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 
(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
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?
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?
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
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
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
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
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 ,
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.
World Ocean Circulation Experiment 
Launch 
Float Data