Autonomous Lagrangian Circulation Explorer Research Articles

How Does Labrador Sea Water Enter the Deep Western Boundary Current?

Abstract Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current as it transits the Labrador Sea. The formation of LSW directly in the DWBC and the eddy heat flux between the interior Labrador Sea and the DWBC may play leading roles in setting the interannual variability of the exported water mass.

Estimating Eddy Heat Flux from Float Data in the North Atlantic: The Impact of Temporal Sampling Interval

Abstract Acoustically tracked float data from 16 experiments carried out in the North Atlantic are used to evaluate the feasibility of estimating eddy heat fluxes from floats. Daily float observations were bin averaged in 2° by 2° by 200-db-deep geographic bins, and eddy heat fluxes were estimated for each bin. Results suggest that eddy heat fluxes can be highly variable, with substantial outliers that mean that fluxes do not converge quickly. If 100 statistically independent observations are available in each bin (corresponding to 500–1000 float days of data), then results predict that 80% of bins will have eddy heat fluxes that are statistically different from zero. Pop-up floats, such as Autonomous Lagrangian Circulation Explorer (ALACE) and Argo floats, do not provide daily sampling and therefore underestimate eddy heat flux. The fraction of eddy heat flux resolved using pop-up float sampling patterns decreases linearly with increasing intervals between float mapping and can be modeled analytically. This implies that flux estimates from pop-up floats may be correctable to represent true eddy heat flux.

Deep Cyclonic Circulation in the Gulf of Mexico

Abstract The anticyclonic Loop Current dominates the upper-layer flow in the eastern Gulf of Mexico, with a weaker mean anticyclonic pattern in the western gulf. There are reasons, however, to suspect that the deep mean flow should actually be cyclonic. Topographic wave rectification and vortex stretching contribute to this cyclonic tendency, as will the supply of cold incoming deep water at the edges of the basin. The authors find that the deep mean flow is cyclonic both in the eastern and western gulf, with speeds on the order of 1–2 cm s−1 at 2000 m. Historical current-meter mooring data, as well as profiling autonomous Lagrangian circulation explorer (PALACE) floats (at 900 m), suggest that vertical geostrophic shear relative to 1000 m gives a surprisingly accurate result in the interior of the basin. The temperature around the edges of the basin at 2000 m is coldest near the Yucatan Channel, where Caribbean Sea water is colder by ∼0.1°C. The temperature increases steadily with distance in the counterclockwise direction from the Yucatan, consistent with a deep mean cyclonic boundary flow.

A Statistical Method for Correcting Salinity Observations from Autonomous Profiling Floats: An ARGO Perspective

Abstract The Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float is used to implement the Array for Real-Time Geostrophic Oceanography (ARGO). This study presents a statistical approach to correct salinity measurement errors of an ARGO-type fleet of PALACE floats. The focus is on slowly evolving drifts (typically with time scales longer than a few weeks). Considered for this case study is an ensemble of about 80 floats in the Irminger and Labrador Seas, during the 1996–97 period. Two different algorithms were implemented and validated based on float-to-float data comparison at depth, where the water masses are relatively stable over the time scales of interest. The first algorithm is based upon objective analysis of the float data, while the second consists of a least squares adjustment of the data of the various floats. The authors’ method exhibits good skills to retrieve the proper hydrological structure of the case study area. It significantly improves the consistency of the PALACE dataset with in situ data as well as with satellite altimetric data. As such, the method is readily usable on a near-real-time basis, as required by the ARGO project.

Statistical Behavior of ALACE Floats at the Surface of the Southern Ocean

Autonomous Lagrangian Circulation Explorer (ALACE) floats were designed to measure subsurface velocities throughout the global ocean. In order to transmit their data to satellite, they spend 24 h at the ocean surface during each 10–25-day cycle. During this time the floats behave as undrogued drifters. In the Southern Ocean, floats tend to advect downwind and, in accordance with Ekman theory, slightly to the left of the wind during their time at the surface. Mean displacements are likely to carry floats northward and, correspondingly, with each cycle, the Southern Ocean floats will move into warmer water with higher dynamic height. Because of large variability, the northward trend may not be discernible for any single float: in 2 years' worth of 10-day cycles, a typical float will be displaced 100 ± 270 km northward relative to a float that never surfaces. Float surface velocities and wind speed are statistically correlated at the 95% confidence level. Compared with drogued drifters, floats tend to move more rapidly, are advected more strongly downwind, and are more sensitive to changes in wind speed. Regression coefficients estimated from the differences between float and drogued drifter velocities suggest that floats may be used to estimate the mean upper ocean currents in regions where drogued drifter data are not available.

