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  Description of WOCE Float Experiments
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The North Atlantic Float Program

Specific projects of PIs are, in some cases, abstracts of their plans , so actual implementations may be different in detail. See under each PI (in the table) links to their own web pages or other references.

Links from the main headings lead to a table for the whole section, eg ACCE

ACCE 1996-99
The following  paragraphs immediately relevant to the float program are extracted from the Plans for the Atlantic Climate Change Experiment (ACCE)-US WOCE Report 1996. hyperlink

 Goals of the ACCE

    • To provide a quantitative four-dimensional observational description of the pathways and material property fluxes of the meridional overturning circulation (MOC) within the North Atlantic Ocean that vary on time scales from interannual to at least decadal.
       
    • To improve understanding and modeling of the relationships between the rates and natural variability of the MOC, internal ocean properties, sea surface temperature (SST), and the variability of the overlying atmosphere.
       
    • To identify and initiate measurements to be continued beyond the ACCE observational period to monitor the variability of important elements of the MOC and its relation to global climate variability.

     

    Because of funding constraints, all of these components will not be studied simultaneously. The subpolar gyre study will begin first.The U.S. is planning two cruises to the northeastern Atlantic (with 30-mile station spacing) in fall 1996 and spring 1997. These cruises will occupy two lines between East Greenland, Ireland and the Azores. Ship work will include basic hydrography, lowered and underway acoustic Doppler current profiler measurements, and tracer sampling for chlorofluorocarbons, 3H/3He and 18O.

    Simultaneous with the hydrographic cruises, a fleet of RAFOS and PALACE floats will be launched. PALACE floats are profiling versions of the Autonomous Lagrangian Circulation Explorer (ALACE) floats that have been so successful in providing absolute 1000-m current velocities. The addition of a temperature sensor with the pressure sensor means that the floats also will furnish a temperature profile on each ascent; conductivity sensors also may be added to allow derivation of a salinity profile. One hundred fifty PALACE floats will be deployed north of 40°N on a nominal 2° x 2° grid at depths of 400 and 1500 m in fall 1996 and spring and fall 1997 using U.S., British and Canadian research cruises. The floats will be set to cycle every 15 days. An additional 40 floats are funded by the Office of Naval Research (ONR) for deployment in the Labrador Sea.

    The RAFOS floats will be launched in two separate experiments. The first experiment (in the eastern Atlantic) will consist of 68 floats ballasted to remain at the 27.5 sigma-theta level. Forty of the floats will be launched on the two U.S. hydrographic cruises along 28°W, and the remainder will be deployed in the eastern boundary region in coordination with French deployments. The second RAFOS float experiment will consist of 80 floats. These will be deployed from merchant vessels on zonal lines either side of the North Atlantic Current in the western basin as well as on four quarterly deployments across the current at 38°W. These floats will be ballasted to the same density level as those in the eastern basin. All RAFOS deployments will be for two years, with daily positional tracking. The eastern basin experiment will begin in fall 1996 and the western basin experiment in spring 1997.

    U.S. field work in the tropical and subtropical surface layer is confined essentially to float deployments. Most, if not all, will be deployed in 1997 or later. There are three separate components to the float work:

         54 ALACE/PALACE floats will be deployed on a 600-km grid between 6°S and 40°N, cycling every two weeks, at 800- to 1000-m depth. (an additional 19 floats will be contributed by NOAA/AOML)

         70 PALACE floats will be deployed in the 25° to 35°N latitude band to study the formation of 18° water in the upper 1000m of the water column.

         40 S-PALACE floats hyperlink (PALACE floats equipped with a salinity sensor) will be deployed between 6° and 16°N to examine the salinity balance of the tropical region. About half will be deployed at 800 m and contribute to the 600-km grid of the North Atlantic; the remainder will provide enhanced resolution where the hydrological cycle is most intense and cycle to 1800 m.

More details of the actual implementation can be found under the individual PIs.

