Indian Journal of Marine Sciences
Vol. 37(4), December 2008, pp. 373-385
The role of Southern Ocean in past, present and future climate: A strategy for
the International Polar Year
Stephen R Rintoul
CSIRO Wealth from Oceans National Research Flagship, Antarctic Climate and Ecosystems Cooperative Research Centre,
GPO Box 1538, Castray Esplanade, Hobart, Australia 7001, Australia.
*[Email: [email protected]]
The present study outlines a strategy for Southern Ocean climate research during the International Polar Year (IPY). The
research is organized in five themes: Antarctica and the Southern Ocean in the global water cycle; Southern hemisphere
teleconnections; Climate processes at the Antarctic continental margin; Climate – ecosystem – biogeochemistry interactions
in the Southern Ocean; and Records of past Antarctic climate variability and change. To address these themes, an integrated
IPY Southern Ocean observing system is needed. The observing system will include synoptic, multidisciplinary ocean
transects; time series measurements; enhanced atmospheric measurements; and new paleoclimate data sets. The Southern
Ocean IPY will leave a legacy of a targeted, affordable, sustained observing system and a circumpolar snap-shot to serve as
a benchmark for the assessment of past and future change. The observations will inform development of models capable of
improved projections of future change.
[Keywords: Southern Ocean; Climate Change; Biogeochemical cycle; International Polar Year]
Introduction
Prior to 1990, most of our knowledge of the
Southern Ocean was based on detailed measurements
in Drake Passage and occasional coarse-resolution
hydrographic sections at other locations. The past
fifteen years have seen a significant expansion in
observations of the Southern Ocean, including
circumpolar surveys conducted as part of large
international experiments (e.g the World Ocean
Circulation Experiment (WOCE), the Joint Global
Ocean Flux Study (JGOFS) and the Climate
Variability and Predictability (CLIVAR) program),
new and more powerful satellite instruments and the
development of autonomous floats.
The Southern Ocean is now understood to have a
global reach, playing a critical role in the overturning
circulation and large-scale budgets of heat, carbon
and freshwater. However our ability to understand
and predict how the Southern Ocean circulation will
change in the future and the impact of such changes
on regional and global climate, on biogeochemical
cycles and on marine ecosystems, is inadequate. The
International Polar Year (IPY) provides an
opportunity to harness the resources of the
international community to tackle these scientific
challenges. The present study reviews recent progress
in understanding the Southern Ocean, identifies gaps
in understanding and outlines a strategy to address
key unknowns during the IPY.
The role of the Southern Ocean in the climate
system
The unique geography of the Southern Ocean has a
profound influence on the global ocean circulation
and climate1. The largest ocean current, the Antarctic
Circumpolar Current (ACC), passes through this
ocean channel. The Sverdrup balance which holds in
the major ocean gyres cannot apply to the ACC due to
the lack of continental boundaries to support pressure
gradients. Hence eddies and topography play a more
central role in the dynamical balances in the Southern
Ocean. The strong quasi-zonal flow of the ACC
connects the ocean basins, smoothing out differences
in water properties between basins. It allows climate
anomalies to propagate from one region to another
and influence regional climate downstream.
The lack of boundaries to support zonal pressure
gradients means that there can be no net meridional
geostrophic mean flow across the Southern Ocean
channel, at depths shallower than the topography. By
inhibiting north-south flow, the ACC isolates
Antarctica from the warmer waters to the north and
contributes to the cold climate of the continent.
However, a meridional circulation does exist and
374
INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008
plays a critical role in the closing the loop of the
global overturning circulation.
The atmosphere-ocean-ice interactions transform
water from one density class to another, connecting
the meridional exchange in different density classes.
It is now understood that the conversion of deep water
to intermediate water in the Southern Ocean connects
the deep and shallow layers of the global-scale
overturning (or thermohaline) circulation1,2. The
global overturning circulation, in turn, is primarily
responsible for the transport and storage of heat and
carbon by the ocean and so has a strong influence on
climate.
The major currents of the Southern Ocean are
illustrated in Fig.1. The Antarctic Circumpolar
Current (ACC) carries 137 ± 9 × 106 m3 s-1 of water
from west to east through Drake Passage3, increasing
to 147 ± 9 × 106 m3 s-1 south of Australia4. Hence, the
transport and variability of ACC is important for
understanding global heat and water budgets. Recent
work has shown that the southern hemisphere wind
field and the limited southern extent of the African
and Australian continents allow the subtropical gyres
of the three southern hemisphere basins to connect to
form a “supergyre”5. To constrain basin-scale budgets
of mass, heat and freshwater, measurements of
Southern Ocean transports need to extend northward
to each of the southern hemisphere continents to
sample the contribution of the supergyre to the net
interbasin exchange.
The meridional circulation across the Southern
Ocean is illustrated in Fig. 2. Divergent wind-driven
Ekman transport causes upwelling of deep water.