Float Observations of the Southern Ocean. Part II: Eddy Fluxes

Abstract Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine eddy fluxes in the Southern Ocean. Eddy fluxes are calculated from differences between ALACE float data and mean fields derived from hydrographic atlas data or objectively mapped float observations. Heat fluxes indicate an average poleward eddy heat transport across the Antarctic Circumpolar Current (ACC) of about 3–7 kW m−2 at 900-m depth. Because analysis of current meter data suggests that ALACE's 9–25-day averaging underestimates the total heat flux, the initial ALACE estimates are rescaled to account for this undersampling. This results in a total corrected heat flux of 5–10 kW m−2 at 900 m, depending on the mean field used for the calculations. If the cross-ACC heat flux is assumed to vary exponentially through the water column with an e-folding depth of 1000 m, then the implied net poleward heat flux across the ACC is between 0.3 ± 0.1 and 0.6 ± 0.3 (×1015 W). These estimates are in agreement with previous Southe...

Float Observations of the Southern Ocean. Part I: Estimating Mean Fields, Bottom Velocities, and Topographic Steering

Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine mean flow and eddy fluxes at 900-m depth in the Southern Ocean. Mean temperature and dynamic topography from float data are consistent with earlier estimates from hydrographic surveys, although floats imply warmer temperatures and narrower frontal structures than do atlas data. Differences between hydrographic and ALACE dynamic topography suggest the presence of eastward bottom velocities of about 2 cm s−1 below the eastward-flowing jets of the Antarctic Circumpolar Current. Flow is steered by bathymetry and can be represented as an equivalent barotropic system with an e-folding depth of about 700 m.

Current Measurements of the Japan Sea Proper Water and the Intermediate Water by ALACE Floats

The subsurface current of the Japan Sea was observed by two Autonomous Lagrangian Circulation Explorer (ALACE) floats. One float, having a 20-day cycle, was deployed on 29 July 1995 in the eastern Japan Basin and drifted in the northeastern part of the basin until 15 September 2000. The other float, with a 10-day cycle, was deployed on 4 August 1995 in the western Japan Basin and drifted in the western Japan Basin, in the Yamato Basin and around the Yamato Rise until it reached its life limit in midMay 2000. An anticlockwise circulation in the eastern Japan Basin was observed and it was assumed to be in the upper portion of the Japan Sea Proper Water (UJSPW) or in the intermediate water. The spatial scale of the circulation increased as the depth decreased. A clockwise circulation was observed around the Yamato Rise in the UJSPW. Smaller clockwise and anticlockwise rotations were observed in the western Japan Sea, where a seasonal variation was seen in drift speed with different phase by depth. The correlation coefficient between drift speeds of two floats indicated little coherence among the subsurface circulation between the east and the west of the Japan Basin, or between the north and the south of the subpolar front.

Warming of the Southern Ocean Since the 1950s

Autonomous Lagrangian Circulation Explorer floats recorded temperatures in depths between 700 and 1100 meters in the Southern Ocean throughout the 1990s. These temperature records are systematically warmer than earlier hydrographic temperature measurements from the region, suggesting that mid-depth Southern Ocean temperatures have risen 0.17 degrees C between the 1950s and the 1980s. This warming is faster than that of the global ocean and is concentrated within the Antarctic Circumpolar Current, where temperature rates of change are comparable to Southern Ocean atmospheric temperature increases.

