The 4th iAnZone Program: Convection

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...


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