RAFOS Research Articles

Variability of Iceland Scotland Overflow Water Across the Reykjanes Ridge: 2‐Years of Moored Observations in the Bight Fracture Zone

AbstractThis study presents the first continuous observations of Iceland Scotland Overflow Water (ISOW) passing through the Bight Fracture Zone (BFZ), the northernmost deep bathymetric channel across the Reykjanes Ridge between the Iceland and Irminger Basins in the subpolar North Atlantic. Data from two 2‐year moorings, measuring temperature, salinity, and current velocity from 2015 to 2017, along with a set of deep ISOW‐embedded RAFOS floats, are used to investigate ISOW transport and water property variability through the BFZ, as well as advective pathways between the Iceland and Irminger Basins. The mooring‐derived record‐mean ISOW transport through the BFZ was −0.59 ± 0.27 × 1e6 m3/s (westward) and varied seasonally with weaker transport in winter and stronger transport in summer. Flow direction of ISOW through the BFZ was consistently westward except in winter, when week‐long flow reversals were frequently observed. The previously reported subpolar North Atlantic freshening event of the 2010s is evident in the BFZ mooring records beginning about January 2017. About one‐quarter of floats deployed in ISOW at 1800‐m depth upstream in the Iceland Basin show a direct advective pathway into the BFZ that appears to be primarily determined by bathymetry. Another quarter of the floats crossed over the ridge to the Irminger Sea through other gaps prior to reaching the Charlie‐Gibbs Fracture Zone.

Advective pathways and transit times of the Red Sea Overflow Water in the Arabian Sea from Lagrangian simulations

The present study investigates the advective pathways and transit times of virtual particles released in the Red Sea outflow area as a proxy for the poorly understood spreading of the Red Sea Overflow Water (RSOW) in the Arabian Sea. This work uses the Parcels toolbox, a Lagrangian framework, to simulate tens of thousands of trajectories under different initial conditions. Six different Lagrangian simulations are performed at isobaric and isopycnal surfaces within the RSOW layer. All simulations are based on the eddy-rich GLORYS12 reanalysis that merges almost all in-situ (temperature–salinity) and satellite observations collected over the last two decades into a dynamical framework. This study shows that GLORYS12 reproduces relatively well the climatological seasonal cycle of the RSOW to the Gulf of Aden and essential characteristics of the exchange at the Strait of Bab al-Mandab. Statistical comparisons between synthetic trajectories and RAFOS floats in the Gulf of Aden corroborate the quality of GLORYS12 velocity fields used for the Lagrangian simulations. Six main advective pathways are uncovered (by order of preference): Southwest, Northwest, Socotra Passage, Central, Eastern, and Southern. Trajectories from Argo floats give observational support for some of these paths. Although most particles are exported out of the Arabian Sea off Somalia, the simulations reveal robust connectivity of the RSOW to the Arabian Sea interior and its eastern boundary. The fact that particles have long trajectories in the interior increases the potential of RSOW mixing with the fresher and oxygen-poor ambient waters. Thus, these pathways may have profound implications for the salt and oxygen budgets in the Arabian Sea and beyond since the RSOW is also part of the global overturning circulation and exported out of the Indian Ocean via the Agulhas Current. Transit time distributions indicate that it takes about six months for outflow-originated particles to spread over the entire Gulf of Aden and one to three years to be exported along the western boundary, toward Somalia (Socotra Passage and Southwest pathways) and off the Yemeni–Omani coast (Northwest Pathway). In contrast, reaching the eastern boundary takes much longer. North of 14°N, the most frequent time is around 10–15 years, and about 20–25 years at the southeastern Arabian Sea. Hence, the RSOW can often carry oxygen to the western boundary but not to the eastern basin. This may contribute to the eastern shift of the Arabian Sea Oxygen Minimum Zone, a subject that deserves investigation.

