FUTURE PROJECTS - GOODHOPE

1. Strong cyclonic eddy activity CURRENT PROJECTS – AGULHAS CURRENT SYSTEM Inter-ocean exchange of heat and salt around southern Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It is currently thought to exist of three major parts: the advection of Agulhas filaments, the shedding of Agulhas rings and the transport of Intermediate water between the South Indian and the South Atlantic Oceans. Most of the Indian Ocean leakage into the South Atlantic takes place near the Agulhas Retroflection region where large current rings pinch off and translate into the South Atlantic. Mixture of cyclonic and anticyclonic eddy activity Fig. 1: A schematic figure showing the general circulation south of South Africa Fig. 2: Snapshot of surface temperature and currents from Biastoch and Krauss (1999) regional model which is currently being run at UCT. The snapshot clearly shows the Agulhas Current system, Subtropical Convergence and the advection of Agulhas rings into the South Atlantic. ARGO floats deployed in the retroflection region clearly show the generation of Agulhas rings (Fig. 3). These Agulhas rings result in the leakage of warm, salty subtropical water masses into the South Atlantic. Present estimates suggest that 6 rings form per year with volumes ranging between 0.5 – 1.5 Sv per ring. Estimates of total heat fluxes involved in the inter-ocean exchange range between 0.1 and 0.8 PW. Salt fluxes range between 0.9 x 105 and 38 x 105 kg/s assuming an average of 6 rings are shed annually. Observations have shown that ~30% of rings remain within the Cape Basin, while the remainder are able to propagate a westerly path towards the western boundary of the South Atlantic subtropical gyre. Fig 3: Observation of an Agulhas ring from an ARGO float. Temperature and salinity sections clearly show the subtropical characteristics of such a ring. (taken from S. Garzoli) - DYNAMICS OF EDDY IMPACTS ON MARION’S ECOSYSTEM (2002 - 2004) Mesoscale eddies and meanders are one of the dominant sources of flow variability in the ocean. Regions of high mesoscale variability correlate closely with either the terminal region of a major western boundary current such as the Agulhas Current or the Gulf Stream, or where the Antarctic Circumpolar Current (ACC) interacts with prominent bottom topography such as in the Drake Passage, or the Crozet and Kerguelen Plateaux in the Southern Ocean (Fig.4). Altimetry and hydrographic data in the vicinity of the South-West Indian Ridge have identified this as another region of high variability, which extends eastwards from 30ºE towards the Prince Edward Islands at 37ºE. Hydrographic data collected during the SWINDEX surveys in 1993 and 1995 has shown that close to the South-West Indian Ridge the ACC fragments into several branches possibly resulting in enhanced eddy generation in the lee of the ridge. Fig. 5: Intense eddy activity immediately downstream of the South-West Indian Ridge Fig. 4: Southern Ocean Eddy Kinetic Energy (Gille et al., 2000) The unusually high variability in flow near the South-West Indian Ridge seems to be a direct result of an intensification of the ACCand convergence of frontal zones as the ACC is steered through a narrow fracture zone at 30° E. As a consequence a large number of both anticyclonic and cyclonic eddies are formed in the lee of the ridge and advect downstream into the vicinity of the Prince Edward Islands (Fig. 5). An ARGO float was deployed within the survey region during DEIMEC II (April 2003). Surprisingly, mesoscale features observed from drifter data (Fig. 5) are not apparent in the ARGO data suggesting that these features are shallower than the ARGO parking depth of 2000 m (Fig. 6). Of particular interest is the intrusion of cold fresh water masses close to the Prince Edward Islands, suggesting the presence of AASW as a result of either the meandering nature of the APF or the advection of a cold cyclonic eddy north of the APF (Fig. 6). Further supporting claims of high dynamic variability in this region and the need for further ARGO deployments within the vicinity of the South-West Indian Ridge Annual Cruise Opportunities. THE GOODHOPE TEAM Fig. 7: Annual re-supply voyages to the South African base stations on Gough Island (Yellow, August – September) Marion Island (Green, April – May), SANAE (Red, December – January) and South Sandwich Islands drifter deployments (Red box, February) and GOODHOPE (Blue dash, February – March) Fig. 6: Trajectory of an