Deep water that upwells near the coast of Antarctica
Fig. 1—A schematic view of the main circulation features in the Southern Ocean1. Depths shallower than 3500 m are shaded. The two
major cores of the Antarctic Circumpolar Current (ACC) are shown, the Subantarctic Front and the Polar Front. (F = front, G = gyre).
RINTOUL: THE ROLE OF SOUTHERN OCEAN IN PAST, PRESENT AND FUTURE CLIMATE
375
Fig. 2—A schematic cross-section of the Southern Ocean, illustrating the two overturning cells in the vertical – meridional plane8.
spreads poleward and is converted to denser Antarctic
Bottom Water. Water that upwells beneath the
westerly wind regime is driven north in the surface
Ekman layer and transformed to lighter density by the
addition of heat, precipitation and ice melt. The
density transformations and meridional transports act
to connect the deep and intermediate layers of the
ocean and close the overturning cell1,2. However,
large uncertainties remain regarding the rate and
nature of the water mass transformations taking place
in the Southern Ocean.
Subantarctic Mode Water and Antarctic
Intermediate Water formed at the sea surface are
subducted to intermediate depths and exported to the
north. This upper cell of the Southern Ocean
overturning makes an important contribution to the
heat, freshwater and nutrient balance. Heat
sequestered by the upper branch of the overturning
circulation determines the rate and pattern of southern
hemisphere sea-level rise due to thermal expansion.
The equatorward flow of relatively fresh intermediate
waters from the Southern Ocean is the primary return
path for moisture carried poleward in the atmospheric
branch of the hydrological cycle6. Nutrients exported
from the Southern Ocean in Subantarctic Mode Water
have been shown to support 75% of global export
production7.
The Southern Ocean overturning circulation
contributes to ocean uptake and storage of carbon
dioxide. About 40% of the total ocean uptake of
anthropogenic carbon dioxide is found on the northern
flank of the Southern Ocean, between 30 °S and the
Antarctic Circumpolar Current (ACC)9 (Fig. 3).
Climate models suggest the Southern Ocean uptake of
carbon dioxide will decrease as a result of changes in
circulation and stratification caused by enhanced
greenhouse warming. This provides a potential
positive feedback for climate change10. Changes
in the rate of upwelling, stratification and the
extent to which sea ice limits air-sea exchange may
also impact the future atmospheric concentration of
carbon dioxide, as is believed to have occurred in the
past11,12.
376
INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008
Fig. 3—The water column inventory of anthropogenic carbon dioxide. Approximately 40% of the total inventory is found south
of 30° (S98).
Climate models suggest the overturning circulation
may be sensitive to climate change13. Enhanced
greenhouse warming is expected to drive a more
vigorous hydrological cycle, with increased
precipitation at high latitudes and increased
evaporation at low latitudes. The increased
precipitation lowers the surface salinity and reduces
the formation of dense water at high northern and
southern latitudes. The dense water that sinks in both
hemispheres to supply the lower limb of the
overturning circulation has freshened in recent
decades14-17. In the high latitude Southern Ocean, this
freshening appears to be driven primarily by an
increase in melt of floating glacial ice18. The IPY
provides an opportunity to make the observations and
model improvements required to assess the resonse of
the overturning circulation to changes in the
freshwater balance.
Antarctic sea ice has a significant influence on the
climate system and Antarctic ecosystems. The sea ice
covers an area of 18 million km2 at maximum extent,
an area larger than that of the Arctic ice cover or the
Antarctic continent19. The presence or absence of sea
ice influences climate through the ice-albedo
feedback. The release of brine during the formation of
sea ice increases the density of Antarctic shelf waters
and drives the formation of dense Antarctic Bottom
Water. The seasonal formation and melting of sea ice
is the dominant term in the freshwater budget of the
high latitude ocean and controls the stratification of
the upper ocean. Sea ice cover inhibits the air-sea
exchange of heat and gases such as carbon dioxide.
The sea ice pack also plays important roles in the
ecology of the Southern Ocean and variability in ice
extent has been linked to variability in krill and
penguin numbers and distributions20.
Antarctic sea ice has apparently not changed or has
slightly increased during the satellite era21. Studies
based on proxy information, such as whaling records22
and chemical concentrations in ice cores23; suggest a
possible decline in sea ice extent between 1950 and
the start of the satellite observations in the 1970s.
The Antarctic continent has experienced little
change in temperature in the last 50 years other than
the northern Antarctic Peninsula. The peninsula has
warmed more rapidly than almost anywhere on earth
0.5 (C per decade over the last five decades)24. The
increase in summer melt as a result of higher air
temperatures has been identified as the likely cause of
the dramatic collapse of the Larsen B ice shelf in
200225 (Fig. 4) although enhanced melting by warmer
ocean temperatures may also have contributed.
The cause of the warming of the Antarctic
Peninsula remains a topic of debate. The Southern
Annular Mode (SAM), the dominant pattern of
variability in the southern hemisphere atmosphere,
has trended toward its positive index state over the
same time period (in part reflecting a change in the
RINTOUL: THE ROLE OF SOUTHERN OCEAN IN PAST, PRESENT AND FUTURE CLIMATE
seasonality of the mode)26. The positive phase
coincides with a strengthening and poleward
contraction of the circumpolar vortex. Much of the
recent trend in surface winds and air temperatures can
be explained by a strengthening of the polar vortex
associated with a positive SAM index27 (Fig. 5).
