RAFOS Float Data Research Articles

Observed and modeled pathways of the Iceland Scotland Overflow Water in the eastern North Atlantic

The spreading of Iceland Scotland Overflow Water (ISOW) in the eastern North Atlantic has largely been studied in an Eulerian frame using numerical models or with observations limited to a few locations. No study to date has provided a comprehensive description of the ISOW spreading pathways from both Eulerian and Lagrangian perspectives. In this paper, we use a combination of previously unreported current meter data, hydrographic data, RAFOS float data, and a high resolution (1/12°) numerical ocean model to study the spreading pathways of ISOW from both of these perspectives. We identify three ISOW transport cores in the central Iceland Basin (∼59°N), with the major core along the eastern boundary of the Reykjanes Ridge (RR) and the other two in the basin interior. Based on trajectories of observed and/or numerical floats seeded along 59°N, we also describe the ISOW spreading pathways and quantify their relative importance. Within 10 years, 7–11% of ISOW from 59°N escapes into the Irminger Sea via gaps in the RR north of the Charlie Gibbs Fracture Zone (CGFZ); the water that moves through these gaps principally originates from the shallower ISOW layer along the RR eastern boundary. 10–13% travels further southward until the CGFZ, where it crosses westward into the western subpolar gyre. 18–21% of ISOW spreads southward along the eastern flank of the Mid-Atlantic Ridge into the Western European Basin (WEB). Most of the remaining water stays in the Iceland Basin over the 10-year period. A model-based investigation provides a first look at the temporal variability of these ISOW pathways. We find that the fraction of southward water exported into the WEB is anti-correlated with the export through the CGFZ, a result assumed to reflect these pathways’ interactions with the North Atlantic Current in magnitude and/or position shift.

Circulation and Stirring in the Southeast Pacific Ocean and the Scotia Sea Sectors of the Antarctic Circumpolar Current

AbstractThe large-scale middepth circulation and eddy diffusivities in the southeast Pacific Ocean and Scotia Sea sectors between 110° and 45°W of the Antarctic Circumpolar Current (ACC) are described based on a subsurface quasi-isobaric RAFOS-float-based Lagrangian dataset. These RAFOS float data were collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The mean flow, adjusted to a common 1400-m depth, shows the presence of jets in the time-averaged sense with speeds of 6 cm s−1 in the southeast Pacific Ocean and upward of 13 cm s−1 in the Scotia Sea. These jets appear to be locked to topography in the Scotia Sea but, aside from negotiating a seamount chain, are mostly free of local topographic constraints in the southeast Pacific Ocean. The eddy kinetic energy (EKE) is higher than the mean kinetic energy everywhere in the sampled domain by about 50%. The magnitude of the EKE increases drastically (by a factor of 2 or more) as the current crosses over the Hero and Shackleton fracture zones into the Scotia Sea. The meridional isopycnal stirring shows lateral and vertical variations with local eddy diffusivities as high as 2800 ± 600 m2 s−1 at 700 m decreasing to 990 ± 200 m2 s−1 at 1800 m in the southeast Pacific Ocean. However, the cross-ACC diffusivity in the southeast Pacific Ocean is significantly lower, with values of 690 ± 150 and 1000 ± 200 m2 s−1 at shallow and deep levels, respectively, due to the action of jets. The cross-ACC diffusivity in the Scotia Sea is about 1200 ± 500 m2 s−1.

A Method for Obtaining the Mean Transports of Ocean Currents by Combining Isopycnal Float Data with Historical Hydrography

This article presents a method for obtaining the mean structure of the temperature, specific volume anomaly, and velocity of an ocean current, using isopycnal float data combined with gravest empirical mode (GEM) fields calculated from historical hydrography. A GEM field is a projection on a geostrophic streamfunction space of hydrographic data, which captures most of the vertical structure associated with frontal regions. This study focuses on the North Atlantic Current–subpolar front (NAC–SPF) current system, but the float–GEM method has broad applicability to baroclinic ocean currents in general. The NAC–SPF current system is of climatic interest, being an important conduit of warm salty waters into the northern North Atlantic. It constitutes the upper limb of the thermohaline circulation of the Atlantic Ocean and plays a crucial role in the moderation of European climate, but uncertainties regarding its transport and corresponding heat fluxes remain, mainly because the structure of the system is not well known. This paper shows how isopycnal floats can be used to obtain such estimates. The performance of the float–GEM method is tested in two ways. First, two synoptic hydrographic sections (one across the NAC and the other across the SPF) are reconstructed from simulated isopycnal float pressure measurements. The baroclinic transports of volume and temperature (relative to 1000 dbar) across the sections are well reproduced by the method: the float–GEM transport estimates have an accuracy of ±20% and a precision of ±15% or less, which result in deviations of less than ±10% from the “real” values. In the second test, horizontal maps of pressure and temperature on the δ = −12.7 × 10−8 m3 kg−1 specific volume anomaly surface (σθ ≈ 27.5 kg m−3) are produced, using RAFOS float data from two experiments that sampled the region from 1993 to 2000. These maps compare well with similar maps constructed in previous studies and establish the consistency of the method. The good performance of the float–GEM method gives confidence in this novel way of using isopycnal floats to obtain information on the structure of the ocean. Combined with the velocity measured by the floats, it has the potential to estimate absolute transports and heat fluxes along the NAC–SPF system.

Two Circulation Regimes of the Mediterranean Outflow Revealed by Lagrangian Measurements

The Eurofloat experiment was a joint initiative to examine the large-scale spreading of Mediterranean Water (MW) and Labrador Sea Water in the northeast North Atlantic. RAFOS float data from the southern (MW) portion of the Eurofloat experiment have been examined in conjunction with historical float data in order to calculate quasi-Eulerian means in an effort to separate and quantify the constituents of the spreading of the MW tongue east of the Mid-Atlantic Ridge. While recent studies focussed chiefly on the role of meddies in the shaping of the MW tongue, this analysis also examines the tongue's second constituent, that is, the “background” (non-meddy advective and diffusive) flow. The results suggest the existence of two regimes approximately to the north and south of the 36°N parallel (i.e., the latitude of the Gulf of Cadiz), which are distinguished by different types of dominant spreading mechanisms for MW. To the south of the Gulf of Cadiz, the background flow shows an incoherent and weak mean, whereas the mean velocity of the salt enhanced meddies is strong and to the southwest. In contrast, to the north of 36°N the mean velocity of the meddies seems to be less pronounced and the background flow is shown to be a major component in the northwestward spreading of the MW tongue. The two regimes are separated by the Azores Current, which previously has been hypothesized to act as a dynamic barrier to the southward advective spreading of the background regime, which the meddies are able to penetrate because of their high kinetic energy. Overall, the meddies are calculated to contribute to approximately half of the total salinity anomaly flux.

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...

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...