On the eddy statistics in a ring-rich area: A hypothesis of bimodal structure

A hypothesis is introduced in constructing the statistics of an eddy field, which distinguishes Gulf Stream rings as a separate anomalous feature from the rnesoscale eddy field. Analysis of XBT sections between New York City and Bermuda with rings removed indicates that the therrnocline deepens toward the Gulf Stream from Bermuda and the r.rn.s. displacement of the 15°C isotherm is 50 m or less...

The \u3cem\u3eOleander\u3c/em\u3e Project: Monitoring the Variability of the Gulf Stream and Adjacent Waters between New Jersey and Bermuda

An overview of the first 4.5 years of operation of a program to monitor the structure and variability of the Gulf Stream (GS) is presented. A container vessel that operates on a weekly schedule between Port Elizabeth, New Jersey, and Hamilton, Bermuda, is equipped with a 150-kHz narrowband acoustic Doppler current profiler to measure currents from the surface to ~300 m depth. A major objective of the multiyear program is to study the annual cycle and interannual variations in velocity structure and transport by the GS. In this survey the focus is on the transport and lateral structure of the current at 52-m depth. The velocity maximum is constant at 2.07 ± 0.24 m s−1 (4 kt) with a seasonal range of ~0.1 m s−1 . Seasonal and interannual variations in total transport are observed but appear to be limited to the edges of the current, apparently reflecting low-frequency variations in the intensity of the recirculating waters adjacent to the stream. The transport by the central core of the current, defined as those waters moving at 1 m s−1 or faster, equals 0.9 × 105 m2 s−1, has no seasonal signal, and is constant to within a few percent when averaged in half-year intervals. If the central core of the current is viewed as “insolated” from the effects of meandering, this result implies substantial stability to the large-scale wind-driven and thermohaline circulations during the observation program. Variations in poleward heat transport probably originate less in the GS and more from changing heat loss patterns at higher latitudes. Other issues concerning the potential vorticity field and energy conversion rates are also discussed. This ongoing program illustrates the role commercially operated vessels can play in making repeat observations of the velocity structure (and other parameters) of the ocean on a regular basis

The Volunteer Observing Ship and Future Ocean Monitoring

Widespread and sustained in situ ocean measurements are essential to an improved understanding of the state of the ocean and its role in global change. Merchant marine vessels can play a major role in ocean monitoring, yet apart from routine weather observations and upper-ocean temperature measurements, they constitute a vastly underutilized resource due to lack of suitable instrumentation. Examples of ways in which vessels can assist include profiling techniques of physical properties, chemical sampling via automated water samplers, optical techniques to measure various biological parameters, and ground truth measurements for remote sensing from orbiting and geostationary satellites. Further, ships can act as relays between subsurface instrumentation and satellite communication services. To take advantage of the opportunities that the maritime industry can provide, two steps must be taken. The first is to initiate an instrumentation development program with emphasis on techniques optimized for highly automated use onboard ships at 15-20-kt speeds. The second is to forge partnerships or links between academic and government laboratories and the maritime industry for the institution and maintenance of such monitoring programs. No doubt significant resources will be required, but in the long run the improved ability to monitor the state of ocean in situ will make the effort more than worthwhile

A Miniature Acoustic Device for Tracking Small Marine Animals or Submerged Drifters

This paper presents an acoustic archival tag capable of tracking small marine animals. It is also a technology that can be ported to other platforms, such as the next-generation acoustic and Argo floats as well as gliders. Tracking is achieved by standard RAFOS triangulation using the arrival times of unique sound signals emitted by moored sources. At the core of the tag is a custom microchip that controls all system operations. It incorporates the critical acoustic arrival time detector, a thermal sensor, and a pressure sensor interface. All the electronic components are housed inside a cylindrical hydrophone of 25.4-mm length and 10.7-mm diameter. The collected data are archived in nonvolatile memory chips with a total capacity of 4 Mb, sufficient storage to record position, temperature, and pressure on an hourly basis for 2 years. The tag consumes 4–5 μW in standby mode and between 60 and 90 μW while the sound arrival time detector is in operation. The power is provided by two button cell silver-oxide batteries, which enable an active tag lifetime of approximately 2 years

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

A Simple Method for Measuring Deep Convection

The glass-pipe technology used for RAFOS floats is applied to the monitoring of convection in deep mixed layers. The velocity of a vertical current is estimated from the relationship between the drag force exerted on a float by the vertical current and the buoyancy force induced by the float\u27s resultant displacement from hydrostatic equilibrium. Tests conducted in the winters of 1990 and 1991 in the 18°C waters of the northwestern Sargasso Sea reveal definite convective events. Vertical velocities of both upwelling and downwelling plumes are estimated to approach maxima nearing 0.05 m s−1, with durations of up to 2 h. One float that crossed the Gulf Stream and entered the Newfoundland Basin showed evidence of very active vertical currents in the near-surface waters with maximum velocities greater than 0.09 m s−1

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...

The RAFOS System

The RAFOS float is a small neutrally buoyant subsurface order, which, like its big brother the SOFAR float, uses the deep sound (or SOFAR) channel to determine its position as a function of time. Whereas the SOFAR float transmits to moored receivers, the ∼12 kg glass pipe RAFOS float listens for accurately timed signals from moored sound sources to determine its position. The acoustic signal detection and norm of data are all handled by a CMOS microprocessor in the float. The data are recovered at the end of its mission when the float surface and telemeters its memory contents to Systeme Argos, a satellite-borne platform location and data collection system. Just a few sound sources provide navigation for an arbitrary number of floats

Atlantic water transformation along its poleward pathway across the Nordic Seas

The warm and salty Atlantic Water is substantially modified along its poleward transit across the Nordic Seas, where it reaches deeper isopycnals. In particular, the Lofoten Basin, exposed to intense air‐sea interactions, plays a crucial role in the transformation of Atlantic Water. Averaged over a seasonal cycle, Atlantic Water releases approximately 80 W/m2 of heat to the atmosphere over a large area, leading to winter mixed layer depths of up to 500 m (locally exceeding 1,000 m in the Lofoten Basin Eddy, a permanent vortex located in the basin center) and substantial water mass transformation. We investigate spiciness injection (temperature and salinity increase) by winter mixing, by performing an isopycnal analysis using a comprehensive observational data set covering the 2000–2017 period. Compared to the Atlantic Water properties at the Svinøy section, representative of the inflowing Atlantic Water, some isopycnals reveal an important warming (up to 1.5°C) and salinification (up to 0.2 g/kg). Key areas for spiciness injection are the western Lofoten Basin and west of Bear Island. The modified spicy Atlantic Waters coincide with low potential vorticity with strongly density‐compensated layers at their base, allowing double‐diffusion processes to occur farther downstream toward the Arctic. Despite its limited spatial extent, the Lofoten Basin Eddy exhibits the greatest spiciness injection, as well as the deepest mixed layer and thickest low potential vorticity layer of the Norwegian Seas. The Atlantic Water spiciness at Svinøy shows a downstream correlation in the Lofoten Basin and farther north toward the Arctic with a lag of 1 to 1.5 years.publishedVersio