Lagrangian Geography of the Deep Gulf of Mexico

AbstractUsing trajectories from acoustically tracked (RAFOS) floats in the Gulf of Mexico, we construct a geography of its Lagrangian circulation within the 1500–2500-m layer. This is done by building a Markov-chain representation of the Lagrangian dynamics. The geography is composed of weakly interacting provinces that constrain the connectivity at depth. The main geography includes two provinces of near-equal areas separated by a roughly meridional boundary. The residence time is about 4.5 (3.5) years in the western (eastern) province. The exchange between these provinces is effected through a slow cyclonic circulation, which is well constrained in the western basin by preservation of f/H, where f is the Coriolis parameter and H is depth. Secondary provinces of varied shapes covering smaller areas are identified with residence times ranging from about 0.4 to 1.2 years or so. Except for the main provinces, the deep Lagrangian geography does not resemble the surface Lagrangian geography recently inferred from satellite-tracked drifter trajectories. This implies disparate connectivity characteristics with potential implications for pollutant (e.g., oil) dispersal at the surface and at depth. Support for our results is provided by a Markov-chain analysis of satellite-tracked profiling (Argo) floats, which, while forming a smaller dataset and having seemingly different water-following characteristics than the RAFOS floats, replicate the main aspects of the Lagrangian geography. Our results find further validation in independent results from a chemical tracer release experiment.

Horizontal mixing in the Southern Ocean from Argo float trajectories

AbstractWe provide the first observational estimate of the circumpolar distribution of cross‐stream eddy diffusivity at 1000 m in the Southern Ocean using Argo float trajectories. We show that Argo float trajectories, from the float surfacing positions, can be used to estimate lateral eddy diffusivities in the ocean and that these estimates are comparable to those obtained from RAFOS floats, where they overlap. Using the Southern Ocean State Estimate (SOSE) velocity fields to advect synthetic particles with imposed behavior that is “Argo‐like” and “RAFOS‐like” diffusivity estimates from both sets of synthetic particles agreed closely at the three dynamically very different test sites, the Kerguelen Island region, the Southeast Pacific Ocean, and the Scotia Sea, and support our approach. Observed cross‐stream diffusivities at 1000 m, calculated from Argo float trajectories, ranged between 300 and 2500 m2 s−1, with peaks corresponding to topographic features associated with the Scotia Sea, the Kerguelen Plateau, the Campbell Plateau, and the Southeast Pacific Ridge. These observational estimates agree with previous regional estimates from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) near the Drake Passage, and other estimates from natural tracers (helium), inverse modeling studies, and current meter measurements. These estimates are also compared to the suppressed eddy diffusivity in the presence of mean flows. The comparison suggests that away from regions of strong topographic steering suppression explains both the structure and magnitude of eddy diffusivity but that eddy diffusivities in the regions of topographic steering are greater than what would be theoretically expected and the ACC experiences localized enhanced cross‐stream mixing in these regions.

Vertically averaged velocities in the North Atlantic Current from field trials of a Lagrangian electric-field float

A subsurface Lagrangian float that utilizes motional induction to calculate vertically averaged velocities was tested in the North Atlantic Current (NAC), taking advantage of existing cruises and infrastructure. The Electric Field Float (EFF) is a RAFOS float with horizontal electrodes that measures its own velocity by RAFOS tracking and calculates vertically averaged velocities when merged with the electrode system. The observations showed depth-averaged velocities that were fast in the core of the NAC (0.6–0.9ms −1) and moderate in adjacent recirculations and eddies (0.3–0.4ms−1). A float at 850dbar moved at close to the depth-averaged velocity, while shallower floats followed surface intensified flow on top of the depth-averaged motion. Integral time scales of depth-averaged velocity (1.3–1.6±0.4d) are slightly shorter than time scales of float velocity (1.6–2.0±0.3d), while integral length scales of depth-averaged water velocity (35±10km for u, 18±6km for v) are slightly shorter than length scales of float motion (53±12km for u, 28±6km for v). Velocity spectra of depth-averaged velocity show significant variance at inertial periods. Quantitative and qualitative validation with multiple independent data sets confirms the accuracy of the instrument and sampling strategy in the NAC, advancing the limited observational knowledge of depth-averaged circulation in subpolar regions.

