The goal of iAnZone is: through development
and coordination of observational and modeling programs, to advance
our quantitative knowledge and modeling capability of climate
relevant processes, their seasonal cycle, their inter-annual and
decadal variability, within the Southern Ocean's Antarctic Zone
(region poleward of the Antarctic Circumpolar Current), with emphasis
ocean and atmosphere coupling in regions influenced by sea ice,
and to the link between the Antarctic Zone and the global ocean
and climate system.
There have been a number of national programs that address these
goals, and the iAnZone banner is associated with several
national expeditions into the Weddell Gyre during the 1980s &
1990s. However, there have been three coordinated, multi-investigator,
international iAnZone Weddell Gyre programs which were
explicitly developed at the previous iAnZone meetings.
Ice Station Weddell (ISW): The first
iAnZone program was ISW in 1992, involving US, Russian
and Finland contributions. The ISW project explored the western
rim of the Weddell Sea, investigating its contributions to newly
formed deep and bottom layers. A description of the program was
presented in an AGU EOS article (ISW Group, 1993). The German
Alfred Wegener Institution (AWI) Antarctic program obtained a
Polarstern section in the western rim of the Weddell in 1993 (Fahrbach
et al. 1995) is considered a valuable extension of 1992 ISW observations.
The German WOCE current meter monitoring program (Fahrbach et
al, 1994) coupled with the ISW observations have greatly increased
our understanding of the forms and vigor of the Weddell Sea ventilation
processes.
AnzFlux
: The second iAnZone program
was AnzFlux in 1994. AnzFlux, involving US and Germany contributions,
obtained a more quantitative understanding of the surface and
deep water interactions in the region of Maud Rise. AnzFlux gave
a new appreciation of the complex and highly variable thermohaline
fluxes within the upper ocean, and its coupling to the sea ice
cover and atmosphere. This program is described by McPhee et al.
1996 and in the iAnZone Biosphere Report.
Dovetail: The third iAnZone program
is underway, called Dovetail, in 1997-1998, which includes US,
Spain and Germany contributions. Dovetail's objective is to measure
the integrated outflow of ventilated water masses from the Weddell
Sea. Description of Dovetail is given in two Antarctic Journal
articles (in press), copies of which may be obtained from the
authors: Muench, 1997 and Gordon et al. 1997, and is further described
in the iAnZone Biosphere Report.
The first three iAnZone programs involved just a few nations
and occurred in the Weddell Gyre. With the enlarged scope of iAnZone
resulting from the SCOR affiliation, it was agreed that
an iAnZone program requires broader involvement, and must consider
the full circumpolar Antarctic Zone. A theme for iAnZone
experiment #4 that fully addresses the iAnZone goals,
that can be met at a variety of sites around Antarctica, and for
which there is wide interest deals with the convection,
a thrust which is fully endorsed within the southern ocean component
(D5) of the CLIVAR draft Implementation Plan. A preliminary name
for iAnZone Exp. #4 is Convection.
While much research has
been directed at the formation and spreading of Antarctic Bottom
Water (AABW) in all of its varied forms, we still do not have
a firm assessment of the total, circumpolar production of AABW,
and even less of an understanding of varied processes that sustain
production.
The Objective of Convection are:
To obtain a quantitative estimate of the effect of Antarctic
zone water mass modification on the global thermohaline circulation
and to establish the basis of an observational system which allows
to improve and validate the representation of Southern Ocean convection
in large scale models.
It is anticipated that there will be a series of Convection
field programs during the period 2000 to 2004 that address these
components. Field operations may occur at the same time, but difference
sectors of the Antarctic Zone. To quality as an iAnZone program
the field program must involve more than one national program
and be subject to discussion at iAnZone meetings. Three
components of Convection are considered (Fig. 1), which
involve furthering aspects of the previous three iAnZone
experiments:
1. Continental Margin Convection: This component is directed
at the formation of the ëparentí shelf water masses
contributing to deep reaching convection, mixing within the continental
margin frontal structures, and the plume convection feeding the
deep and bottom water ventilation. This component includes:
a- shelf waters coupling to atmospheric and sea ice (and coastal
polynya) forcing as well as shelf water interaction with the ice
shelves, ice bergs and ice tongues;
b- the variety mixing processes, including coastal waves and tidal
action, and equation of state subtleties and their associations
with bottom topography and slope circulation and fronts that enable
deep reaching convection; and
c- the dynamics and entertainment characteristics of the convective
plumes characteristic of many continental slopes of Antarctica.