Changes in the SAM have been attributed to depletion
of ozone in the stratosphere27,28 and the enhanced
greenhouse effect29,30. Natural variability may also
contribute31.
The short instrumental record in the Southern
Ocean is also starting to provide evidence of lowfrequency variability and trends in temperatures.
Comparison of floats and historical data suggest the
circumpolar Southern Ocean has warmed at a depth of
900m since 195032 (Fig. 6) The accumulation of heat
on the northern side of the ACC has made the largest
contribution to observed changes in zonally integrated
ocean heat content33,34. Shelf and bottom waters
formed in the Indian and Pacific sectors have
377
freshened markedly since 197016-18,35. The freshening
has been attributed to increased melt of glacial ice18,
which in turn has been linked to warmer ocean
temperatures36. The short satellite record supports the
inference of mass loss from floating continental ice in
the southeast Pacific and southeast Indian sectors of
the Southern Ocean37. While evidence is growing that
changes are underway in the Southern Ocean, the
sparse observations make it difficult to distinguish
between natural variability and climate change.
Recent research has also sparked interest in
teleconnections linking the high southern latitudes to
lower latitudes. El Nino – Southern Oscillation
(ENSO) signals are evident in a number of Antarctic
and Southern Ocean variables, but the strength of the
connection appears to vary in time and the
mechanisms are poorly understood38. The strongest
interannual variability is located in the southeast
Pacific. The Southern Ocean response to the ENSO
teleconnection consists of a dipole pattern, with
Fig. 4—Collapse of the Larsen B ice shelf in March 2002. Images from MODIS on (from left to right) January 31, February 17, February
23 and March 5, 2002. The dark spots on the left-most image correspond to pools of melt water on the surface of the ice shelf. The melt
water is believed to enhance the propagation of crevasses, destabilizing the ice shelf25.
Fig. 5—Left plot: recent trends in surface air temperature and surface wind (December to May, 1969 – 2000). Right plot: the part of the
trend that is congruent with the Southern Annular Mode27.
378
INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008
anomalies of opposite sign in the southeast Pacific
and southwest Atlantic, known as the Antarctic
Dipole39. Once established, the Antarctic Dipole is
maintained by anomalous heat flux by the mean
meridional circulation in the atmosphere (Fig. 7).
Other dominant modes of variability in southern
hemisphere climate include the semi-annual
oscillation (SAO) and Southern Annular Mode
(SAM). A major challenge for the IPY is to
understand the physical mechanisms behind
teleconnections and modes of variability linking the
high and low latitudes of the southern hemisphere,
and the impact of these modes on Antarctic climate
and ecosystems.
The Southern Ocean harbors a series of unique and
distinct ecosystems in biogeographic zones defined by
fronts of the ACC. A series of recent experiments
have demonstrated that addition of iron can fuel a
dramatic increase in phytoplankton biomass40.
Southern Ocean primary production supports a vast
population of krill, which in turn supports a large
population of higher predators. Climate variability
and change are likely to have significant but poorly
understood impacts on Southern Ocean ecosystems
and biogeochemical cycles41. A primary goal of the
IPY should be to obtain sufficient understanding of
the
links
between
climate,
ecosystems,
biogeochemical processes and biodiversity to
Fig. 6—Temperature at 900 m depth from climatology (left), from WOCE floats (centre) and the temperature change between recent float
data and historical ocean profile data32.
Fig. 7—A schematic illustration of a possible mechanism for the initiation and maintenance of the Antarctic Dipole, corresponding to the
warm phase of ENSO (courtesy of X. Yuan).
RINTOUL: THE ROLE OF SOUTHERN OCEAN IN PAST, PRESENT AND FUTURE CLIMATE
determine the response of the Southern Ocean system
to climate variability and change.
A number of modelling studies and paleoclimate
proxy records have suggested that changes in northern
and southern hemisphere climate are linked on glacial
– interglacial time-scales. Cooling in Greenland is
generally associated with warming in Antarctica,
giving rise to the concept of a “bipolar seesaw.”
Recently the seesaw concept has been extended to
account for the response of ocean heat transport to a
freshwater anomaly introduced in the North Atlantic42
The model does an impressive job of explaining many
of the climate variations in both hemispheres during
the last glacial period and emphasizes the strong
coupling between the hemispheres. Recent studies of
well-synchronised ice cores from Antarctica and
Greenland have further demonstrated the one-to-one
coupling between climate events in the two polar
regions43
A Strategy for the Southern Ocean IPY
The discussion above identified a number of
outstanding research issues concerning the role of the
Southern Ocean in past, present and future climate.
IPY activities to address these unknowns can be
organized into five research themes.