Meddy dynamics and interaction with neighboring eddies southwest of Portugal: Observations and modeling

During the SEMANE 2000 experiment southwest of Portugal, two meddies were found in near contact. These meddies had hydrological radii of about 20 and 30 km, thickness of 900 m, maximum temperatures of 12.45°C and 13.45°C, and maximum salinities of 36.52 and 36.78. The smaller meddy with more pronounced thermohaline anomalies was clearly double cored (at 750 and 1300 m depths) while the wider one was more diffuse and more homogeneous. The associated geostrophic velocities (referenced at 2000 m) locally reached 0.5 m/s in the smaller meddy, and 0.2 m/s in the wider one. Three RAFOS floats and two deep‐drogued surface drifters, seeded in the two meddies, rapidly gathered in the more intense meddy. This meddy trajectory, revealed by the float motion, was first eastward, then southward. Maps of sea level anomaly indicate that this motion did not correspond to the long‐term evolution of the initial positive sea level anomaly signature of the meddies, and that neighboring cyclones must have played a role in the meddy evolution. To determine the role of each eddy in the observed evolution, several scenarios were studied with a three‐layer quasi‐geostrophic numerical model. The interaction of two meddies in isolation did not result in the observed meddy trajectories on the long term. The interaction of these two meddies with successive neighboring cyclones provided a more realistic trajectory of the meddy containing the floats.

On westward transport processes off central California revealed by RAFOS floats

[1] Westward transport processes off central California are quantified by exit time statistics for RAFOS floats launched at mid-depths (275–375 m). The random displacement model in the diffusion limit was used to interpret results. The alongshore-averaged westward motion of floats represented inhomogeneous eddy diffusion interspersed with short-duration advective motions. The across-shore diffusion coefficient grew as a power law with distance from the coastline (the power exponent was close to one). This is evidence for offshore growth of eddy kinetic energy and may be explained through flow barotropization accompanied by a quasi-geostrophic turbulent regime. Typical values of the float drift velocity were about 1.8–2 cm/s but, offshore, drift velocities were as high as 4–6 cm/s during the upwelling season. Between 150 and 250 km from shore, a transition zone between nearshore and offshore was clearly identified in spring but was either very narrow or undetectable in other seasons.

Intercomparing drifts from RAFOS and profiling floats in the deep western boundary current along the Mid-Atlantic Ridge

The Lagrangian nature of cycling floats is evaluated in the framework defined by the Deep Western Boundary Current of the Mid-Atlantic Ridge. In a statistical approach, speeds and drifts are estimated for an APEX cycling float and compared with the velocities inferred from a park ensemble of four eddy-resolving RAFOS floats. They were deployed at the same location and ballasted for drifting at the same mission depth. Displacement errors induced by geostrophic shear and wind forced currents are analyzed. We observe that the velocity estimated from the RAFOS floats is not statistically different from the velocity estimated from the APEX float. Likewise, the initial separation between the cycling float and a simultaneously deployed RAFOS float has been studied in terms of the turbulent diffusivity. Though the performance of this study in comparable cases without a mean current field may be limited, these oceanic observations support exploiting the Lagrangian nature of the cycling floats.

Characteristics of intermediate water flow in the Benguela current as measured with RAFOS floats

Abstract Seven floats (not launched in rings) crossed over the mid-Atlantic Ridge in the Benguela extension with a mean westward velocity of around 2 cm / s between 22S and 35S. Two Agulhas rings crossed over the mid-Atlantic Ridge with a mean velocity of 5.7 cm / s toward 285°. This implies they translated at around 3.8 cm / s through the background velocity field near 750 m . The boundaries of the Benguela Current extension were clearly defined from the observations. At 750 m the Benguela extension was bounded on the south by 35S and the north by an eastward current located between 18S and 21S. Other recent float measurements suggest that this eastward current originates near the Trindade Ridge close to the western boundary and extends across most of the South Atlantic, limiting the Benguela extension from flowing north of around 20S. The westward transport of the Benguela extension was estimated to be 15 Sv by integrating the mean westward velocities from 22S to 35S and multiplying by the 500 m estimated thickness of intermediate water. Roughly 1.5 Sv of this are transported by the ∼3 Agulhas rings that cross the mid-Atlantic Ridge each year (as observed with altimetry). This value of the Benguela extension transport is the first one to have been obtained from long-term (two-year) observations and across the full width of the Benguela extension.

