The global importance of the Southern Ocean, and the key role of its freshwater cycle

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Ocean Challenge, Vol. 23, No.2 (publ. 2019) 27
‘How inappropriate to call this planet Earth when it is quite clearly Ocean.’ This quote,
beloved of oceanographers and others who care about the sea, comes from Arthur C.
Clarke, famed writer of science fiction. To many, it encapsulates perfectly the pre-eminence
of the ocean in everything to do with our planet – sustaining life, controlling our climate,
feeding our populations. Clarke’s use of ‘Ocean’ as a singular noun also foreshadows the
first principle of Ocean Literacy (http://oceanliteracy.wp2.coexploration.org) which is that
‘Our planet has one big ocean with many features’. The idea of a single interconnected
ocean with many features becomes accessible visually when the world is viewed on a
Spilhaus Projection (Figure 1), designed by Athelstan Spilhaus in 1942 as a side project
whilst developing the early bathythermograph. The projection not only emphasises this
interconnectedness, but also highlights the centrality of the Southern Ocean in the global
ocean circulation. Flowing around Antarctica, the Antarctic Circumpolar Current – the
world’s largest current system – connects the Atlantic, Pacific and Indian Ocean basins,
and moves huge quantities of heat and freshwater, along with carbon and other climatically
and ecologically important substances, between them.
The global importance of the Southern Ocean
transcends even this connecting role, however. The
Southern Ocean is the site of much atmospheric
and cryospheric forcing that drives the global
circulation. It is the key site globally where old,
deep waters rise to the surface, and are thus able to
interact with the atmosphere and cryosphere. These
deep waters were last in contact with the atmos-
phere hundreds of years ago, so this represents the
main interaction globally of the industrial-era atmos-
phere with the pre-industrial ocean. Large amounts
of heat and carbon are exchanged at the surface,
before the water sinks back into the interior, both
in a less dense form in intermediate layers, and in a
denser form as bottom waters (Figure 2 overleaf).
Figure 1 The globe viewed on a Spilhaus
projection; in contrast to conventional misleading
projections, this portrays the ocean fringed by
land. The global thermohaline circulation is shown
in cartoon form, with upper-layer flow in red and
lower-layer flow in blue.
This article is
based on the
lecture given by
the author on
receipt of the
Challenger Medal
in September
2018
The key role of the Southern
Ocean in the global circulation
becomes very clear when the
Spilhaus projection is
used

Ocean Challenge, Vol. 23, No. 2 (publ. 2019)
28
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The bar charts show
the distribution of
observations by
longitude (top panel)
and by latitude
(right-hand panel).
The smooth curve
in the right-hand
panel is the expected
distribution of those
ocean observations
if they were spread
equally on a sphere;
the irregular curve
is the expected
distribution of ocean
observations also
taking into account
the shape and size
of the Earth’s land
masses.
Figure 3 The Southern Ocean data desert, as seen in the global distribution of temperature observations between
1955 and 2017. The dominance of the Northern Hemisphere in the data is clear; by contrast, the Southern Hemisphere
south of 60° S is especially poorly covered. Whilst innovations such as the Argo programme are addressing this data
desert, it remains as problematic as ever for parameters that cannot yet be measured autonomously.
As well as being
the key connecting
region for the global
ocean, the Southern
Ocean is the most
important region for
the reprocessing of old,
deep waters into newer
water masses, both
denser and less dense
Figure 2 Above 3D map of Antarctica and the
Southern Ocean (yellow corresponds to continental
shelf/tops of ridges), showing schematically deep
water rising to the surface to be converted to
intermediate and bottom water within the Antarctic
Circumpolar Current. (Adapted from Meredith, 2016)
Left Cross-section of the Southern Ocean to
show how the reprocessing of deep water to form
intermediate and bottom waters results in heat and
carbon (including that produced by human activity)
being removed from the atmosphere. The carbon
is removed both in dissolved form in sinking water
and via high primary productivity largely supported
by nutrients being brought to the surface around
Antarctica.
Reprocessing of
water masses around
Antarctica has a
profound effect on
global climate by
removing heat and
carbon from the
atmosphere and storing
it in the deep ocean for
centuries or longer
intermediate
deep
bottom

Ocean Challenge, Vol. 23, No.2 (publ. 2019) 29
The world’s largest data desert
It is thus beyond question that the Southern
Ocean deserves special attention, so that we
can better understand it and better predict its
changes and its future impacts on the rest of the
world. However, the Southern Ocean is also argu-
ably the biggest data desert on the planet. Global
shipping tends to avoid the Southern Ocean,
focussing instead on major trade routes that
lie predominantly in the Northern Hemisphere.