Regions of potential field measurements are Weddell Sea, Ross
Sea, Prydz Bay, Adelie coast. What are the common threads of these
regions that make them sites of slope convection?
2. Open Ocean Convection: Of interest in this component
are the vertical transfer processes within the Antarctic Zone
thermohaline stratification and their response to wind, buoyancy
forcing and polynyas. The perturbation of the regional stratification
and vertical fluxes by the processes imposed by topography, such
as Maud Rise.
The objectives of the project are:
a - to determine the time scales and intensity of the variability
in characteristics and amount of the inflowing Circumpolar Deep
Water - to determine the time scales and intensity of the variations
of the atmospheric forcing in seasonal to interannual time scales
- to determine the time scales and intensity of the variation
in water mass properties
b- to determine the potential correlation of the variations of
water mass properties and ice or atmospheric forcing - to estimate
the effect of topographic features like Maud Rise to intensify
vertical transports
c- to determine the potential of remote and local effects to induce
variability in the atmosphere-ice-ocean interaction - to determine
the contribution of open convection the modify the source waters
of shelf processes
d- to estimate the contribution of open ocean convection in the
Antarctic zone to the ventilation of the global ocean
e- to estimate the potential of abrupt changes
A component of this component of Convection is the Weddell Polynya
Quick Response Program described elsewhere in the iAnZone Biosphere
Report.
3. Monitoring the convective products: Coupled to the two
process oriented components of Convection is the establishment
of a time series measurement program of the outflow of deep and
bottom water masses formed within the Antarctic Zone which ventilate
the global ocean.
This component includes:
a- Defining the primary pathways and processes that drain the
products of cold Antarctic zone water masses;
b- Measurement of the thermohaline and volume flux variations
of the outflow of Antarctic Deep and Bottom Water at key sites
around Antarctica;
Inspection of bottom water characteristics can be used to identify
a few key regions to establish a time series. In consideration
of the Rossby deformation scales at high latitude and of the small
residence time of newly formed water masses in the western Weddell
Sea and elsewhere, the time series array must have appropriate
spatial and temporal resolution. The iAnZone and previous
research has shown that the northwest Weddell Sea topographic
features make it is a key site for monitoring of the outflow of
Weddell Sea convective products. Other sites have similar topographic
features that confine the outflow thus enabling monitoring strategies.
What Next?
During the course of 1998 various working groups set up at
the meeting will develop more details (via e/mail). They should
provide: a description of the key research issues and detail their
importance to the larger scale climate role of the Antarctic Zone;
and prepare concise objectives and activities that would contribute
to each of the Convection components. The development of iAnZone
Experiment #4 will be the primary topic of discussion at the next
(April or May 1999) iAnZone meeting.
The lead persons for each of the three components are as follows,
others will be asked for input. The reports will be made available
to the full iAnZone group
WG1: Continental Margin Convection. A. Gordon and S. Rintoul
(co-chairs), J. Klinck, J Morison, L. Padman, H. Hellmer, S. Jacobs,
more...
WG2: Open Ocean Convection: E. Fahrbach and M. McPhee (co-chairs),
J. Launiainen, M. Drinkwater, D. Martinson, Norway rep, more...
WG3: Monitoring the convective products: M. Visbeck and
N. Bindoff (co-chairs), R. Muench, W. Smethie, more...
References:
Fahrbach, E., G. Rohardt, M. Schroder, and V. Strass, Transport
and structure of the Weddell gyre, Ann. Geophys., 12, 840-855,
1994
Fahrbach, E., G. Rohardt, N. Scheele, M. Schroder, V. Strass,
and A. Wisotzki, Formation and discharge of deep and bottom water
in the northwestern Weddell Sea, Jour. Mar. Res., 53(4), 515-538,
1995.
Gordon, A. L., M. Visbeck and B. Huber Export of Weddell
Sea water along and over the South Scotia Ridge. Ant Jour of the
US, in press
ISW Group, Ice Station Weddell 1 Explores the Western Edge of
the Weddell Sea, Eos 74(11), 121, 124-126, 1993.
McPhee, M. S. Ackley, P. Guest, B. Huber, D. Martinson, J. Morison,
R. Muench, L. Padman, and T.Stanton, The Antarctic zone flux experiment,
Bull Amer. Met. Soc. 77, 1221-1232, 1996.
Muench, R.D. Deep ocean ventilation through Antarctic intermediate
layers: the DOVETAIL program. Ant Jour of the US, in press