Antarctica and the Southern Ocean in the global water cycle
The goal of Theme 1 is to quantify the highlatitude contributions to the global water cycle, to
determine the sensitivity of the water cycle to climate
change. . The largest uncertainties in the high latitude
water balance (e.g. sea ice thickness) are of such a
scale that a coordinated multi-disciplinary, multinational effort is required to address them. The water
balance is considered to include the atmosphere (e.g.
precipitation, evaporation and circulation), the
cryosphere (e.g. sea ice and glacial ice) and the
hydrosphere (e.g. ocean circulation and stratification,
run-off). A similar IPY initiative has been proposed
for the northern polar regions.
Southern hemisphere teleconnections
The objectives of Theme 2 are to understand the
climate connections between low and high latitudes,
including both atmospheric and oceanic pathways; to
determine the role of air-ice-ocean interactions in
southern hemisphere variability and change; and to
assess the sensitivity of the modes of variability to
future change.
379
Climate processes at the Antarctic continental margin
The objectives of Theme 3 are to improve our
understanding and models of ocean-ice-atmosphere
interactions at high southern latitudes. This will
include a snapshot of the circumpolar distribution of
the complex system of coastal, shelf and slope
currents along the periphery of Antarctica. The
production rate of Antarctic Bottom Water will be
quantified over an annual cycle and an observing
system for long-term monitoring of AABW export
will be tested. The circumpolar volume of sea ice will
be measured for the first time. The impact of warmer
ocean temperatures on ice shelf stability will be
investigated.
Climate – ecosystem – biogeochemistry interactions in the
Southern Ocean
The objective of Theme 4 is to understand the
impact of climate variability and change on Southern
Ocean ecosystems and biogeochemical cycles. This
will provide an opportunity to investigate how
patterns of biodiversity are likely to change with
changing climate.
Records of past Antarctic climate variability and change
The objectives of theme 5 are to use high
resolution proxy records to determine the natural
modes of climate variability on time-scales from years
to millennia. This will improve our understanding of
the mechanisms of abrupt climate change in the past,
including the role of northern versus southern
hemisphere.
Key Elements of an Integrated IPY Southern Ocean
Observing System
The observations needed to address the themes
described above are best obtained as part of an
integrated, multidisciplinary observing network. Such
a network should include the following elements.
Synoptic multi-disciplinary transects
During IPY a set of meridional transects will
provide the first synoptic snapshot of the circulation,
stratification and biogeochemical status of the
Southern Ocean. The transects will include physical
(e.g. CTD/O, LADCP, tracers), biogeochemical (e.g.
nutrients, trace elements and micronutrients, carbon,
isotopic measurements of export flux, DMS)and
biological (e.g. primary production, pigments, biooptics,
fast
repetition
rate
fluoromoter,
molecular/genetic techniques, biomarkers, targeted
trawls, acoustic) measurements. The sections should
380
INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008
extend from north of the ACC to the Antarctic coast,
including the sea ice zone and the continental slope
and shelf.
Sea ice volume
A primary focus of the IPY should be to make the
first measurement of the volume of Antarctic sea ice
throughout the annual cycle. A variety of tools will
need to be used to meet this challenge: AUV’s and
fixed-point moorings with ice-profiling sonars,
acoustically-tracked floats, ship-board observations,
remote sensing and data-assimilating models all have
a role to play. Remote sensing from new satellites and
airborne sensors and data-assimilating models will
play an important role in integrating the in situ
observations.
Enhanced sea ice drifter array
Information on the intense and highly variable
ocean – ice – atmosphere interactions taking place in
the Antarctic sea ice zone is sparse due to lack of
observations. Numerical weather predictions to the
south of 60S suffer due to lack of surface pressure
observations from the seasonal ice zone. Hence flux
products derived from atmospheric reanalyses are also
uncertain. An enhancement of the circumpolar array
of sea ice buoys during the IPY will provide a one
year snapshot of sea ice drift around the whole of
Antarctica. This will complement efforts to measure
sea ice thickness and will greatly improve southern
hemisphere meteorological analyses. Dense clusters
of platforms will be deployed in some locations for
detailed studies of ice dynamics and deformation.
will provide profiles and current velocities from key
ice-covered seas. In other areas of Antarctica, floats
will be programmed to continue to profile and store
data beneath ice, but not to surface. Once the floats
detect open water, the stored profiles will be
transmitted.
Monitoring of key passages
Key passages and boundary currents should be
instrumented to obtain transport estimates. Tide
gauges and bottom pressure recorders have been
shown to provide a cost-effective means of
monitoring the transport of the Antarctic Circumpolar
Current.
Environmental sensors on marine mammals
Recent experiments have demonstrated the
viability of using temperature and conductivity
sensors mounted on large marine mammals for
oceanographic monitoring. Elephant seals, for
example, dive repeatedly to depths of up to 1500 m
and spend the autumn and winter in the marginal ice
zone. The seals also provide high resolution transects
across the Antarctic Circumpolar Current during trips
between subantarctic islands and the sea ice zone. The
tags also provide valuable information on the
relationship between seal distributions and
oceanographic features. The technology is likely to
develop further before the IPY. A deployment of
50 to 70 tags during the IPY would provide a unique
data set to complement the more traditional
oceanographic sensors.
