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GATEWAYS
Balancing global climate research
Despite its role as the Southern Hemisphere equivalent of the Gulf Stream, the Agulhas Current has received
little international attention until recently. Professor Rainer Zahn discusses research into the phenomenon
are brought to the table by your
interdisciplinary group?
The GATEWAYS consortium consists of
scientists from Spain, Germany, Israel,
The Netherlands, the UK and South Africa
and forms an interdisciplinary partnership
of physical oceanography, ocean and
atmospheric numerical modelling, and
paleoceanography and paleoclimatology.
Analytical skills encompass laboratory-based
methodologies, high-resolution climate
modelling, and advanced numerical data
processing techniques.
How are members of the project redefining
ocean reconstructions and climate models?
Is the team using any particularly unique or
interesting methods for collecting data?
Over the last 10 years, the greater
Agulhas system around southern Africa
has stirred curiosity in many researchers.
What knowledge has been gained about
the transportation of heat and salt from
this area?
The Agulhas Current carries warm and salty
tropical waters from the equatorial Indian
Ocean to the tip of Africa. This occurs at a
rate similar to the water transported by the
Gulf Stream in the North Atlantic, and the
associated heat and saltwater transports
have implications for the local climate and
the world ocean circulation. Reconstructions
of the Agulhas Current show that over the
past several hundred thousand years the
water transport from the Indian Ocean
to the Atlantic, the so-called Agulhas
leakage, strengthened substantially during
periods of global climate changes. These
observations underscore the sensitivity of
the Agulhas Current and suggest a key role
of the Agulhas leakage in global climate
developments.
Who comprises the GATEWAYS
consortium, and what skill sets
The project is innovative because it combines
new and established analytical methodologies
not normally available in single research
projects; it links modern ocean and climatic
processes with the analysis of materials
collected from water column sampling and
sediment traps in order to verify the degree
to which the collected materials represent
ocean processes. The research work applies
coherent analytical protocols on a wide range
of materials and links marine with terrestrial
palaeoclimatic expertise to perform the often
postulated, while rarely conducted, land-ocean
palaeoclimatic linking.
Finally, the project combines these procedures
with state-of-the-art numerical modelling that
employs palaeoclimatic data fields to cuttingedge high-resolution modelling. Model runs of
this quality require supercomputing facilities,
and modellers do not normally feel encouraged
to apply these models in palaeoclimatic
research. Hence GATEWAYS establishes an
interdisciplinary collaboration of a dimension
seldom taken up in climate research.
What will your research mean for current
Intergovernmental Panel on Climate Change
(IPCC) models used to predict the next 100
years of climate change?
IPCC activities so far have focused largely (but
not exclusively) on the ocean and climate
regimes of the North Atlantic region because
these are considered particularly sensitive and
therefore a prime region to monitor the effects
of ongoing global warming. The Southern
Hemisphere regime, however, has long been
ignored. GATEWAYS research focuses on the
major ocean current system in the Southern
Hemisphere and its role in and impact on ocean
circulation and climate.
The Agulhas water transports and leakage
cause a positive density anomaly in the South
Atlantic that is approximately an order of
magnitude larger than the negative density
anomaly currently observed in the subpolar
North Atlantic. Assessing the dynamics of the
Agulhas system therefore provides important
constraints on the sensitivity of the Atlantic
meridonial overturning circulation (AMOC).
Credible forecasts of Atlantic overturning
changes and associated climate change
can no longer ignore Southern Hemisphere
processes.
Finally, what more can be done to promote
awareness about the residual effects of the
Agulhas Current on climate change?
Global climate change has outstanding visibility
in the media owing to its immediate societal,
economic and political significance. But the
processes involved with global change are
complex and proceed on multiple levels, making
it difficult to convey the details to a wider nonspecialist audience. The general public deserves
a realistic description, in simple yet scientifically
adequate words, of the processes involved in
climate change. The Agulhas system is one such
regime that exemplifies the climatic relevance
of ocean circulation and its dynamic linking
with global climate.
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GATEWAYS
Modelling the southern hemisphere climate
The Agulhas Current has a profound effect on both the regional climate in southern Africa and the global ocean
circulation. GATEWAYS is using cutting-edge modelling techniques to generate important insight on the system
THE AGULHAS CURRENT is the major western
boundary current of the Southern Hemisphere,
carrying water from the tropical Indian Ocean
along the east coast of southern Africa. The
Current impacts eastern and southern African
climates and forms a key component of the
global ocean ‘conveyor’ circulation. It controls
warm water return flow to the Atlantic Ocean
that compensates for the renewal of deep
water in the subpolar North Atlantic and its
export to the rest of the world Ocean. As such,
it is increasingly recognised as a key player
in ocean thermohaline circulation, feeding
into the upper arm of the Atlantic meridional
overturning circulation (AMOC) through the
leakage of warm, saline waters from the Indian
Ocean to the Atlantic.