Other U.S. and International Participation

As stated above, ONR has funded a study of the Labrador Sea during 1996-1997, and possibly into 1998. This work will include the deployment of 40 PALACE and 50 RAFOS floats. It also will include 30 Lagrangian floats that can monitor water movement in three dimensions. Together, these floats will cover the upper 2000 m of the water column. Additionally, about 50 surface drifters will be deployed, and current meter moorings and a meteorological mooring will be set. An associated modeling component also is planned.

There has been a great effort to integrate U.S. contributions with those of other nations. In the subpolar gyre, the Canadians are planning a series of hydrographic and tracer cruises to cover the Labrador Sea in summer and fall 1996 and perhaps also in spring 1997. These will be used to launch many of the U.S. instruments supported by ONR. The Germans also plan a tomography experiment in the Labrador Sea and will collect hydrographic and mooring data there. Other German work includes a number of hydrographic cruises, including sections at 48° and 55°N. Additionally, they plan to study circulation in the region of the Denmark Straits, the Iceland Basin and the Irminger Sea using hydrography, float deployments, and current meter moorings. British scientists will carry out hydrographic, float and current meter work in the northeast subpolar gyre between the U.K., Iceland and Greenland; the Nordic countries will concentrate on work in the Norwegian Basin and around the Faroe Islands.

Further south, in the subtropical gyre, the emphasis will be on hydrography and float deployments. French scientists have a major program in the eastern basin of the North Atlantic, between 40° and 50°N and east of 33°W. They will be deploying  floats at multiple depths beginning in October 1996. This work will dovetail with the U.S. float deployments in the eastern basin further north. The French also have funded a tomography experiment to be carried out between the Azores and the Canary Islands. Work in this region will be enhanced by contributions from Germany, Portugal and Spain; it will include hydrography and current meter moorings and will concentrate on the eastern boundary current and the outflow of Mediterranean Sea water.



Profiling ALACE Floats for the Subpolar Gyre in Support of the Atlantic Circulation and Climate Experiment (ACCE)- Davis,Owens (joint projects)

In Companion project by Dr. Brechner Owens of the Woods Hole Oceanographic Institution, and by Dr. Russ Davis of the Scripps Institution of Oceanography,150 profiling ALACE (Autonomous Lagrangian And Circulation Explorer) floats will measure the absolute  velocity and evolution of the temperature and salinity structure of the upper 1500 m in the subpolar gyre as part of the Atlantic Circulation and Climate Experiment (ACCE). Absolute velocities will be mapped at nominal depths of 400 and 1500 meters' depth using float arrays deployed uniformly over the basin. The velocity data will address three major pathways of the meridional overturning circulation (MOC) within the subpolar gyre: the shallow inflow of water into the regions of water mass modification; the convection that forms new dense waters; and the outflow of the resultant intermediate water from this source region. Temperature and salinity profiling will describe time evolution of the upper1500 m, allowing quantification of the rates of water-mass modification in the warm-water limb of the MOC that supplies water for formation of deep and intermediate water. With 150 profiles observed roughly every ten days over two years, these observations will describe evolution with a combined temporal and spatial resolution greater than can be provided by all the other ACCE observations, especially over the winter-time convection season when other observations are extremely scarce. Analysis of these data will provide time series of three-dimensional maps of the velocity, temperature, and salinity fields within the upper 1500 m of the subpolar gyre over the duration of the ACCE field experiment. Additionally, statistical analyses will provide spatial distributions of mean circulation and quantities like eddy kinetic and potential energies, eddy mixing, and fluxes of heat and freshwater. These gyre-wide estimates will provide strong constraints for the future climate models that must be developed to adequately include the physics of the subpolar gyre, particularly convection. The absolute velocity measurements will also be used to reference large-scale geostrophic shears obtained from hydrography in this region where strong barotropic components are common.