Indeed, there are large regions of the Southern
Ocean that remain virtually unvisited each year,
and from which very few direct ocean measure-
ments are obtained (Figure 3).
The problem is especially severe in winter, when
some of the strongest winds on the planet drive
massive seas, and when the Antarctic continent
effectively doubles in size due to the expansion
of sea ice. This makes collecting data from the
Southern Ocean using conventional ship-based
methods extremely challenging (cf. Figure 4).
Robotic and other innovative techniques are
beginning to fill this data void, and the advent
of floats capable of operating under ice and
long-duration gliders offers great potential for
the collection of the sustained, systematic ocean
datasets that are required. Nonetheless, many of
the measurements we need cannot yet be made
using automated techniques, and still rely on
collection of discrete water samples for process-
ing and analysis; this makes such data as are
collected from the Southern Ocean, especially in
winter, of disproportionately high value.
A sustained, year-round ocean series in
Antarctic waters: RaTS
One example of a sustained, coherent measurement
programme in the Southern Ocean is the Rothera
Time Series (RaTS), which is conducted by the
British Antarctic Survey (BAS) and operates out of
Rothera Research Station on the Antarctic Peninsula
(Figure 5). This series has been providing quasi-
weekly ocean data for more than two decades.
Figure 5 Map of the Antarctic Peninsula, showing
the location of Rothera Research Station on Adelaide
Island, the home of RaTS. The RaTS sampling site is
within Ryder Bay on the eastern side of the island.
(Meredith et al., 2017)
Figure 4 Left View forward from RRS James Clark Ross, as she moves south into the Weddell Sea in April 2016
(cruise JR15006). Right A CTD being recovered in a blizzard. (Photos by the author)
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There are many
challenges to
collec ting data
in the Southern
Ocean
The Rothera
Research Station
alllows year-round
sampling at the
RaTS site
90°W
0°
90°E
180°

Ocean Challenge, Vol. 23, No. 2 (publ. 2019)
30
Naturally occurring oxygen is composed of three stable isotopes,
16O, 17O and 18O, with 16O being the most abundant (99.76% of
the total, compared with 0.04% 17O and 0.20% 18O). Water that
evaporates from the ocean eventually condenses as cloud droplets
and falls as rain or snow. When seawater evaporates, molecules
with the lighter oxygen isotope (H2
16O) evaporate more readily, so
atmospheric water vapour is relatively enriched in 16O. When water
vapour condenses and is precipitated back into the ocean, water
containing the heavier isotope (H2
18O) condenses preferentially. Both
processes therefore deplete water vapour over the ocean in H2
18O
relative to H2
16O. When 18O-depleted water vapour is precipitated
as snow, the snow will also be depleted in 18O relative to the oceans
– and the same will be true of ice sheets, glaciers and icebergs.
Conversely, when sea ice melts into the ocean, it provides freshwater
that is isotopically much more similar to the seawater into which it
melts. This is because, aside from a small fractionation factor, sea
ice acquires the isotopic signature of the seawater from which it was
formed. Thus, whilst sea ice, snow, glacial melt and iceberg melt all
provide waters with similar salinities to the ocean (zero, or nearly
zero), we can distinguish them from each other by also measuring
the seawater’s isotopic composition. The measurement made is
of 18O, being the ratio of H2
18O to H2
16O in seawater, relative to a
known standard.
Distinguishing the origins of freshwater input
to the Southern Ocean
RaTS core variables include a range of physi-
cal and biogeochemical parameters, including
temperature, salinity, phytoplankon uorescence
(to measure chlorophyll concentration), size-frac-
tionated chlorophyll, macronutrients (nitrate, phos-
phate and silicate) and many others. RaTS also
provides the scientic context and infrastructure
for numerous collaborative investigations both
nationally and internationally; these have sup-
ported collection of measurements to answer spe-
cic hypothesis-driven questions, including those
relating to trace metals, viruses and climatically
active gases.
Tracing different freshwater inputs at RaTS
One of the core RaTS variables is 18O, a measure
of the ratio of stable oxygen isotopes in seawater
(see Box). This is measured from water samples
which are returned to the UK and analysed at
the NERC Isotope Geosciences Laboratory (BGS
Keyworth), and is a tracer which provides valuable
insight into the sources of freshwater injected
into Antarctic waters. When measured along with
salinity, 18O provides information on whether the
freshwater present in a seawater sample derives
from sea-ice melt, or from other sources (namely
glacial discharge and precipitation). This is critical
information freshwater from dierent sources can
aect the ocean in dierent ways. For example,
glaciers (which originate as compressed snow)
can scour underlying rock and sediment, and
thus they can contain signicant concentrations
of trace metals such as iron, which are released
when the glaciers, ice sheets, and the icebergs
that break o from them, melt. Accordingly, in
regions such as the Southern Ocean, where pri-
mary production is limited over large areas by low
Figure 6 Left The author conducting a RaTS
profiling operation by hand-winching a CTD
upwards from 500 m depth in Ryder Bay, adjacent
to Rothera Research Station (see Figure 5). Dr Hugh
Venables (BAS) looks on in amusement.