Enhanced meteorological observations
The ocean circulation and structure beneath the
Antarctic sea ice remains largely unknown. The
strategy for sub-ice observations in the Antarctic will
relay heavily on technology being developed for the
Arctic. However, the challenges are significantly
greater in the Antarctic. The area of the Antarctic sea
ice pack is much greater than that of the Arctic. Many
areas are more remote; and the divergence and strong
seasonality of the sea ice pack makes ice-tethered
stations more difficult to maintain. Therefore, in the
Antarctic the IPY will focus on one or more “wellmeasured” regions or basins (e.g the Weddell Sea).
The IPY should serve as a test of the impact of an
enhanced atmospheric observing system on Antarctic
and southern hemisphere weather forecasts. The
enhanced observations should include additional
automatic weather stations and remote profilers, sea
level pressure observations from ice and ocean
drifters (see above) and aircraft (manned and unmanned).
During
the
IPY,
state-of-the-art
meteorological sensors (e.g. IMET systems) should be
installed on as many Antarctic research, supply and
tourist ships as possible to provide validation data for
the next generation of flux products from reanalyses
and satellites.
Enhanced Southern Ocean Argo
Ice cores from high accumulation rate coastal regions
A primary tool for obtaining a snapshot of ocean
conditions during the IPY period will be the Argo
array of profiling floats. Acoustically tracked floats
The short duration of the instrumental record poses
a huge challenge when attempting to understand
southern hemisphere climate variability and change.
Ocean circulation under sea ice
RINTOUL: THE ROLE OF SOUTHERN OCEAN IN PAST, PRESENT AND FUTURE CLIMATE
Ice cores from high accumulation rate coastal sites
will be of immense value in reconstructing a record
of past change on time-scales from years to
millennia23,44. The IPY should be used to accelerate
the collection of shallow and intermediate cores from
a circumpolar distribution of coastal sites.
Sediment cores
New sediment cores from medium to high
accumulation rate regions will help to identify
changes in Southern Ocean circulation and structure
during the course of past glacial cycles. These cores
will provide estimates of past changes in sea ice
extent and shifts in ocean fronts and help to clarify the
relationship between changes in the northern and
southern hemispheres.
Remote sensing
Satellite measurements will play a critical role in
the IPY. Key instruments include satellite altimeter
and scatterometer, infrared and microwave
radiometers. This will be used for sea surface
temperature, gravity missions, ocean colour and
cryosphere satellites to measure ice extent, ice
thickness and snow thickness (e.g. ICESat, CryoSat,
EOS Aqua). A major hurdle for use of the new
cryosphere sensors is the requirement for groundtruth. A major goal of the IPY should be to obtain the
field measurements needed to validate the new
generation of cryosphere satellites.
Process studies
The role of Antarctica and the Southern Ocean in
the global climate system requires focused process
studies. Exchange of water masses across the
Antarctic Slope Front is an important, but poorly
understood, process in the formation of dense water
on the continental shelf. The complex interactions
between the ocean and ice shelves remain largely
unobserved. The distribution of diapycnal mixing
remains a central issue in oceanography. Recent
studies suggest the Southern Ocean may be a hot-spot
for diapycnal mixing45. Microstructure measurements
should be conducted during the IPY to test this
hypothesis.
Implementation
The plan for Antarctic and Southern Ocean climate
research during IPY outlined here is too broad to
provide an effective structure for implementation. A
number of focused themes or clusters have been
381
created, each of which in turn serves as an umbrella
for a large number of individual projects. The two
themes of most relevance to physical oceanography
are Climate of Antarctica and the Southern Ocean
(CASO) and Synoptic Antarctic Shelf-Slope
Interactions (SASSI). The CASO and SASSI
programs are closely integrated with a number of
other IPY programs, including GEOTRACES (trace
elements and isotopes), ICED (climate – ecosystem
dynamics), CRYOS (cryosphere) and CAML (Census
of Antarctic Marine Life).
CASO is focused on the open ocean region. CASO
involves scientists from 18 nations, including India.
The objectives of CASO are (1) to obtain a synoptic
circumpolar snapshot of the physical environment of
the Southern Ocean and (2) to enhance understanding
of the role of the Southern Ocean in past, present and
future climate. CASO will deliver improved climate
predictions, from models that incorporate a better
understanding of southern polar processes; proof of
concept of a viable, cost-effective, sustained
observing system for the southern polar regions
(including the ocean, atmosphere and cryosphere);
and a baseline for the assessment of future change.
The full-depth hydrographic sections proposed to
be occupied by CASO are shown in Fig. 8. The field
phase of CASO will include:
• A circumpolar array of full-depth multidisciplinary
hydrographic
sections
and
XBT/XCTD sections, extending from the
Antarctic continent northward across the
Antarctic Circumpolar Current, including key
water mass formation regions.
• An enhanced circumpolar array of sea ice
drifters, measuring a range of ice, ocean and
atmosphere parameters.