Labrador Sea Water: Pathways, CFC Inventory, and Formation Rates

In 1997, a unique hydrographic and chlorofluorocarbon (CFC: component CFC-11) dataset was obtained in the subpolar North Atlantic. To estimate the synopticity of the 1997 data, the recent temporal evolution of the CFC and Labrador Sea Water (LSW) thickness fields are examined. In the western Atlantic north of 50°N, the LSW thickness decreased considerably from 1994–97, while the mean CFC concentrations did not change much. South of 50°N and in the eastern Atlantic, the CFC concentration increased with little or no change in the LSW thickness. On shorter timescales, local anomalies due to the presence of eddies are observed, but for space scales larger than the eddies the dataset can be treated as being synoptic over the 1997 observation period. The spreading of LSW in the subpolar North Atlantic is described in detail using gridded CFC and LSW thickness fields combined with Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float trajectories. The gridded fields are also used to calculate the CFC-11 inventory in the LSW from 40° to 65°N, and from 10° to 60°W. In total, 2300 ± 250 tons of CFC-11 (equivalent to 16.6 million moles) were brought into the LSW by deep convection. In 1997, 28% of the inventory was still found in the Labrador Sea west of 45°W and 31% of the inventory was located in the eastern Atlantic. The CFC inventory in the LSW was used to estimate the lower limits of LSW formation rates. At a constant formation rate, a value of 4.4–5.6 Sv (Sv ≡ 106 m3 s−1) is obtained. If the denser modes of LSW are ventilated only in periods with intense convection, the minimum formation rate of LSW in 1988–94 is 8.1–10.8 Sv, and 1.8–2.4 Sv in 1995–97

Observations of Open-Ocean Deep Convection in the Labrador Sea from Subsurface Floats*

Abstract The occurrence and extent of deep convection in the Labrador Sea in winters 1996/97 and 1997/98 is investigated from measurements of over 200 neutrally buoyant subsurface Profiling Autonomous Lagrangian Circulation Explorer (PALACE) and Sounding Oceanographic Lagrangian Observer (SOLO) floats. In addition to providing drift velocity data and vertical profiles of temperature and salinity, 55 floats are equipped with vertical current meters (VCMs). Time series of vertical velocity (derived from measured pressure and vertical flow past the float) and temperature are obtained from the VCM floats. Mixed layer depths estimated from profile measurements indicate that convection reached depths greater than 1300 m in 1997, but no deeper than 1000 m in 1998. Deep mixed layers were concentrated in the western basin, although a number of deep mixed layers were observed southwest of Cape Farewell and also north of 60°N. The highest variance in vertical velocity and the lowest mean temperatures were found in t...

The Evaluation of Salinity Measurements from PALACE Floats

Abstract Seven PALACE (Profiling Autonomous Lagrangian Circulation Explorer) floats were deployed in October 1996 in the Irminger Basin of the Atlantic Ocean as a U.K. contribution to the World Ocean Circulation Experiment. Of these floats, four were fitted with a conductivity–temperature–depth package. The floats were ballasted to drift at a depth of about 1500 m, above the Labrador Sea Water (LSW) cold and fresh extreme, and programmed to surface every 14 days. The floats made a profile during each ascent to the surface. The authors present a method to evaluate the performance of the conductivity sensors and to calibrate the float salinity data. Since the LSW appears to be relatively stable over a timescale of ∼1–2 months and a length scale of ∼50–100 km, the authors were able to make direct comparisons between the first year of float data and accurate ship-based measurements and, therefore, were able to correct for errors of the conductivity sensors. A correction was applied in all cases. The conductiv...

Profiling ALACEs and Other Advances in Autonomous Subsurface Floats

Abstract Over the past decade more than 1200 autonomous floats have been deployed worldwide. In addition to velocity as marked by lateral movement, many of these floats measured quantities like profiles of temperature and salinity, temperature microstructure, and time series of vertical velocity. The authors' laboratory's implementation of profile measurements in what is called a Profiling Autonomous Lagrangian Circulation Explorer is described. Biofouling and degradation of antifouling coatings on the conductivity sensor both cause drifts that mean accurate salinity measurements will depend on corrections based on known temperature–salinity relations. A second generation autonomous float called the Sounding Oceanographic Lagrangian Observer (SOLO) has been developed to provide enhanced reliability and to provide complete two-way depth control. A dual hydraulic-pneumatic buoyancy system reduces the energy cost of vertical cycling and buoyancy generation at the surface. A SOLO Vertical Current Meter has be...

