the magellan-antarctic connection: links and frontiers at southern high latitudes

380 Pages·2013·46.71 MB·English

THE MAGELLAN-ANTARCTIC
CONNECTION: LINKS AND
FRONTIERS AT SOUTHERN
HIGH LATITUDES
Edited by
Wolf E. Arntz, Gustavo A. Lovrich and Sven Thatje
Scientia Marina, 69 (Suppl. 2)
Institut de Ciencies del Mar, CMIMA-CSIC, Barcelona, Spain

Foreword
The second IBMANT Symposium & Workshop was held in Ushuaia (Argentina) between
October 19 and 24, 2003. It was organised jointly by the Alfred Wegener Institute for Polar and
Marine Research (AWI, Bremerhaven, Germany) and the Centro Austral de Investigaciones
Cientificas y Tecnicas (CADIC, Ushuaia, Argentina), and was combined with the ANDEEP
Symposium dealing with recent deep-sea research in the Weddell and Scotia seas (see Deep-Sea
Res. II Vol. 51, 2004). IBMANT II, following a first conference held in Punta Arenas, Chile in
1997 (see Scientia Marina Vol. 63, Suppl. 1, 1999) focussed on continental shelf and slope work
over a wide latitudinal gradient between the northern Magellan region and the high Antarctic
continental coasts. A total of 117 scientists from 13 countries participated in the combined meet
ing, and presentations were offered from 17 countries.
The idea of combining Antarctic deep-sea with shelf and slope studies, and including a wider
geologic and oceanographic background, turned out to be very useful during this conference.
The IBMANT/ANDEEP meeting was the first in which the two aspects were combined, leading
to novel insights and innovative perspectives for research in the years to come.
The presentations and discussions during the meeting focussed on various central issues,
most of which emerged repeatedly. In summary, major subjects discussed during the conference
included the following items:
The final separation and isolation of the Antarctic continent, going along with the establish
ment of a vigorous circum-Antarctic current system and the origin of the Polar Front. These key
events are supposed to have shaped the evolution, biogeography and biodiversity of the South
ern Ocean. However, the various geological alternatives for these processes—in particular the
opening of Drake Passage—have yet to be reconciled with molecular data on major extinction
and radiation events. Perhaps the much greater similarity between the Antarctic and Magellan
marine biota, as compared to the overlap between the Antarctic and Australia, could be a bio
logical key in this context.
Advances and retreats of the Antarctic and Patagonian ice caps as further major driving
forces on evolutionary time scales, and the impact of sea ice, glaciers and icebergs, i.e. present
disturbance leading to ecological change. How much do we know about these processes?
The "barrier" effect of the Polar Front (Antarctic Convergence). To what extent is it reduced
by eddies causing a certain amount of cross circulation? What role in these processes do the
northern and southern branches of the Scotia Arc play? Do they act as stepping stones for biot-
ic exchange between the Magellan region and the Antarctic?
The Circumpolar Current (West Wind Drift) and its role as an efficient transport vehicle
towards the east. Is this function restricted to species with well-developed dispersal stages such
as pelagic larvae? Are there other (including anthropogenic) means of dispersal? Is there any
transport in E-W direction, e.g. by the East Wind Drift?
The degree of similarity between the marine biota of the Magellan region and their Antarc
tic relatives. How much overlap is there at species (genus, family) level in different taxa? What
is the age of the present marine ecosystems on either side of the Drake Passage? Are there still
effects resulting from the last glaciation?
The predominantly circum-Antarctic benthic distribution pattern on the Antarctic continental
shelf has become a paradigm. However, what percentage of this "identical" fauna are cryptic
species, as suggested by molecular methods? Is the high Antarctic an isolated ecosystem under
present conditions, with speciation occurring rather by isolation than by exchange?
The ways of biotic exchange in the past, the role of the deep sea in these processes, emer
gence and submergence, radiation in situ. What was the respective role of these processes, par
ticularly that of the deep-sea floor for Antarctic benthos?
The possibility of increased invasions due to global climate change. Present warming might
enhance invasions into areas south of the Polar Front, because survival might be favoured due
to reduced temperature differences. Are there any indications for such a development? In what
way might it influence the existing communities, and what are the timescales we are dealing
with?