• Profiling floats deployed throughout the
Southern Ocean, including acoustically-tracked
floats in ice-covered areas.
• Current meter moorings to provide time series of
ocean currents and water mass properties at key
passages, in centres of action of dominant modes
of variabilityand in areas of bottom water
formation and export.
• Environmental sensors deployed on marine
mammals.
• Direct measurements of diapycnal and isopycnal
mixing rates in the Southern Ocean.
382
INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008
Fig. 8—Synoptic multi-disciplinary hydrographic transects to be occupied by CASO (www.clivar.org/organization/southern/CASO).
• Analysis of ice cores, sediment cores and deep
corals to extend observations of Southern Ocean
variability back beyond the instrumental era.
• Bottom pressure gauges will be used near Drake
Passage to monitor ocean currents, validate tidal
• modelsand improve regional corrections to
satellite altimeter products.
• Automatic weather stations, flux measurements
in the boundary layer and drifters to measure
atmospheric variability (pressure, winds, heat
and freshwater flux).
• The observations will be integrated closely with
modelling studies using a variety of approaches
SASSI will:
• Obtain a circumpolar synoptic view of Antarctic
shelf and slope oceanography (Fig. 9).
• Assess quantitatively the properties and amount
of inflow of warm, saline deep water into the
continental shelf, with a focus in regions known
to be active sites for water transformation.
• Assess the role of onshore oceanic heat transport
in melting sea ice and ice shelves.
• Determine where, when and how this oceanic
inflow is transformed, through net cooling and
freshwater fluxes during the seasonal sea ice
melting/freezing cycle over the shelf domain
into dense Shelf Water and its subsequent
derivative Antarctic Bottom Water.
• Assess the importance of ice shelves in the net
upper ocean freshening process including
iceberg calving and meltingand determination of
basal melt rates.
• Assess the importance of coastal polynyas to
water mass transformations.
• Better understand the dynamics of the coastal
current and slope front systemsand how they
influence the exchanges between sea ice, glacial
ice, coastal and deep ocean waters.
• Quantify
freshwater
transports
around
Antarctica through both currents and
atmosphere-ocean-ice interaction.
• Determine down-slope dynamics and associated
meridional transports, integrating physical,
geological and geophysical records with the
currents in the bottom boundary layer.
• Assess the degree to which present coupled
ocean-ice models represent the shelf system and
its variability.
• Design a long-term monitoring system over the
Antarctic continental margins that can act as an
early indicator of global climate-related changes.
• Identify key Antarctic shelf/slope processes that
should be included or parameterised in future
climate models.
• Explore and document the geology, chemistry
and biology of underwater volcanic hot vents.
RINTOUL: THE ROLE OF SOUTHERN OCEAN IN PAST, PRESENT AND FUTURE CLIMATE
383
Fig. 9—Hydrographic sections (lines) and mooring arrays (dots) contributing to SASSI (roughy.tamu.edu / sassi). SASSI will focus on
ocean – atmosphere – cryosphere interactions along the Antarctic continental margin. SASSI involves researchers from 12 nations.
SASSI will make observations in geographical regions never intensively studied
• Obtain a swath bathymetry map of the Antarctic
continental shelf and slope, including beneath
ice shelves.
• Assess the role of the microbial biomass and
processes in regulating the carbon biological
pump efficiency for the carbon sequestration on
the Antarctic continental shelf.
• Understand the bio-optical processes that affect
the ocean colour signal in the Southern Ocean.
Summary
Progress in understanding the influence of the
Southern Ocean region (including the ocean,
atmosphere and cryosphere) on climate has been rapid
in recent years. It is known that the Southern Ocean
connects the shallow and deep layers of the
overturning
circulation
and
participates
in
teleconnections linking high and low latitudes. It
experiences a number of prominent modes of
variability and controls the ocean uptake, storage and
transport of heat, freshwater and carbon.
Nevertheless, substantial unknowns remain. The IPY
offers an opportunity to build on recent progress to
deliver a major advance in our understanding of the
role of Antarctic and the Southern Ocean in past,
present and future climate. The strategy outlined here
is targeted directly at the four major goals of the IPY:
(1) to determine the present environmental status of
INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008
384
the polar regions by quantifying their spatial and
temporal variability; (2) to quantify, and understand,
past and present environmental and human change in
the polar regions in order to improve predictions; (3)
to advance our understanding of polar – global
teleconnections on all scales and (4) to investigate the
unknowns at the frontiers of science in the polar
regions.
5
The legacy of the IPY 2007-2008 will be a
sustained, cost-effective observing network for
Antarctica and the Southern Ocean. The system will
rely heavily on autonomous instruments to provide
long time-series from remote and inaccessible
locations. During the IPY new observational
approaches will be tested for their suitability as part of
a sustained system. The IPY will deliver an observing
system capable of providing year-round sampling
with spatial and temporal sampling at least an order of
magnitude greater than presently available.