Warm Water Pathways and Intergyre Exchange in the Northeastern North Atlantic- Bower,Richardson

In this project, the PIs will study the North Atlantic Current, which transports subtropical water northeastward from the Gulf Stream, and the Poleward Eastern Boundary Current, which carries Mediterranean Outflow Water northward along the European continental slope. These two current systems are thought to be the two most likely sources of warm, salty water that is transformed into intermediate and deep water in the subpolar region..

Specific objectives are to: 1) provide a quantitative description of the bifurcation of the North Atlantic Current east of the Mid-Atlantic Ridge; 2) assess the importance of meridional eddy fluxes, compared to large-scale advection, in the northward flux of heat and salt in the northeastern North Atlantic; and 3) establish the degree of continuity of the Poleward Eastern Boundary Current to the entrance to the Norwegian Sea and the fate of the Mediterranean Outflow  Water carried by this current.

To meet these objectives, a total of 68 isopycnal, acoustically-tracked RAFOS floats will be deployed in the eastern Basin of the North Atlantic, forty in and adjacent to the North Atlantic Current, and twenty-eight near the eastern boundary. The floats will drift submerged for two years, sampling three cooling seasons, with tracking provided once daily using moored sound sources.

This study is a contribution to the US AtlanticCirculation and Climate Experiment (ACCE), and to the International World Ocean Circulation Experiment (WOCE).  The high-resolution RAFOS floats will specifically address important features of the circulation that have relatively short  time and/or spatial scales, such as bifurcations, eddy mixing and boundary flows.


A Study of the Extension of the North Atlantic Current and Pathways of Exchange   H. Thomas Rossby,Mark Prater

As a contribution to the Atlantic Circulation and Climate Experiment (ACCE) of the World Ocean  Circulation Experiment (WOCE), the Pis in this project will seek to identify the pathways of the warm-water North Atlantic Current (NAC) and the sites and mechanisms of frontal exchange across it using appoximately 80 isopycnal RAFOS floats to be launched at various locations in the vicinity of the NAC. Specific objectives include: (1) exploring  the structure of the front which separates the cold, fresh subpolar water to the north and the warm, saline subtropical water to the south, and (2) attempting to quantify the mechanisms and rates of exchange between the two gyres. In  conjunction with the field program of Drs. Bower and Richardson of the Woods Hole Oceanographic Institution, they will also expand the study of the bifurcation of the NAC as it enters the eastern North Atlantic. The North Atlantic Current (NAC) is recognized as a crucial element of the meridional overturning cell, both as the primary northward-penetrating upper-ocean current and as a region where thermodynamic exchange occurs between the subtropical and subpolar gyre. Its character changes from an intense, topographically controlled jet in the western boundary, to a less- defined, perhaps filamented eastward flow after veering east north of the Flemish Cap. It is not clear whether the NAC is a discrete front that translates north and south, a collection of fronts, or simply a broad swath of eastward flow, nor is its fate fully understood. Although most of the flow is to the north and towards Europe, some recirculation occurs as well, and this partitioning of pathways is important in attempting to understand and quantify the controlling factors of the overturning cell.


Subsurface Float Observations in the Upper Layers of the Tropical and Subtropical North Atlantic-Leaman,Schmitt,Riser (joint projects)