Preliminary results from directly measuring middepth circulation in the tropical and South Pacific

As part of the World Ocean Circulation Experiment, 300 Autonomous Lagrangian Circulation Explorer floats were deployed in the tropical and South Pacific to measure the general circulation at 900 m depth. Most floats measured average currents over 25 submerged days between ascents to the surface for locating and data relay. By the end of 1997, over 12,500 observations spanning 840 float years were obtained. Accuracy of the average velocity is limited by variability, some of which is found to arise from steady flows with spatial variations smaller than averaging scales. Observed 25‐day velocities are serially uncorrelated except near the equator where energetic zonal variability has seasonal timescales and appears to be described by a data‐assimilating ocean circulation model run by the National Centers for Environmental Prediction. Space‐time averages of float velocities disclose a general pattern of middepth flow that resembles the surface circulation with the strongest flows of O (3 cm/s) in the deep Antarctic Circumpolar Current and East Australian Current. The weaker elements of the flow, including an interior subtropical circulation with O (millimeters per second) flows, are described by an objective analysis whose accuracy is limited by sampling noise rather than uncertainty of the analysis parameters. The subtropical circulation has a gyres‐within‐a‐gyre structure including boundary currents along Australia and east of New Zealand and a concentrated equatorial limb with surprising strength in the western basin. This current is also involved in a tropical gyre with generally eastward flow along the equator in the western basin at least. Because of strong seasonal‐scale variability, the equatorial zonal currents are not well measured yet but data are still being accumulated. Comparison with Reid's [1997] absolute circulation shows more areas of agreement than of disagreement.

Comparison of Autonomous Lagrangian Circulation Explorer and fine resolution Antarctic model results in the South Atlantic

The motions of eight Autonomous Lagrangian Circulation Explorer (ALACE) floats released near 750 m depth in Drake Passage and followed through the South Atlantic are described and compared with emulations made by advecting model floats through 12 monthly snapshots of velocity from the fine resolution Antarctic model (FRAM). Both ALACEs and FRAM reproduce the major features of the general circulation as follows: strong intermediate depth flow in Drake Passage, bifurcation of the Antarctic Circumpolar Current (ACC) passing over the Falkland Plateau, a strong Falkland Current, its confluence with the Brazil Current, and moderate zonal flow across the South Atlantic. FRAM versus ALACE comparisons are made in both the Eulerian frame and using observed and modeled trajectories. In Drake Passage, where float velocities agree with earlier observations, FRAM velocities are about twice too big. Both FRAM and ALACE velocities are consistent with an O(100 Sv) Falkland Current. In the central South Atlantic the few available float observations indicate the ACC and South Atlantic Current (SAC) to be more localized than in the model. Eddy kinetic energy is much stronger in the observations than in FRAM. Float dispersion in both the model and observations is due primarily to mean shear. Initial RMS particle separation of 100 km grows to nearly 1000 km after 1 year, but most of this is associated with floats that take different paths of the general circulation. The observations indicate that eddy effects are particularly important near the Falkland‐Brazil Current confluence in allowing Antarctic Intermediate Water to transfer from the ACC to the SAC, from which they may enter the subtropical gyre.

The Autonomous Lagrangian Circulation Explorer (ALACE)

Abstract The autonomous Lagrangian circulation explorer (ALACE) is a subsurface float that cycles vertically from a depth where it is neutrally buoyant to the surface where it is located by, and relays data to, System Argos satellites. ALACEs are intended to permit exploration of large-scale low-frequency currents and to provide repeated vertical profiles of mean variables. ALACEs periodically change their buoyancy by pumping hydraulic fluid from an internal reservoir to an external bladder, thereby increasing float volume and buoyancy. Because positioning and data relay are accomplished by satellite, ALACEs are autonomous of acoustic tracking networks and are suitable for global deployment in arrays of any size. While providing only a sequence of displacements between surfacing intervals, ALACEs are efficient in gathering the widely spaced long-term observations needed to map large-scale average flow. The primary technical challenges met in the ALACE design are maximizing energy efficiency to achieve a l...