ARGO float deployed during April 2003 and its associated temperature and salinity sections. Monitoring the ocean's south of Africa - the need for ARGOI.J.Ansorge1, S.Speich1,2, C. J. Reason1, J.R.E. Lutjeharms11Oceanography Department, University of Cape Town, Rondebosch, 7701, South Africa2 LPO/UBO University of Brest, Brest, CEDEX, France FUTURE PROJECTS - GOODHOPE While the Southern Ocean dynamics is suspected to have a major role in the global ocean circulation and present day climate, our understanding of its three-dimensional dynamics, variability, and the impact of such variability on the climate system, is rudimentary. The GoodHope project aims to partially fill in this knowledge gap by periodic observations along a line between the African and Antarctic continents (Fig. 8). The objectives are fourfold: • A better understanding of Indo-Atlantic interocean exchanges (in terms of water masses, heat and fresh water budgets) and their impact on the global thermohaline circulation and present day climate. • A better understanding of the impact of interocean exchanges on the local climate of the African continent. • A monitoring of the variability of particular dynamical features of the Southern Ocean (Antarctic Circumpolar Current, frontal systems) • A study of the local air-sea heat exchanges and their role on the global heat budget (with emphasis on the intense exchanges in the Agulhas Retroflection region). Fig. 8: Map showing the GOODHOPE monitoring line between Cape Town and Neumayer station. The same line was occupied during WOCE (SR2). Triangles show the launching positions of the Coriolis profilers, dots those of the WECCON moorings. SCIENTIFIC RATIONALE • The Southern Ocean plays a unique role in coupling the ocean to the atmosphere and cryosphere. Variations in the mechanisms responsible for this coupling are expected to influence global climate variability. • The most relevant processes are: • The Antarctic Circumpolar Current (the only current that connects all three major ocean basins, thereby providing an essential heat and fresh water pathway). • The production of Antarctic Intermediate Water and Subantarctic Mode Water. They spread northward injecting cool low salinity water into and along the base of the main thermocline helping close the hydrological cycle. • Upwelling of Circumpolar Deep Water poleward of the Antarctic Circumpolar Current provides the site for major venting of deep oceanic heat into the atmosphere. • The production of cold dense Antarctic Bottom Water. • The large-scale coherent variability of the atmospheric circulation over the Southern Ocean and the mechanisms of these variations are directly involved in the propagation of anomalies across various climate zones (e.g., ACW and AAO). Fig. 9: The Southern Ocean observational “gap” : (a) Position of deployment of all profiling floats as it is for January 2003; (b) XBTs lines accomplished during the year 1998. The state of observations and modelling of the Southern Ocean is not as developed as in other regions of the ocean and atmosphere. While major achievements were made during the WOCE/JGOFs era, we still only have a very incomplete “glimpse” of the mean state and variability of the Southern Ocean, its coupling with the atmosphere and cryosphere, and the zonal and meridional fluxes. Observations are dramatically incomplete in space and time (Fig. 9), consequently, further emphasis on exploratory investigations in the Southern Ocean needs to be placed than in better-sampled ocean basins. b a MONITORING STRATEGY – AFRICA-ANTARCTICA SECTION • The advantages of the GoodHope-SR2 line from Cape Town to the German Antarctic Base station (Fig. 8) are: • It follows the TOPEX/POSEIDON-JASON1 altimeters flight path. • The southern fraction of this line (south of 50ºS) has already been sampled for several years by moorings of the German WECCON project (Fig. 10). • Its northern part overlaps with the USA ASTTEX programme (Fig. 10), thus linking the Southern Ocean dataset with that collected in the Benguela region. • Two PIES moorings have already been deployed along this line. The data collected during the monitoring programme will support the PIES data set. Two more PIES moorings are envisaged. • The monitoring line lies close to the annual "ferry service" of the SA Agulhas from Cape Town to the German Antarctic base Neumayer. No more than an extra day will be required to accommodate this line. Fig. 10: Location of WECCON moorings and ASTTEX array.