Latitudinal clines in species richness. There does not seem to be a common pattern for all
taxa, but is the paradigm of the "bell-shaped curve" with decreasing species numbers toward the
pole true for some groups? To what extent do regional hotspots refer to sampling intensity rather
than real differences in species diversity?
Characteristics of cold-water life histories, population dynamics and reproduction. Are there
particular patterns in the Antarctic, and is there a latitudinal gradient from the Magellan region
to the high Antarctic shelf? How strict are these rules?
Characteristics of cold-water physiology, especially stenothermy vs. eurythermy. What are
the advantages and disadvantages of increasing stenothermy on the latitudinal gradient? Are
there exceptions to such a cline, and how do species manage? Do things differ under the aspect
of global warming? To what extent may temperature tolerance have influenced biogeography
and biodiversity of the Southern Ocean in Earth history?
The impact of increased UV-B radiation on marine biota along the latitudinal gradient. Have
increased UV values led to damage in shallow water organisms? What are the protective mech
anisms? Are such mechanisms more frequent at higher latitudes?
Benthic assemblage structure along the latitudinal gradient. Are there consistent clines along
this gradient, e.g. in abundance, biomass, productivity, distribution and diversity? Can data
banks be helpful in managing the enormous amount of data assembled in the last few years?
Of course, not all of these questions were answered comprehensively during the Conference,
but many of them received some clarification during the presentations and the discussions fol
lowing them, and most were discussed in detail during the workshop that followed the oral and
poster sessions (see Summary Review at the end of this volume). While it turned out advanta
geous to have colleagues from a wider frame of disciplines included in these discussions, it was
felt that a future IBMANT meeting should be even more multidisciplinary, including an ade
quate share of physical oceanographers, geologists, palaeontologists and glaciologists.
The editors of this volume hope that the Ushuaia IBMANT Symposium and Workshop will
have a strong impact on future activities in ecological and evolutionary research in the Southern
Ocean. They would like to express their sincere thanks to all the organisations and institutions
that contributed substantially to the success of the meeting and these proceedings.
The following international and national organisations and institutions provided support for
travel and stay of keynote speakers and other participants:
- Intergovernmental Oceanographic Commission (IOC) of the UNESCO
- Belgian Federal Office for Scientific, Technical and Cultural Affairs
- Fundacion Antorchas
- Scientific Committee of Antarctic Research (SCAR)
- Inter American Institute for Global Change Research (IAI)
- International Bureau (IB) of the Federal Ministry of Education and Research (BMBF) of
Germany
- Census of Marine Life (CoML)
- Government of the Province of Tierra del Fuego
- Las Hayas Resort Hotel, Ushuaia
- Hamburg University
- British Antarctic Survey, Cambridge
- Alfred Wegener Institute, Bremerhaven
Further support for running a smooth meeting and providing multiple facilities is gratefully
acknowledged from:
- Centro Austral de Investigaciones Cientificas y Tecnicas (CADIC)
- Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET)
- Museo Maritimo de Ushuaia
- Secretaria de Turismo de Tierra del Fuego
- Banco de Tierra del Fuego
- Antarpply Expeditions
- Municipalidad de Ushuaia
- Consejo Deliberante de la Ciudad de Ushuaia
For financial support to publish the proceedings volume, we would like to thank the Alfred
Wegener Institute for Polar and Marine Research, Bremerhaven, and the International Bureau
(IB) of the Federal Ministry of Education and Research (BMBF). Our sincere thanks go to the
Publishing Department of "Scientia Marina", particularly to Dr. Pere Abello, Ms. Victoria Mar
tinez de Albeniz and Mr. Jordi Corbera, who in a very competent and friendly way responded to
all our queries and patiently solved all editorial problems. We would also like to thank all those
colleagues, who have assisted us in the peer review process, and have substantially improved the
contributions. Final corrections were made, and the subject index prepared, during a stay of the
senior editor at the Instituto de la Patagonia of the Universidad de Magallanes in Punta Arenas,
where the first IBMANT meeting had been held in 1997.