   Dr K. Leaman of the University of Miami, Dr. S.Riser of the University of Washington and Dr. R. Schmitt of the Woods Hole Oceanographic Institution, approximately 145 profiling P-ALACE (Profiling Autonomous Lagrangian and Circulation Explorer) floats (at least half of them with salinity) will be used to study specific water-mass transformation processes in the tropical and subtropical Atlantic as part of the Atlantic Circulation and Climate Experiment (ACCE). The basin-wide grid will be used to provide relatively uniform sampling to track upper layer salinity and heat content changes throughout the basin. Higher resolution arrays will be deployed in important local regions where water masses such as the high-salinity Subtropical Underwater (Leaman), relatively uniform 18-degree water (Riser) are formed or transformed, or where important air-sea exchanges are highly variable such as under the ITCZ (Inter-Tropical Convergence Zone) (R. Schmitt). The higher-resolution will allow tracking of seasonal and interannual changes in the volume and structure of the upper ocean features, and relate these to modeled/observed estimates of surface heat and fresh water fluxes. Analysis of these data will provide time series of three- dimensional maps of the velocity, temperature, and salinity fields within the upper 1500 m of the tropical and subtropical gyres over the duration of the ACCE field experiment. Additionally, statistical analyses will provide spatial distributions of mean circulation and quantities like eddy kinetic and potential energies, and eddy mixing, in addition to the flux estimates.
     These gyre-wide estimates will provide strong constraints for the future climate models that must be developed to adequately include the physics of the tropical and subtropical Atlantic, particularly the fresh water budget.
 

Molinari,Garzoli (AOML)

The Atlantic Circulation and Climate Experiment (ACCE) is directed at increasing our understanding of the interaction between the Atlantic Ocean and global climate. As a contribution to ACCE, NOAA joins several other NSF-funded investigators in deploying a North Atlantic wide PALACE float array to study the processes important in establishing sea-surface temperature (SST) variability. NOAA's area of interest is the tropical Atlantic and 19 PALACE floats were deployed along 6°S and the equator to provide current vectors at the sea surface and 1000m and temperature profiles every 10 days. These data not only will provide a regional perspective to address ACCE objectives, but when combined with other floats and observations and models, a basinwide perspective will be available.



Labrador Sea 1995-98

The main goals of this experiment (extracted from the comprehensive linked site above) are

  • Plume Scale (100 m - 1km)

What are the characteristic scales and properties of convective plumes?
How do the plume parameters depend on the forcing and their local environment?
What are the fluxes of mass, heat and salt resulting from the ensemble of plumes?

  • Mesoscale (5 km - 100 km)

How is convection related to its mesoscale environment?
What is the relative importance of lateral (eddy) versus vertical (convective) flux of heat and salt inside/outside the convective region?

  • Gyre Scale (50 km - 1000 km)

What controls the volume and T/S properties of created water  masses?
How is the convected water mass accommodated in to the general circulation?
What is the mean and seasonal variation in the gyre circulation?

PIs with floats deployed in the experiment -Davis,Owens,Prater,Schmitt,Schott

In addition  BIO (Lawrence) has deployed Palaces.


Eurofloat 1995-99
The notes are largely extracted from the web pages of the European Commission's Eurofloat Project hyperlink
 

    Principal Scientific Questions

    • What is the mean circulation  of the Mediterranean Water and the Labrador Sea Water in the intergyre region of the  eastern North Atlantic Ocean?

    • Is there a lagrangian drift or eddy mixing of the Mediterranean Water and the Labrador Sea Water?

     Analysis
          According to Saunders' (1982) description of the circulation in the eastern North Atlantic, there is, roughly, southward flow above 500m, northward flow in the depth range 500-1000m north of 40°N, and southward flow below 1000m. His study was based on the analysis of hydrographic data, with flow referenced such that the vertically integrated transport agreed with Ekman pumping, generally weak in this area. Recent analyses of historical data in the region by Paillet (1996) refine and support the earlier work, and provide quantitative estimates of geostrophic transport in various density classes.

          The Med Water itself has been suggested to have a dynamical impact on the eastern Atlantic through the flow generated by mixing or double diffusion. Mixing tends to stretch water columns locally by permitting a diapycnal velocity to arise, generating vorticity and meridional flow. The Med outflow from the Strait of Gibralter tends to flow north along the topography, feeding the interior with salt and mass via instabilities. Modeling and laboratory studies are underway by a number of groups to understand the diverse dynamics involved in the overflow, eastern boundary current, and interior circulation. Ekman suction in the subpolar gyre is one mechanism for creating meridional flow at mid-depths since isopycnal surfaces within the Med Water rise to the surface in the subpolar gyre. Other mechanisms may be relevant near the eastern boundary where topographic waves occur. Also, the basic instability of the large-scale flow in the eastern Atlantic could lead to the mixing of Med Water with surrounding water and generate salt fluxes. Cruise and Deployment

    Deployments

    During three research cruise from October 1995 to January 1997, 21 Marvor and 20 RAFOS floats were deployed in the North East Atlantic:
     

    • October     1995 - 15 IfM/Seascan floats at 1000m depth in the northern Canary Basin.