Right BAS Marine Assistant Zoë Waring preparing
to collect a seawater sample using a Niskin bottle.
Behind her, two Ryder Bay residents are decidedly
unfazed by the ground-breaking science happening
nearby. (Photo by Rich Rowe, British Antarctic Survey)
Measurements
of temperature
and salinity, and
water samples,
are regularly
collected at the
RaTS station
Rothera’s coastal location allows scientists and sup-
port sta based there to access the ocean weekly
and year-round, with sampling conducted from small
boats in summer (Figure 6), or through holes cut in
the ice when ice cover precludes boating operations
The systematic collection of data and samples during
wintertime is almost unique in the Southern Ocean,
making them extremely valuable.

Ocean Challenge, Vol. 23, No.2 (publ. 2019) 31
concentrations of micronutrients, the spatial pat-
terns and temporal changes in glacial discharge
are very important biogeochemically, ecologically
and even climatically. Other freshwater sources
are important in other contexts; for example, sea-
ice formation and melt impacts strongly on upper-
ocean stratication and dense water production
(cf. Figure 2), and provides a seasonally varying
ecological habitat that is exploited by a range
of species in manifold ways. On a global scale,
changing glacial discharge from the Antarctic con-
tinent can have signicant impacts on sea-level
rise, whereas sea-ice melt exerts only a minimal
eect. Accordingly, it is of great importance to
discriminate between the sources of dierent
freshwater inputs to the ocean, even when they
result in comparable salinity changes.
Measuring salinity and 18O concurrently allows
us to quantitatively separate freshwater into an
amount that derives from sea-ice melt, and an
amount that derives from meteoric water (i.e.
water originating from the atmosphere, being
the sum of glacial discharge plus precipitation).
The RaTS series of these quantities are shown
in Figure 7. Numerous features are apparent
in these series, and have different significance
depending on their time scales.
Marked seasonal variability in the amount of
sea-ice melt in the upper ocean at the Antarctic
Peninsula is clear; this is to be expected, given
the profound seasonality in the area covered by
sea ice. Arguably more surprising is that the sea-
sonality in meteoric water is equally as strong as
sea-ice melt, reecting both seasonality in glacial
discharge to the ocean, and melt of snow which
has accumulated during winter either on land or
on top of the sea ice. Thus, whilst the advance
and retreat of sea ice around Antarctica is often
referred to as the biggest seasonal signal on the
planet, its impact on the water column at RaTS
is matched by those caused by changes in fresh
water from other sources.
On interannual time-scales, there are years of
marked extremes in the concentration of both
sea-ice melt and meteoric water in the ocean
at RaTS. For example, sea-ice melt showed a
strong peak (more than 2%) in early 2005 and
an even stronger peak in 2014. Previous work
has revealed the sensitivity of this part of the
Southern Ocean to large-scale coupled modes of
climate variability, including the El Niño/Southern
Oscillation (ENSO) phenomenon, the Southern
Annular Mode (SAM), and others, each of which
can affect the different components of the fresh-
water budget, in addition to upper-ocean strat-
ification and mixing. These modes operate over
very large spatial scales; in the case of ENSO, the
signature in the time series confirms the impact
that it can have on even the remotest regions.
On decadal time scales, the series are not yet
sufficiently long to draw unambiguous inferences
concerning trends. However, the sea-ice melt
tends to be higher in the latter part of the record
compared with the earlier part, indicative of a
shift from the region being one of net sea-ice
production to one of net sea-ice melt. Further, the
concentration of meteoric water tends to be lower
in the latter part of the record compared with the
earlier part; this has been traced to changes in
ocean stratification, specifically wintertime mixed
layer depth, which affects the vertical distribu-
tion of the freshwater in the water column and
hence alters the freshwater concentration at the
15 m sampling depth. Such climatic signals have
strong relevance for ecosystems by, for example,
affecting the concentrations of glacier-supplied
micronutrients in the photic zone, with long-term
consequences for primary production.