Last but not least, we would like to express our gratitude to those staff members of AWI,
CADIC and UBA who provided a perfect organisation, a most enjoyable procedure and a sound
balance of this second IBMANT Conference: Daniel Aureliano, Andrea Bleyer, Claudia Boy,
Javier Calcagno, Jorge Calvo, Nahuel Erikson, Silvia Gigli, Marcelo Gutierrez, Vesna Karic,
Gabriela Malanga, Victoria Mattenet, Elba Morriconi, Patricia Perez-Barros, Andrea Raya-Rey,
Carolina Romero, Adrian Schiavini, Federico Tapella, Marcelo Turus and Fabian Vanella.
Wolf E. Arntz 1, Gustavo A. Lovrich 2, Sven Thatje 1
1 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
2 Centro Austral de Investigaciones Cientificas y Tecnicas, Ushuaia, Argentina

SCI.MAR.,69(Suppl.2):7-15 SCIENTIA MARINA ^
THE MAGELLAN-ANTARCTIC CONNECTION: LINKS AND FRONTIERS AT HIGH SOUTHERN LATITUDES.
W.E. ARNTZ, G.A. LOVRICH and S. THATJE (eds.)
Mesoscale eddies in the Subantarctic Front -
Southwest Atlantic*
PABLO D. GLORIOSO l, ALBERTO R. PIOLA2 and ROBERT R. LEBEN3
1 British Antarctic Survey, Cambridge, UK; present address: Av. Mila Reial 113 D4, Torre en Conill, Betera
Valencia 46117, Spain. E-mail: [email protected]
2Depto. Oceanografia, Servicio de Hidrografia Naval and Depto. Cs. de la Atmosfera y los Oceanos,
Universidad de Buenos Aires, Buenos Aires, Argentina.
3 Colorado Center for Astrodynamics Research, University of Colorado, Boulder, USA.
SUMMARY: Satellite and ship observations in the southern southwest Atlantic (SSWA) reveal an intense eddy field and
highlight the potential for using continuous real-time satellite altimetry to detect and monitor mesoscale phenomena with a
view to understanding the regional circulation. The examples presented suggest that mesoscale eddies are a dominant fea
ture of the circulation and play a fundamental role in the transport of properties along and across the Antarctic Circumpolar
Current (ACC). The main ocean current in the SSWA, the Falkland-Malvinas Current (FMC), exhibits numerous embedded
eddies south of 50°S which may contribute to the patchiness, transport and mixing of passive scalars by this strong, turbu
lent current. Large eddies associated with meanders are observed in the ACC fronts, some of them remaining stationary for
long periods. Two particular cases are examined using a satellite altimeter in combination with in situ observations, sug
gesting that cross-frontal eddy transport and strong meandering occur where the ACC flow intensifies along the sub-Antarc
tic Front (SAF) and the Southern ACC Front (SACCF).
Keywords: southwest Atlantic, mesoscale, eddies, transport, Falkland-Malvinas Current.
RESUMEN: EDDIES DE MESOESCALA EN EL FRENTE SUBANTARTICO, ATLANTICO SUDOESTE. - Observaciones in situ y sateli-
tales en el sudoeste del Oceano Atlantico Sur revelan un intenso campo de eddies e indican la utilidad de la altimetna sate-
lital para detectar, monitorear y mejorar la compresion de fenomenos de mesoescala en la region. Los ejemplos presentados
sugieren que los eddies de mesoescala son una caracten'stica dominante de la circulation y juegan un papel fundamental en
el transporte de propiedades a lo largo y a traves de la Corriente Circumpolar Antartica (CCA). Al sur de 50°S la principal
corriente en esta region, la Corriente de Malvinas exhibe numerosos eddies, los que pueden contribuir al patron de manchas,
y al transporte y la mezcla de trazadores pasivos de esta intensa y turbulenta corriente. En los frentes de la CCA se obser-
van grandes eddies asociados con meandros, algunos de estos eddies permanecen estacionarios durante largos periodos de
tiempo. Se analizan dos casos particulares empleando altimetna satelital en combination con observaciones in situ que
sugieren que donde la CCA se intensifica, a lo largo del Frente Subantartico y el Frente Sur de la CCA, se producen inten-
sos transportes a traves de la corriente y fuertes meandramientos.