Structure of the Mediterranean Undercurrent and Mediterranean Water spreading around the southwestern Iberian Peninsula

[1] RAFOS float and hydrographic data are analyzed to investigate the pathways, rates, and mechanisms of Mediterranean Water (MW) spreading into the North Atlantic from the Gulf of Cadiz, with an emphasis on processes other than meddies. We find that the transition of the Mediterranean outflow from a density current to an intermediate-depth jet occurs as it enters Portimao Canyon south of Portugal due to rapidly steepening topography and associated turbulent entrainment. This feature was not seen in recent plume model simulations due to their use of smoothed topography. In addition to the traditional lower salinity maximum (or core), a denser, deeper continuous vein of MW was also traceable from Portimao Canyon to Cape St. Vincent. The mean velocity structure of the Mediterranean Undercurrent changes from a ∼30-km-wide jet with peak mean westward speed of ∼0.2 m s−1 along the southern Iberian slope, to a broader, much weaker northward flow with maximum mean speed of only ∼0.06 m s−1 along the western Iberian slope, due in part to changes in the bottom slope. Lower core MW spread mainly (1) southward into the interior Gulf of Cadiz, (2) northward along the western Iberian slope, and (3) westward into the deep Tagus Abyssal Plain. The average spreading rate was 0.01–0.02 m s−1 toward the west–northwest direction. Meddies forming near the slope were found to have an indirect but profound impact on MW spreading, often diverting MW approaching from upstream into the interior. The results are compared to recent hydrography-based studies of MW spreading pathways.

Observations of an intense anticyclonic warm eddy in the Newfoundland Basin

An intense anticyclonic warm eddy was sampled in February 1997 in the Newfoundland Basin. Its presence, detected during a hydrographic survey, was further corroborated through ERS/TOPEX sea surface height anomaly and satellite tracked drifters. This 120 km wide eddy was very homogeneous in temperature and salinity down to 800 m and its transport (15–1800 dbar) reached 27 Sv. Its location, 49°N 40°W, in the warm and saline waters of the Northwest Corner, the northernmost position reached by the North Atlantic Current, is clearly exceptional. Although intense anticyclonic eddies have previously been observed with satellite tracked buoys and RAFOS floats, this is the first eddy (to our knowledge) ever documented at such a high latitude in the North Atlantic Current system. Its formation is discussed in term of thermohaline processes.

An Alternative Hypothesis for the Origin of the “Mediterranean” Salt Lens Observed off the Bahamas in the Fall of 1976

Abstract A hypothesis is presented that the original salt lens, or “meddy,” observed off the Bahamas in the fall of 1976 may have been formed, not near the Mediterranean outflow, but instead in the vicinity of the northwest corner (51°N, 43°W) of the North Atlantic Current. An eddy was observed near the northwest corner by an isopycnal RAFOS float deployed during the 1993–95 North Atlantic Current Experiment, and had nearly identical temperature/salinity properties as those of the Bahamas lens. Hydrographic evidence of thick homogeneous layers with similar properties near the northwest corner suggest a possible formation mechanism by which surface eddies containing warm and saline waters are cooled and subducted. A plausible scenario is made whereby a northwest corner eddy might be advected southward in the Newfoundland Basin by the flow around the high pressure ridge east of the North Atlantic Current and then enter the recirculation gyre immediately south of the Gulf Stream. Such an eddy could be advect...