    • September 1996 - 21 Tekelec Marvor floats at 1750m depth in the North East Atlantic.

    • December  1996 - 5 additional IfM/Seascan floats at 1000m depth in the northern Canary Basin



ARCANE 1996-99

The ARCANE project hyperlink is a partnership of scientists with a common goal of investigating the circulation of Mediterranean Water in the eastern North Atlantic Ocean between 40 and 50N by French civil and military scientists.

A companion project, Eurofloat, is focussed on the Labrador Sea Water as well as Mediterranean Water in the interior only, east of the Mid-Atlantic Ridge.

Much of the work will be east of 14W up to the 200m isobath, although some float work will occur out to 25W to link with the proposed U.S. RAFOS deployments in this region. The eastern boundry work is a study of the circulation and dispersion of the North Atlantic Central water and Mediterranean water on the continental slope and on the abyssal plane. Plans call for the release of 60 RAFOS floats (by the SHOM)  together with 40 MARVOR floats (by IFREMER).
They are coordinated closely with the sub-tropical component of the US ACCE project.


CANIGO 1996-99
The main goal of CANIGO is an improved understanding of the functioning of the marine system in the Canary-Azores-Gibraltar region of the Northeast Atlantic Ocean and its links with the Alboran Sea through comprehensive interdisciplinary basin scales models.

 CANIGO is divided in four sub-projects including hydrographic surveys and work on carbon and particle fluxes,paleoceanography, remote sensing, float and drifters and modelling. The project involves 49 partners from Spain, theEuropean Community and other countries.Floats are being launched by IfM (Germany), AINCO(Spain) and  Instituto de Oceanografia (Portugal)



North Atlantic Basin

Irminger Sea (1996-98) Gould,Bacon

The design of the experiment addresses the assessment of flow near the "level of no motion" in the Irminger Sea as well as the performance of conductivity sensors on floats. An initial report hyperlink is available in pdf format and is 1 Mbyte in size.

 Water mass transformation in the eastern basin  (1997-98) Zenk

The long-term aim of the project hyperlink is the investigation and understanding of transformation rates of Overflow and Labrador Sea Water on their paths through the Iceland Basin. These two major water masses spread into the Iceland Basin as part of the subpolar circulation.In the float component of the programme the on-going Eurofloat initiative for the observation of Mediterranean and Labrador Sea Water spreading was complemented by additional 17 RAFOS floats. They will re-inforce the international WOCE float fleet in the North Atlantic.

Koltermann (1998)

North Atlantic Current (1993-95) Rossby
The objective of the RAFOS float program hyperlink, jointly supported by the Office of Naval Research and the National Science Foundation, was to study the structure of the currents in the North Atlantic Currrent (NAC) region and the exchange of waters between the subtropical and subpolar gyres in the Newfoundland Basin. One hundred RAFOS floats were deployed on two density surfaces corresponding to sigma-theta = 27.2 and 27.5, respectively.


Mediterranean Outflow
 

A Mediterranean Undercurrent Seeding Experiment (AMUSE 1993-94)- Armi,Bower

      In this project, PI's will study the dispersion of Mediterranean Water into the North Atlantic by seeding the Mediterranean Undercurrent near its source with lagrangian drifters. Forty RAFOS floats will be tracked for a period of eighteen months, with the principal objective of distinguishing between the formation of Mediterranean mesoscale eddies (Meddies), and simple advection/ diffusion. In a collaborative study, scientists from Portugal will carry out hydrographic surveys and maintain an array of current meters in the region.