Palabras clave: Atlantico suroccidental, mesoescala, eddies, transporte, Corriente de Falkland/Malvinas.
INTRODUCTION Remote sensing and satellite-tracked drifting buoys
have reinforced this view by resolving the broad-
The view of ocean currents as smooth, stream- band temporal and spatial variability associated with
lined flows in geostrophic balance has been replaced turbulent scales of motion (Stammer, 1997).
by the modern view of the ocean as a turbulent fluid. Mesoscale turbulence arising from baroclinic and
barotropic instability, wind forcing and topographic
*Rectived January i, 2004. Accepted June 21,2005. interactions influence the variability of ocean cur-
SOUTHWEST ATLANTIC MESOSCALE EDDIES 7
FIG. 1. - Topographic map of the southern southwest Atlantic (SSWA) depicting the Shag Rocks Passage (SRP). The red lines indicate the
mean position of the sub-Antarctic Front (SAF), the Polar Front (PF) and the Southern Antarctic Circumpolar Current Front (SACCF) as
described in Orsi et ah, 1995). The XBT transect consisted of 32 launches and was carried out on 21-22 Feb 2003. Transect T4 consisted of
CTD casts and XBT launches. XBT 17 and CTDs 6 and 7 in T4 are indicated by the red stars. The dashed line is the 1500 m isobath.
rents and generate eddies, fronts and jets, which are graphic signal in the SW South Atlantic, which is a
significant contributors in the transport of properties distinct biogeochemical province (Longhurst,
such as heat, momentum and nutrients. 1995). The FMC originates in the northern Drake
Mesoscale processes also have a large influence Passage as a branch of the ACC associated with the
on the development and diversity of biological SAF, and then flows northward, around the Burd-
species at different trophic levels, from phytoplank- wood Bank, steered by the edge of the Patagonian
ton blooms to the life cycle of whales (Davis et al, shelf (Piola and Gordon, 1989).
2002). For instance, according to recent findings Further south, the ACC tends to be concentrated
(Thatje and Fuentes, 2003), planktonic larvae from in frontal jets steered by the bottom topography
South America may have reached Antarctic waters (Nowlin et al, 1977; Pollard et al, 2002). The input
by an intrusion of Subantarctic warm-water rings of eastward momentum by the wind stress that gen
across the ACC fronts. Moreover, mesoscale phe erates the ACC is transferred vertically to the deep
nomena are crucial to our understanding of ocean ocean, where it is dissipated by bottom friction, and
circulation and climate change, and to a large extent hence the system is dynamically balanced (see Gent
may determine the location and variability of fish et al, 2001 and references therein). This transfer of
eries. Here we present evidence that mesoscale tur momentum is possible due to the generation of tran
bulence in the SSWA may play a substantial role in sient and standing eddies caused by the presence of
the transport of properties within and across major topographic barriers in the path of the ACC, and is a
currents of the ACC system, such as the Falkland- form of interfacial stress set up by the eddies
Malvinas Current (FMC). This evidence comes (McWilliams et al, 1978; Wolff et al, 1991).
from observations at sea and from remote sensing. Satellite altimetry shows that the SSWA is a
The main ocean current in the SSWA (Fig. 1) is region of enhanced eddy activity east of the Drake
the FMC, a cold stream linked to the Subantarctic Passage (Gille et al, 2000), where eddies might be
Front. To place the FMC in context, it carries about generated by topographic features partially blocking
40 to 70 Sv (1 Sv = 1x10s m3 s1, Peterson, 1992; the ACC (Wolff et al, 1991). The continental shelf
Vivier and Provost, 1999), or 200 times the outflow extending south from Tierra del Fuego, submarine
of the Amazon River, and in concert with the Patag- ridges and seamounts are probably inducing the rel
onian shelf tides, it constitutes the dominant oceano- atively high surface eddy kinetic energy observed
8 P.D. GLORIOSO et al.
downstream of Drake Passage in the northwest Sco OBSERVATIONS
tia Sea.