Spreading of Labrador Sea Water in the eastern North Atlantic

The spreading of Labrador Sea Water (LSW) in the eastern North Atlantic south of 55°N is described on the basis of recent, high‐resolution hydrographic lines and using an inverse model of that basin general circulation. The isopycnic “salinity anomaly” relative to a standard θ/S curve is used to detect the main branches and limits of dominant influence of the intermediate water masses. The LSW crosses the Mid‐Atlantic Ridge at latitudes around 50°N with parts spreading farther eastward and southward. One southward pathway along the eastern side of the Mid‐Atlantic‐Ridge stands out as an eastern basin counterpart of the western Atlantic “Deep Western Boundary Current.” An entry into the Bay of Biscay in its northern part is also evidenced with striking similitudes between two data sets from 1974 and 1989–1990. At the southern limit of the LSW extension, in the vicinity of the Azores‐Biscay Rise, a marked front exists basinwide around 1800 m with the more saline Deep Mediterranean Water (DMW). The inverse model velocity field is consistent with most features of the tracer distributions and allows estimation of LSW transports: 6.3±1.2 Sv of LSW with salinities lower than 34.94 cross the 35°W meridian eastward between 44°N and 54°N, one fourth of which recirculates southwestward in the western basin, while another fourth turns northward across 54°N and the remaining half proceeds southward in the eastern basin. The model circulation suggests that DMW results from mixing between eastern basin LSW and the overlying Mediterranean Outflow Water, most probably through double diffusion. We finally describe several mesoscale structures of anomalously high LSW influence south of the LSW‐DMW front, notably one sampled by a RAFOS float. The velocity and potential vorticity fields suggest that these “LSW‐eddies” are formed through baroclinic instability at the LSW‐DMW front.

Isopycnal RAFOS floats as roving hydrographers in the North Atlantic Current Region

The properties measured by isopycnal floats can be compiled to provide hydrographic information about the sampled surfaces. The height and temperature of two specific volume anomaly surfaces reported by 84 RAFOS floats deployed in the North Atlantic Current (NAC' region are compared here with the climatological hydrographic database. The mean pathway of the NAC and other prominent features of the region are similar in float‐derived and climatological surfaces. The most striking difference between floats and climatology, an extended trough in the NAC path at 44°N in the float‐derived surface, reflects the sampling time scales. The baroclinic transport of the NAC estimated from RAFOS floats using the observed climatological relationship between the potential energy anomaly and the height of the specific volume anomaly surfaces was 42 Sv at 42°N and 24 Sv at 47°N, in reasonable agreement with climatological estimates.

Analysis of Lagrangian Potential Vorticity Balance and Lateral Displacement of Water Parcels in Gulf Stream Meanders

Abstract The balance of potential vorticity components following fluid parcel motion in Gulf Stream meanders was studied using RAFOS float data from the SYNOP Experiment. By introducing curvature dependent variations to the velocity and density fields, the authors relaxed the rigid field assumption used in earlier studies and examined closely 61 floats in the upper layers (13°–16°C) of the main thermocline. Float trajectories were segmented according to transition from crest to trough and trough to crest, and grouped by their positions relative to the current center. A total of 154 segments were collected to estimate the horizontal divergence and the mean lateral displacement of parcels under two distinct regimes: growing and decaying meanders. Both spatial and temporal changes in curvature affect the regions of divergence in a meandering stream. On the one hand, horizontal divergence increases with increasing curvature magnitude, while on the other hand, the divergence pattern itself changes going from g...

Lagrangian Studies of Fluid Exchange between the Gulf Stream and Surrounding Waters

Abstract Between spring of 1988 and winter of 1990, 75 RAFOS floats were released east of Cape Hatteras near the center of the Gulf stream on the 26.8 σt surface, O (15°C). The purpose of this sequential deployment was to investigate the spatial and temporal characteristics of the meandering stream and the Lagrangian properties of fluid motion in the upper thermocline. These new observations were also intended to provide a database for comparative studies of fluid motion in the upper thermocline with similar observations made earlier (1984–85) in the middle and lower thermocline. Sixty-one of the 75 RAFOS float trajectories in the 13°–16°C layers have been analyzed from both a Lagrangian and Eulerian point of view. The results reconfirm the strong baroclinic structure of the Gulf Stream established earlier and reveal enhanced cross-stream motion with increasing depth and over the New England Seamounts, Float trajectories tend to be more convoluted east of the seamounts reflecting the large amplitude meand...