8
The Southern Ocean IPY will leave a number of
other legacies on which the future of Antarctic and
Southern Ocean research will be built: a snap-shot of
the Southern Ocean that will provide a benchmark for
assessments of past and future change; models
capable of simulating Southern Ocean processes and
therefore improved projections of future change; and
a better integrated, interdisciplinary polar research
community.
Acknowledgement
The present paper is based on an IPY discussion
paper prepared by the Southern Ocean Region
implementation panel, sponsered by CLIVAR, CliC
(Climate and the crosphere) and SCAR (Scientific
Committee for Antarctic Research) with input from
individual IPY investigators in many countries.
References:
1
2
3
4
Rintoul S R, Hughes C & Olbers D, The Antarctic
Circumpolar System, in: Ocean Circulation and Climate,
edited by G Siedler, J Churchand & J Gould, (Academic
Press) 2001, pp. 271-302.
Speer K, Rintoul S R & Sloyan B, The diabatic Deacon cell.,
J. Phys. Oceanog., 30 (2000) 3212-3222.
Cunningham S A, Alderson S G, King B A & Brandon M A,
Transport and variability of the Antarctic Circumpolar
Current in Drake Passage, J. Geoph. Res. Oceans, 108 (2003)
Art. No. 8084.
Rintoul S R & Sokolov S, Baroclinic transport variability of
the Antarctic Circumpolar Current south of Australia
(WOCE repeat section SR3), J. Geophys. Res., 106 (2001)
2795-2814.
6
7
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Speich S, Blanke B, de Vries P, Drijfhout S, Döös K,
Ganachaud A & Marsh R, Tasman leakage: A new route in
the global ocean conveyor belt, Geophys. Res. Lett., 29
(2002) Art. No. 1416.
Sloyan B M & Rintoul S R, Circulation renewal and
modification of Antarctic mode and intermediate water, J.
Phys. Oceanog., 31 (2001) 1005-1030.
Sarmiento J L, Gruber N, Brzezinski M A & Dunne J P,
High-latitude controls of thermocline nutrients and low
latitude biological productivity, Nature, 427 (2004) 56-60.
Rintoul S R, Southern Ocean currents and climate, Pap.
Proc. Roy. Soc. Tasmania, 133 (2000) 41-50
Sabine C L, Richard A, Feely, Nicolas Gruber, Robert M,
Key, Kitack Lee, John L, Bullister, Rik Wanninkhof, Wong
C S, Douglas W R, Wallace, Bronte Tilbrook, Frank J,
Millero, Tsung-Hung Peng, Alexander Kozyr, Tsueno
Onoand Aida F, Rios, The oceanic sink for anthropogenic
CO2, Science, (2004).
Sarmiento J L, Hughes T M C, Stouffer R J & Manabe S,
Simulated response of the ocean carbon cycle to
anthropogenic climate warming, Nature, 393 (1998) 245249.
Francois R, Altabet M A, Yu E F, Sigman, D M, Bacon M P,
Frank M, Bohrmann G, Bareille G & Labeyrie L D,
Contribution of Southern Ocean surface-water stratification
to low atmospheric CO2 concentrations during the last
glacial period, Nature, 389 (1997) 929-935.
Sigman D M, Jaccard S L, Haug G H, Polar ocean
stratification in a cold climate, Nature, 428 (2004) 59-63.
IPCC, Climate Change 2001: The Scientific Basis,
(Cambridge Univ. Press, Cambridge, United Kingdom and
New York, NY, USA) 2001, pp 881.
Dickson B, Yashayaev I, Meincke J, Turrell B, Dye S &
Holfort J, Rapid freshening of the deep North Atlantic Ocean
over the past four decades, Nature, 416 (2002) 832-837.
Jacobs S S, Guilivi C F & Merle P, Freshening of the Ross
Sea during the late 20th century, Science, 297 (2002) 386389.
Aoki S, Rintoul S R, Ushio S, Watanabe S & Bindoff N L,
Freshening of the Adelie Land Bottom Water near 140°E,
Geophys. Res. Lett., 32 (2005) L23601, doi:
10.1029/2005FL024246.
Rintoul S R, Rapid freshening of Antarctic Bottom Water
formed in the Indian and Pacific Oceans, Geophys. Res. Lett.,
34, L06606, doi:10.1029/2006GL028550..
Jacobs S S, Observations of change in the Southern Ocean,
Phil. Trans. Roy. Soc. A, 364 (2006) 1657-1681.
Parkinson C L & Cavalieri D J, Sea ice, in: Satellite Image
Atlas of Glaciers of the World: State of the Earth’s
Cryosphere at the Beginning of the 21st Century,edited by R
S Williams & J G Ferrigno, (Geolog. Surv., Washington DC,
US) 2006, in press.
Atkinson A, Siegel V, Pakhomov E, Long-term decline in
krill stock and increase in salps within the Southern Ocean,
Nature, 432 (2004) 100-103.
Parkinson C L, Earth’s cryosphere: Current state and recent
changes, Annu. Rev. Environ. Resou., 31 (2006) 33-60.