Unusual dynamics pervade the motion of this
warm-water current: as it moves west around
the southern tip of Africa, it is retroflected back
east by its own inertia (so-called vorticity) and
the westerly winds offshore the tip of South
Africa. Not all waters are captured by this
sudden diversion of course – parts of the Agulhas
Current leak away into the South Atlantic
Ocean. Meanders, triggered by anticyclonic
eddies deriving from the Mozambique Channel,
propagate downstream with the current. On
reaching the area of retroflection, eddies called
Agulhas rings spin off into the South Atlantic.
COMPUTER MODELLING IS A BACKBONE
OF THE GATEWAYS PROJECT
This leakage of Agulhas water at the southern
tip of Africa causes a salt anomaly in the South
Atlantic that influences the AMOC, including
the strength of the Gulf Stream, thus affecting
the climate in the Northern Hemisphere.
Modern observations and direct measurements
show that Agulhas leakage is increasing under
anthropogenic climate change. Computer
simulations predict an increase by 25 per cent
over the next 100 years, which could stabilise
the AMOC at a time when global warming
and accelerated melting of polar sea ice is
predicted to weaken it. Model simulations
suggest that a persistent change in leakage
could impact the thermohaline properties
of the Atlantic, changing its stratification
and its potential for deep convection, thus
altering the AMOC to a new stable state over
a period of several hundred years, with direct
implications for climate.
MODELLING AND RECONSTRUCTIONS
Despite increasing recognition of their
significance, the dynamics and sensitivity of
these mechanisms are not well understood.
Numerical modelling combined with palaeoand modern observations provide the insight
to identify the controls that define the Agulhas
system behaviour in a warming climate.
Professor Rainer Zahn, from the Autonomous
University of Barcelona in Spain, is leading
an international initiative called Multi-Level
Assessment of Ocean-Climate Dynamics:
A Gateway to Interdisciplinary Training and
Analysis (GATEWAYS). The project is funded by
the EC FP7 Marie Curie ITN scheme and uses
modern observations, ocean reconstructions
and climate modelling to examine the
dynamics of the Agulhas Current and its
relationships with climate.
GATEWAYS tests the sensitivity of the Agulhas
Current to changing climates of the past,
the Current’s influence on southern African
climates, buoyancy transfer to the Atlantic
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INTERNATIONAL INNOVATION
CORING OPERATIONS WITH THE FRENCH RV MARRION DUFRESNE AT THE AGULHAS PLATEAU OFF THE
TIP OF SOUTH AFRICA (APPR. 41˚S)
by Agulhas leakage around southern Africa
and modulation of the Atlantic circulation
by the leakage. “These processes cannot be
detected in oceanographic datasets from direct
measurements, owing to the high degree of
variability of the AMOC that makes it difficult
to confidently isolate the various factors that
drive the Atlantic circulation,” Zahn explains.
“This puts the onus on numerical modelling
and reconstructions of Agulhas leakage and
associated changes of the Atlantic circulation in
the past.” Embedding high-resolution Agulhas
modules into global models, in conjunction with
atmosphere-ocean simulations, allows for the
assessment of impacts of the Agulhas regime on
global oceanic and atmospheric circulation.
THREE KEY QUESTIONS
There are three key questions related to Agulhas
Current dynamics and sensitivity. Firstly, how
does the Agulhas Current react to shifts in wind
fields and regional ocean fronts? Answering
this enables understanding of the primary force
driving the Agulhas Current along the eastern
shores of southern Africa and the impacts that
the ocean climatology south of Africa exerts
on the Current. The next question is how such
changes affect the Agulhas leakage into the
Atlantic. This is connected with the role that the
INTELLIGENCE
LABORATORY ANALYTICAL WORK PROVIDES THE
DATABASE FOR THE GATEWAYS PROJECT
mid-latitude westerly winds and the cold-water
belt around Antarctica and its border with the
warm water sphere of the subtropical South
Indian and Atlantic Oceans play in controlling
the Indian-Atlantic water transport. The final
question is whether the leakage does indeed
perturb the AMOC. This addresses the central
hypothesis that the water transport from the
Indian Ocean to the Atlantic generates a density
perturbation and sub-surface ocean waves
that ultimately impact the strength of deep
water formation in the subpolar North Atlantic,
with far-reaching consequences for a range of
oceanographic and climatic processes.