Instability waves in the Gulf Stream front and its thermocline layer

We carried out linear instability calculations on a three layer Gulf Stream front model in an attempt to elucidate the interaction of the thermocline layer with surface slopewater shoreward of the front. The basic state is geostrophic balance and constant potential vorticity in the two active layers, but the perturbations are ageostrophic. We found the flow to be unstable to long wave perturbations, the wavelength of the most unstable wave to be of the order of 10 radii of deformation. The instability is mainly baroclinic, 75-85% of the energy supply to the growing perturbation coming from basic flow potential energy. Calculated wavelengths, phase speeds and growth rates, using parameters typical of the Gulf Stream, are similar to those observed. The eigenfunctions of the perturbations show peak cross-front thermocline motions near the inflection points of a frontal wave, and a cyclonic eddy with closed streamlines under a trough, an anticyclonic eddy under a crest. The combined flow (basic state plus perturbation) in the thermocline layer follows the surface streamlines closely, except for small cross-stream anomalies, shoreward just upstream of a wave crest, seaward upstream of a trough. Calculated trajectories have characteristics similar to those observed by RAFOS floats, except that they suggest exchange of thermocline waters exclusively with slopewater.

An Efficient Sound Source for Wide-Area RAFOS Navigation

Abstract To meet the expected need for wide-area acoustic navigation for Lagrangian studies of ocean circulation using RAFOS floats, a new and powerful sound source, a resonant pipe projector has been developed. It consists of a free-flooded open steel pipe with a ceramic-steel driver ring at its midsection. Conservatively demonstrated here at a source level of 195.5 dB re 1 µpa @ 1 m and with an energy conversion efficiency of 85% at resonance (260 Hz), useful operating ranges to 4000 km and beyond are possible, depending on ambient noise conditions. A successful 6-month accelerated test of the complete transducer system was recently completed near Bermuda.

Evidence of Cross-Frontal Exchange Processes in the Gulf Stream Based on Isopycnal RAFOS Float Data

Abstract A unique set of Lagrangian observations has recently been collected in the Gulf Stream using the newly developed isopycnal RAFOS float. Between January, 1984 and October, 1985, thirty-seven of these drifters were launched in the main thermocline of the current off Cape Hatteras and lacked acoustically downstream for 30 or 45 days. Temperature and pressure were also recorded along each float trajectory. The isopycnal capability of this drifter allows it to follow fluid parcel pathways quite accurately along the sloping density surfaces of the Gulf Stream. The RAFOS drifters revealed a striking pattern of vertical and cross-stream motion in Gulf Stream meanders. Floats were consistently observed to upwell (downwell) and move onshore (offshore) as they approached anticyclonic (cyclonic) meander crests (troughs). The rms vertical velocity in the center of the stream was observed to be 0.08 cm s−1 on the 12°C surface. No mean vertical motion was detected in the main thermocline of the Gulf Stream betw...

Particle Pathways in the Gulf Stream

An experiment is under way to study the kinematics, dynamics, and path evolution of the Gulf Stream front between Cape Hatteras and 60°W. The Rafos float, which can track the true motion of water parcels along density surfaces which slope steeply across the Gulf Stream, has recently been developed for this study. These instruments are launched in the center of the Gulf Stream every 5–15 days for a 30-day mission. Each float provides a trajectory and a continuous record of temperature and pressure along the trajectory. Our results so far show that: a) cross-stream motion has a significant vertical component (ranging to some 0.1 cm · s−1) compared to vertical velocities in midocean; b) floats systematically shoal (upwell) as they approach anticyclonic meanders and deepen (downwell) as they approach cyclonic meanders; c) more than half of the floats launched so far remained trapped in the Gulf Stream for distances on the order of 1000 km; d) when floats do escape from the stream, it is more likely they will do so to the south; and e) both internal processes, such as the rapid growth of meanders, and external features, such as nearby rings or eddies, can expel or extract floats from the stream. These observations are also being used in numerical simulations of the Gulf Stream to develop predictive skills for the path of the current.