Dela Mare W K, Abrupt mid-twentieth century decline in
Antarctic sea ice extent from whaling records, Nature, 398
(1997) 57-60.
RINTOUL: THE ROLE OF SOUTHERN OCEAN IN PAST, PRESENT AND FUTURE CLIMATE
23
24
25
26
27
28
29
30
31
32
33
34
Curran M A J, van Ommen T D, Morgan V I, Phillips K L &
Palmer A S, Ice core evidence for Antarctic sea ice decline
since the 1950s, Science, 302 (2003) 1203-1206.
Turner J, Colwell S R, Marshall G J, Lachlan-Cope, T A;
Carleton, A M., Jones, P D, Lagun, V, Reid, P A &
Iagovkina, S, Antarctic climate change during the last 50
years, Int. J. Clim., 25 (2005) 279-294.
Scambos T A, Hulbe C, Fahnestock M & Bohlander J, The
link between climate warming and break-up of ice shelves in
the Antarctic Peninsula, J. Glaciol., 46 (2000) 516-530.
Marshall G J, Trends in Antarctic geopotential height and
temperature: A comparison between radiosonde and NCEPNCAR reanalysis data, J. Clim., 15 (2002) 659-674.
Thompson D W J, Solomon S, Interpretation of recent
Southern Hemisphere climate change, Science, 296 (2002)
895-899.
Gillett N P & Thompson D W J, Simulation of recent
Southern Hemisphere climate change, Science, 302 (2003)
273-275.
Fyfe J C, Boer G J & Flato G M, The Arctic and Antarctic
oscillations and their projected changes under global
warming, Geophys. Res. Lett., 26 (1999) 1601-1604.
Cai W J, Whetton P H & Karoly D J, The response of the
Antarctic Oscillation to increasing and stabilized atmospheric
CO2 , J. Clim., 16 (2003) 1525-1538.
Marshall G J, Stott P A, Turner J, Connolley W M, King, J C
& Lachlan-Cope T A, Causes of exceptional atmospheric
circulation changes in the Southern Hemisphere, Geophys.
Res. Lett., 31 (2004) Art. No. L14205.
Gille S T, Warming of the Southern Ocean since the 1950s,
Science, 295 (2002) 1275-1277.
Levitus S, Antonov J, Boyer T, Warming of the world ocean,
1955-2003, Geophys. Res. Lett., 32 (2005) Art. No. L02604.
Willis J K, Roemmich D, Cornuelle B, Interannual
variability in upper ocean heat content, temperatureand
thermosteric expansion on global scales, J. Geophys. Res.
Oceans, 109 (2004) Art. No. C12036.
35
36
37
38
39
40
41
42
43
44
45
385
Jacobs S S, Bottom water production and its links with the
thermohaline circulation, Ant. Sci., 16 (2004) 427-437.
Rignot E & Jacobs S S, Rapid bottom melting widespread
near Antarctic ice sheet grounding lines, Science, 296 (2002)
2020-2023.
Zwally H J, Giovinetto M B, Li J, Cornejo G, Beckley M A,
Brenner A C, Saba J L & Yi D H, Mass changes of the
Greenland and Antarctic ice sheets and shelves and
contributions to sea-level rise: 1992-2002, J. Glac., 51
(2005) 509-527.
Turner J, The El Nino-southern oscillation and Antarctica,
Int. J. of Clim., 24 (2004) 1-31.
Yuan X J, Martinson D G, The Antarctic Dipole and its
predictability, Geophys. Res. Lett., 28 (2001) 3609-3612.
Boyd P W, Watson A, Law C S, Abraham E, Trull T,
Murdoch R, Bakker D C E, Bowie A, Charette M, Croot P,
Downing K, Frew R, Gall M, Hadfield M, Hall J, Harvey M,
Jameson G, La Roche J, Liddicoat M, Maldonado M, McKay
R M, Nodder S, Pickmere S, Pridmore R, Rintoul S R, Safi
K, Sutton P, Strzepek R, Tanneberger K, Turner S, Waiteand
A & Zeldis J, A mesoscale phytoplankton bloom in the polar
Southern Ocean stimulated by iron fertilization, Nature, 407
(2000) 695-702.
Smetacek V & Nicol S, Polar ocean ecosystems in a
changing world, Nature, 437 (2005) 362-368.
Knutti R, Flückiger J, Stockerand T F, Timmermann A,
Strong hemispheric coupling of glacial climate through
freshwater discharge and ocean circulation, Nature, 430
(2004) 851-856.
EPICA Community Members, One-to-one coupling of
glacial climate variability in Greenland and Antarctica,
Nature, 444 (2006) 195-198.
Goodwin D, Van Ommen T D, Curranand M A J &
Mayewski P A, Mid-latitude winter climate variability in the
south Indian and south-west Pacific regions since 1300 AD,
Clim. Dyn., 22 (2004) 783-794.
Garabato A C N, Polzin K L, King B A, Heywood, K J &
Visbeck, M, Widespread intense turbulent mixing in the
Southern Ocean , Science, 303 (2004) 210-213.