DATA-MODEL INTEGRATION
GATEWAYS is structured into five Work Packages
(WPs). In WP1, researchers are determining
long-term variations of ocean climatology off
Southeast Africa and sensitivity to regional
and far-field forcing. WP2 is assessing IndianAtlantic gateway dynamics, inter-ocean
buoyancy transports and AMOC sensitivity. WP3
concentrates on terrestrial palaeoclimatology,
evaluating connectivity between marine
variability and southern African climates. WP4
is combining modern ocean observations with
proxy formation and dynamics. Tasks include
determining ocean variability and the impact on
water column properties and proxy formation
and integrating in situ instrumental observations
of surface-ocean, thermocline and bottom water
climatology with time series of particle settling
fluxes. Finally, WP5 arranges the GATEWAYS
project coordination and WP6 is dedicated to
project synthesis and dissemination of results.
The research employs a range of numerical
and laboratory-based analytical techniques
that are centred on ocean climatology and
sensitivity. Laboratory protocols involve
stable and radiogenic isotope analysis,
quantitative micropaleontology, molecular
biomarker analysis and sediment component
analysis. Long-term observations from
ocean moorings provide the basis for data
versus water column calibration. Advanced
statistical data processing establishes modes
of variability and co-variation between the
different components of the Agulhas system,
and with global climatic drivers. Data-model
integration provides the framework for the
dynamical evaluation of the project database.
TRAINING AND
STRENGTHENING PARTNERSHIPS
GATEWAYS is offering training to 15 EarlyStage (ESR) and three Experienced Researchers
(ER) in paleoceanographic reconstructions
and ocean and atmosphere modelling. It
provides extensive multi-level scientific and
complementary skills training. “The impact
of the project lies with its methodological
innovation in combining established and new
complementary laboratory and data analysis
with cutting-edge high-resolution modelling,”
Zahn enthuses. “The skills resulting from
the project training and research provide
a firm foundation for the trainees’ onward
development.”
Training encompasses novel laboratory-based
methodologies; high-end equipment such as
isotope-ratio and multi-sector inductively
coupled plasma mass spectrometers;
and high-resolution climate modelling,
accessing supercomputing facilities at highperformance computing centres. Combined
with the diverse scientific expertise
converging on a common scientific theme,
this provides the ESR and ER with outstanding
research training opportunities. Trainees
are also developing transferable skills in
networking, languages and inter-institutional
cooperation, and project management.
Beyond the direct benefits of its training
and research programme, the GATEWAYS
project strengthens partnerships between the
research teams, placing their expertise into a
common logistical framework and embedding
them into formalised partnerships.
GATEWAYS
OBJECTIVES
To conduct interdisciplinary climate change
research on an ocean regime of regional
and global significance: the Agulhas Current
off southern Africa. It provides 15 earlystage and three experienced researchers
with extensive multi-level scientific and
complementary skills training in a number of
processes relevant to climatic developments
and projections.
PARTNERS
Royal Netherlands Institute for Sea
Research (NIOZ), The Netherlands • Cardiff
University, UK • Helmholtz-Zentrum
für Ozeanforschung Kiel (GEOMAR),
Germany • Vrije Universiteit Amsterdam,
The Netherlands • Christian-AlbrechtsUniversitaet zu Kiel, Germany • University
of Cape Town, Republic of South Africa •
Geological Survey of Israel, Israel • Climate
Risk Analysis – Manfred Mudelsee, Germany
• Alfred Wegener Institute for Polar and
Marine Research, Germany • University
of Bremen (MARUM), Germany • Hebrew
University of Jerusalem, Israel • L-UP
Project Management and Training, France •
Simultec Environmental Consulting Zurich,
Switzerland • EMDESK GmbH Website and
Project Management, Germany
FUNDING
FP7-PEOPLE-ITN-2008, Marie Curie Actions –
Networks for Initial Training (ITN) – contract
no. 238512
CONTACT
Professor Rainer Zahn
Project Coordinator
Institució Catalana de Recerca i Estudis
Avançats, ICREA
Universitat Autònoma de Barcelona
Institut de Ciència i Tecnologia Ambientals
Departament de Física
E-08193 Bellaterra (Cerdanyola)
Spain
T +34 935 814861
E [email protected]
www.gateways-itn.eu
DR RAINER ZAHN is Professor at the
Institució Catalana de Recerca i Estudis
Avançats and holds affiliations with the
Institute of Environmental Science and
Technology and Department of Physics at
the Universitat Autònoma de Barcelona. He
has over 30 years of experience in marine
climatology.
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