A Study of Mediterranean Water Eddies (Meddies 1993-95)-Richardson

     In this project, 15 RAFOS (SOFARS spelled backwards) floats will be deployed in about 5 different Mediterranean Water eddies (Meddies) as they are discovered during the French Semaphore Experiment in the vicinity of the Azores Front. The trajectories and life histories of these 5 Meddies will be observed to learn more about their typical translation characteristics, life expectancy, and mode of propagation. The Semaphore Experiment has a major
     observational component in 1993 during which extensive Conductivity-Temperature-Depth (CTD) and Expendable Bathythermograph (XBT) surveys will be made in the region 31.5- 36.ON, 20.5-26.OW, where a large number of historical Meddy observations were made. these CTD and XBT casts will be used to identify the salinity and temperature anomalies of the Meddies and will be instrumental in the design of the deployment strategy for the floats.    The long-range objectives of this study are to understand how Mediterranean Water disperses into the North Atlantic  and, specifically, to understand the role of Meddies in the maintenance of the Mediterranean salt tongue.



Tracer Release Experiment 1991-93

Float Component of the 1992 North Atlantic Tracer Release Experiment(NATRE) -Price, Richardson

         This proposal is a central element of the Tracer Release Experiment (TRE), a process study of the World Ocean Circulation Experiment (WOCE). The PI will release and track a suite of mid- depth floats. These will be released in conjunction with the release of measured amounts of a stable chemical species. At selected intervals, the distributions of the chemical will be measured. Over the duration of the experiment, the floats will be tracked to provide a continuous record of the tracer history.



Western Boundary Current 1988-1992

Exploration of the Intermediate and Deep Western Boundary Current and the Meridional Transport of Water in the Tropical Atlantic Using SOFAR Floats-Richardson ,Schmitz

In this project, the PI's will continue and complete a study of the general circulation in the tropical Atlantic using subsurface acoustic floats. 44 neutrally buoyant SOFAR floats were launched in early 1989, 14 at 800m, 15 at 1800m, and 15 at 3600m, corresponding respectively to layers of Antarctic Intermediate Water and Upper and Lower North Atlantic Deep Water. The array of 6 listening stations will be recovered in late 1992, resulting in a 3.5 year data  set describing the Lagrangian characteristics of the flow in these important water masses.



Pathways in the Deep Western Boundary Current Recirculation South of 30 North (Abaco)-Leaman, Vertes

  As the result of work carried out as part of the NOAA-funded STACS  experiment in the late 1980's,  it became evident that the actual structure of the Deep Western Boundary Current(s) observed east of the Bahamas (Abaco) differed in some significant ways from what might have been expected based on earlier studies.
  It was realized that deep subsurface (RAFOS) floats would be the most efficient and illuminating way to study these deep currents .  In particular the floats would be well-suited to track possible recirculations in the 2000-
2500m velocity core where there is little freon signal. For this reason, in late 1989 the Rosenstiel School of Marine and Atmospheric Science (RSMAS) started an in-house program to construct, ballast, launch and analyze data from RAFOS floats, with the goal of seeding different parts of the Deep Western Boundary Current (DWBC) with these floats.  All floats were launched from a local Bahamian fishing boat based in Marsh Harbor.



A Study of Deep Western Boundary Current Variability Using Hydrography, Tracers, and Lagrangian Drifters (BOUNCE 1994-96)-Pickart,Bower

 A field study of the variability of the North Atlantic Deep Western Boundary Current (DWBC) will be conducted using  a combination of experimental approaches: hydrography, chemical tracers and Lagrangian floats. The main objectives of the study are to determine the synoptic velocity and water mass structure of the DWBC and determine the fluid parcel pathways in the DWBC and identify the regions of exchange with the interior.