There are numerous further aspects to the series
shown here, and also the other RaTS and associ-
ated datasets, and the reader is cordially invited
to follow up the references included below, or to
visit https://www.bas.ac.uk/team/science-teams/
oceans/.
f
reshwater %
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Figure 7 Time series showing the percentage of meteoric water (upper curve) and sea-ice meltwater (lower
curve) in water samples collected from a depth of 15 m at the RaTS station, derived from salinity and 18O data.
Note that the contribution of sea-ice meltwater can be both positive and negative, with the former reecting
net sea-ice melt to the water sampled, and the latter reecting net sea-ice production from the water sampled.
(Figure updated from Meredith et al., 2017)
Time series of
salinity and 18 O
data collected at the
RaTS station allow
us to see seasonal,
interannual and
longer-term
variations in the
contribution of
meteoric water and
sea-ice formation/
melt to near-surface
seawater

Ocean Challenge, Vol. 23, No. 2 (publ. 2019)
Mike Meredith is based at the British Antarctic
Survey in Cambridge, where he leads the Polar
ceans research team, with scientic foci on ocean
circulation and climate, ocean–ice interactions,
and the interdisciplinary marine system. Mike’s
own research has focussed on understanding the
changing ocean circulation and properties around
Antarctica, and what those changes mean for
planetary scale systems. mmm@bas.ac.uk
Meredith, M.P., S.E. Stammerjohn, H.J. Venables et
al. (2017) Changing distributions of sea ice melt
and meteoric water west of the Antarctic Peninsula.
Deep-Sea Research Part II: Topical Studies in Ocean-
ography 139, 40–57. doi: 10.1016/j.dsr2.2016.04.019
Venables, H.J., A. Clarke and M.P. Meredith (2013)
Wintertime controls on summer stratication and
productivity at the western Antarctic Peninsula. Lim-
nology and Oceanography 58, 1035–47.
doi: 10.4319/lo.2013.58.3.1035
Venables, H.J. and M.P. Meredith (2014) Feedbacks
between ice cover, ocean stratication and heat
content in Ryder Bay, western Antarctic Peninsula.
Journal of Geophysical Research 119, 5323–36.
doi: 10.1002/2013JC009669
Personal thoughts and thanks
Being awarded the 2018 Challenger Medal is both
delightful and humbling; I am extremely grateful to
those who made and supported the nomination, and
to the Challenger Society for bestowing the honour.
The award reects a team eort by a huge number
of people over many years, including support sta,
ships’ ocers and crews, Antarctic sta and count-
less others. It also reects the eorts of an incredible
group of scientic collaborators, with whom I have
had the privilege of working. Space precludes listing
them individually here – instead, please see below.
They are all thanked profusely for their wisdom,
energy and patience. Jamie Oliver, Peter Fretwell and
Laura Gerrish are thanked for help with the graphics
for this article.
Take-home messages
● Processes occurring in the Southern Ocean are
of global importance, aecting climate, sea level
and all parts of the marine ecosystem.
● Freshwater processes are key to this impor-
tance, and require more than just salinity meas-
urements to be understood. The ratio of oxygen
isotopes in seawater oers key extra insight into
freshwater sources.
● Maintaining long-term, systematic time series is
critical in order to detect, understand and predict
the global impacts of changes in Antarctic waters.
Further Reading
Clarke, A., M.P. Meredith, M.I. Wallace et al. (2008)
Seasonal and interannual variability in temperature,
chlorophyll and macronutrients in Ryder Bay, north-
ern Marguerite Bay, Antarctica. Deep-Sea Research
II 55,1988–2006
Frölicher, T.L., J.L. Sarmiento, D.J. Paynter et al.
(2015) Dominance of the Southern Ocean in
anthropogenic carbon and heat uptake in CMIP5
Models. Journal of Climate 28, 862–86.
doi: 10.1175/JCLI-D-14-00117.1
Meredith, M.P. (2016) Understanding the structure of
changes in the Southern cean eddy eld. Geo-
physical Research Letters 43, 5829–32.
doi: 10.1002/2016GL069677
Meredith, M.P. and J.C. King (2005) Rapid climate
change in the ocean to the west of the Antarctic
Penisula during the second half of the twentieth
century. Geophysical Research Letters 32.
doi: 10.1029/2005GL024042
Meredith, M.P., . Schoeld, . Newman, et al. (2013)
The vision for a Southern Ocean Observing System.
Current Opinion in Environmental Sustainability 5,
306–13. doi: 10.1016/j.cosust.2013.03.002
Word cloud of
scientists with
whom the author
collaborates.
Font size is
proportional to
number of articles
published together,
but all are thanked
equally profusely.
32