in activating an unrecognized overflow path from the Norwegian slope – at which times the overflow is anomalously strong. We further establish that, regardless of upstream pathways, the overflows are mostly carried by a deep jet banked against the eastern slope of the Faroe-Shetland Channel, contrary to previous thinking. This deep flow is thus the primary conduit of overflow water feeding the lower branch of the AMOC via the Faroe Bank Channel

Oleander is more than a flower twenty-five years of oceanography aboard a merchant vessel

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Rossby, T., Flagg, C. N., Donohue, K., Fontana, S., Curry, R., Andres, M., & Forsyth, J. Oleander is more than a flower twenty-five years of oceanography aboard a merchant vessel. Oceanography, 32(3), (2019): 126-137, doi:10.5670/oceanog.2019.319.Since late fall 1992, CMV Oleander III has been measuring upper ocean currents during its weekly trips between Bermuda and Port Elizabeth, New Jersey, by means of an acoustic Doppler current profiler installed in its hull. The overarching objective of this effort has been to monitor transport in the Gulf Stream and surrounding waters. With 25 years of observation in hand, we note that the Gulf Stream exhibits significant year-to-year variations but no evident long-term trend in transport. We show how these data have enabled studies of oceanic variability over a very wide range of scales, from a few kilometers to the full 1,000 km length of its route. We report that the large interannual variations in temperature on the continental shelf are negatively correlated with flow from the Labrador Sea, but that variability in the strength of this flow cannot account for a longer-term warming trend observed on the shelf. Acoustic backscatter data offer a rich trove of information on biomass activities over a wide range of spatial and temporal scales. A peek at the future illustrates how the new and newly equipped Oleander will be able to profile currents to greater depths and thereby contribute to monitoring the strength of the meridional overturning circulation.First and foremost we extend our heartfelt thanks to the Bermuda Container Line/Neptune Group Management Ltd for permission to operate an acoustic Doppler current profiler on board CMV Oleander III, a 150 kHz ADCP between 1992 and 2004, and a 75 kHz ADCP between 2005 and 2018. Their interest and support is gratefully acknowledged. Cor Teeuwen, our initial contact in Holland while the ship was still under construction, played an important role in facilitating the original ADCP installation. His evident interest to make this concept work has stimulated similar activities on other commercial vessels. The interest and willingness of the shipping industry to be supportive of science has been a very positive experience for all of us who have ventured in this direction. Initial funding came from NOAA and the Office of Naval Research. Since 1999, the National Science Foundation has supported the project through funding to the University of Rhode Island and Stony Brook University, and now also to the Bermuda Institute of Ocean Sciences (BIOS), which will be taking over the Oleander operation. NSF is also funding the current transition to the new CMV Oleander. In the early years, G. Schwartze and E. Gottlieb were very helpful with technical support for the project. This included frequent visits to the ship before we had the capability to transfer the data through the Ethernet. We thank Jules Hummon and Eric Firing for adapting the UNOLS-wide UHDAS ADCP operating system to the merchant marine environment. We thank E. Williams and P. Ortner at the Rosenstiel School of Marine and Atmospheric Science, University of Miami, for making the 38 kHz ADCP data from Explorer of the Seas available to us. We also want to thank the NOAA Ship Of Opportunity Program for continued interest in and support of XBT operations along the Oleander section. That support started over 40 years ago and is now stronger than ever. All ADCP data from 1992 through 2018 have been archived at the Joint Archive for Shipboard ADCP (JASADCP), established at the University of Hawaii by NOAA’s National Centers for Environmental Information (NCEI). Averaged yearly data sets can be downloaded in ASCII text or NetCDF formats (http://ilikai.soest.hawaii.edu/​sadcp/main_inv.html). We thank Patrick Caldwell, JASADCP’s manager, for his assistance. All ADCP and XBT data can be obtained at the Stony Brook website: http://po.msrc.sunysb.edu/Oleander/. The URL to the project website is http://oleander.bios.edu—an updated data portal and products will soon be accessible here. An ERDDAP server for Oleander data (in the process of being configured) is at this address: http://erddap.​oleander.​bios.edu:​8080/​erddap/. The following link to BIOS lists over 40 publications that have used the ADCP data one way or another: http://oleander.bios.edu/publications/. We thank the two reviewers for their many interesting and helpful comments and suggestions

The scientific and societal uses of global measurements of subsurface velocity

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Szuts, Z. B., Bower, A. S., Donohue, K. A., Girton, J. B., Hummon, J. M., Katsumata, K., Lumpkin, R., Ortner, P. B., Phillips, H. E., Rossby, H. T., Shay, L. K., Sun, C., & Todd, R. E. The scientific and societal uses of global measurements of subsurface velocity. Frontiers in Marine Science, 6, (2019): 358, doi:10.3389/fmars.2019.00358.Ocean velocity defines ocean circulation, yet the available observations of subsurface velocity are under-utilized by society. The first step to address these concerns is to improve visibility of and access to existing measurements, which include acoustic sampling from ships, subsurface float drifts, and measurements from autonomous vehicles. While multiple programs provide data publicly, the present difficulty in finding, understanding, and using these data hinder broader use by managers, the public, and other scientists. Creating links from centralized national archives to project specific websites is an easy but important way to improve data discoverability and access. A further step is to archive data in centralized databases, which increases usage by providing a common framework for disparate measurements. This requires consistent data standards and processing protocols for all types of velocity measurements. Central dissemination will also simplify the creation of derived products tailored to end user goals. Eventually, this common framework will aid managers and scientists in identifying regions that need more sampling and in identifying methods to fulfill those demands. Existing technologies are capable of improving spatial and temporal sampling, such as using ships of opportunity or from autonomous platforms like gliders, profiling floats, or Lagrangian floats. Future technological advances are needed to fill sampling gaps and increase data coverage.This work was supported by the National Science Foundation, United States, Grant Numbers 1356383 to ZBS, OCE 1756361 to ASB at the Woods Hole Oceanographic Institution, and 1536851 to KAD and HTR; the National Oceanographic and Atmospheric Administration, United States, Ocean Observations and Monitoring Division and Atlantic Oceanographic and Meteorological Laboratory to RL; Royal Caribbean Cruise Ltd., to PBO; the Australian Government Department of the Environment and Energy National Environmental Science Programme and Australian Research Council Centre of Excellence for Climate Extremes to HEP; and the Gulf of Mexico Research Initiative Grant V-487 to LS