This paper addresses the need to characterise Extraordinary evidence of complex mesoscale
mesoscale eddies in the SSWA, given their signifi patterns in the SSWA comes from images taken by
cant influence in the regional circulation and their SeaWiFS (Sea-viewing Wide Field-of-view Sensor).
potential for the transport of properties. We present The example shown in Figure 2 illustrates complex
examples that indicate agreement between remote non-linear patterns of stretching and folding of
sensing and in situ observations, and point out the streaklines evolving around eddies and dipole vor
potential of using continuous real-time satellite tices, indicating that eddies indeed play a significant
altimetry for detecting, monitoring and improving part in the dynamics of the FMC. The FMC itself is
our understanding of mesoscale phenomena in a visible as a lighter band of colour along the shelf
remote region that is beyond the present coverage of slope to the east of the Falkland-Malvinas Islands.
microwave-derived sea surface temperature and is Note the counter-clockwise eddy of the same colour
often covered by clouds, which obscure visible and that appears to have detached from the current and
infrared imagery. drifted to the east. Notably, north of about 50°S the
-49-
"'.-
FIG. 2. - SeaWiFS true-colour image for 11 Nov 2001 of the Falkland-Malvinas Current (FMC), which flows along the shelf edge, offshore
from the Falkland-Malvinas Islands. The abundance of eddies is highlighted by the presence of phytoplankton. White areas on the top- and
bottom-left of the image are clouds. The white contours are the 200 m and 1500 m isobaths. Image provided by the SeaWiFS Project,
NASA/Goddard Space Flight Center, and Orbimage.
SOUTHWEST ATLANTIC MESOSCALE EDDIES 9
FMC appears as a very low eddy energy region every 15 nm, along a transect (see location in Fig. 1)
(Goni and Wainer, 2001), suggesting that the current that was designed to sample the underlying structure
stabilises downstream, presumably as it is steered by of eddies detected in satellite altimetry maps. In this
the steep bottom topography along the western edge figure, the relatively warm Antarctic Intermediate
of the Argentine Basin. Water (AAIW) close to the continental shelf to the
CTD (Conductivity-Temperature-Depth) data west is in sharp contrast with the colder Circumpo-
collected by the Fisheries Department of the Falk lar Deep Water (CDW, carried by the ACC) to the
land Islands on a crisscross grid across the FMC in east. Both water masses appear below a warmer sur
July 1999 gave a first indication of the presence of face layer due to seasonal heating.
eddies embedded in the current (Glorioso, 2002). This XBT section gives a fairly synoptic view of
The extent of these eddies down into the water col the eddies that were encountered. Eddy locations
umn has been described by performing a water mass were monitored with sea surface height (SSH)
analysis using the Optimum Multi-Parameter anomaly fields produced by blending altimeter data
(OMP) method (Tomczak, 1981). The results from, for example, the TOPEX/Poseidon and ERS-
obtained from this analysis have indicated that the 2 satellites (Leben et ah, 2002) to accurately map
observed FMC eddies reached the full water depth the ocean mesoscale variability. Satellite altimeters
(more than 1000 m) and upwelled deep water to the measure changes in SSH due to the redistribution of
surface layer of the ocean. Thus, the FMC eddies the water column integrated mass and changes in
may play a significant role in the vertical redistribu water density, i.e. the barotropic and baroclinic com
tion of properties, supplying nutrients and other sub ponents.
stances to the upper ocean. XBT positions 9 to 15 covered the Shag Rocks
Figure 3 presents expendable bathythermograph Passage (SRP in Fig. 1), transecting the mean path
(XBT) data taken on 21-22 Feb 2003 at 32 positions of the Polar Front (PF). In the vertical temperature
T(°C)
,32 31 30 29 28 27 26 25 24 23 22 21 2019 18 17 16151413 1211 10 9
55 54 53 52 51 50 49 48 47 46 45
longitude (°W)
FIG. 3. - XBT temperature section (see location in Fig. 1 and text for details).
10 P.D. GLORIOSO et al.