Agulhas Return Current Reference Station Vetting Page

Agulhas Return Current Reference Station
Vetting Page

The ARC mooring was deployed 30 November 2010 at 30E, 38.5S and broke away from its anchor on 16 January 2011. A new location needs to be found that will be easier for the mooring to survive. This vetting webpage compiles information provided by the community to help us find the optimal location for the mooring. Items flagged NEW! indicate items posted after Jan 2011.


The Agulhas Return Current (ARC) reference station was proposed through the OceanObs09 Community White Paper (See Poster):

"Monitoring ocean-atmosphere interactions in western boundary current extensions"
by: M.F. Cronin, N. Bond, J. Booth, H. Ichikawa, T. M. Joyce, K. Kelly, M. Kubota, B. Qiu, C. Reason, M. Rouault, C. Sabine, T. Saino, J. Small, T. Suga, L. D. Talley, L. Thompson, and R. A. Weller.
This OceanObs09 CWP was proposed by the U. S. CLIVAR Working Group on Atmosphere Interactions in Western Boundary Currents and vetted at an open workshop, organized by the working group and held in Phoenix AZ in January 2009.

The plan to initiate an ARC reference station was subsequently endorsed by the U.S. CLIVAR Working Group on High Latitude Fluxes, the SCOR/WCRP/IAPSO WG 136 on Climatic Importance of the Greater Agulhas System, and the OceanSITES panel.

The purpose of the ARC mooring would be to provide a reference time series for assessing model and satellite products; for detecting rapid, episodic and long term changes in the climate and ecosystems; and for identifying mechanisms and relationships within the climate and ecosystems.

Strategy: In order to expedite the launch of this site and reduce its risks, the mooring would have two phases.
During Phase 1, the ARC mooring would include a full suite of surface flux and engineering sensors, but minimal subsurface sensors.
During Phase 2, the mooring would be enhanced with subsurface physical and biogeochemical sensors.

The mooring was deployed at 30E, 38.5S on 30 November 2010, but broke away from its anchor on 16 January 2011. Diagnostics on the recovered mooring line still need to be performed. However it appears that the currents at 30E, 38.5S became too strong for the mooring due to an early retroflection that occurred in mid-January 2011.

Optimal location vetting:

The optimal location of the ARC site depends upon many factors including:

  • the presence of intense air-sea interaction, and particularly the presence of a large surface heat flux signal,
  • the ability of the surface mooring to survive (maximum upper ocean currents should be less than 2.0 m/s and preferably less than 1.6 m/s, deep current should be less than 0.3 m/s, especially if combined with a strong upper ocean current),

  • partnerships, and

  • logistics and feasibility of servicing the site.
    • The mooring could be serviced by the FRS Algoa through partnership with ASCLME.

    Pros and Cons of different locations:

    Retroflection region
    Pros: The retroflection has some of the large surface heat losses in the entire region. It is the gateway to the South Atlantic and extremely dynamic.
    Cons: It is not clear that a surface mooring could survive the energetic currents found here. For this reason, this region won't be considered further at this time.

    55E, 42S (proposed site in original CWP)
    Pros: This region has some of the strongest heat fluxes outside of the retroflection region. The currents are likely to be much reduced from those found further west, increasing survivability.
    Cons: This site is far to the east of most activities and thus is less connected with partnerships in the Greater Agulhas System. It would also be much more difficult to service.

    37E, 38S (proposed site in revised CWP)
    Pros: This region has some of strong heat flux (although not the strongest in the region). This site can be considered as an outer boundary for the various observing arrays in the region (e.g. ASCLME, ACT, SAMOC). The site is within the (warm) trough of the second meander in the mean field currents. As such, it will tend to have lower currents and stronger heat loss.
    Cons: The location is within the meander envelope, and therefore there is the potential that the jet will "hit" the mooring and that the mooring will find itself on the cold side of the front where heat fluxes are much reduced. The KEO mooring has survived meanders that have shifted over its site. The ARC site however may have more energetic deep flow. We are searching for more information abou tthe deep flow in this region.

  • NEW!30E, 38.5S (deployed 30Nov10, broke 16Jan11)
    Pros: This site is just east of the Agulhas Plateau, in the trough of the first meander. The plateau causes the first meander to be very stable so that the site is in fact outside the meander envelope for the 3-years of data used to create this figure, according to the Boebel et al. DSR 2003 study. It thus has the lowest probability of experiencing extremely strong jet conditions. The heat fluxes are stronger than they are downstream, at least until the Crozet Basin. This site would monitor the (exit?) boundary of the ASCLME. Likewise, the site is well connected with the ACT array and SAMOC line. The site is the closest to South Africa considered here and would have the shortest transit.
    Cons: The mooring line broke after a little over a month of being deployed. Although the Boebel analysis showed a standing meander, this was based on 3 years of data. Subsequent analyses have shown that the Agulhas Current can occasionally retroflect for short periods, as it did in 2000, 2008, and 2011 (deRuijter pers. comm.). Deep currents can occasionally (during these events?) be greater than 30 cm/s. Due to this interannual variability, this is not a good site. Also, there may be a trade off between the low temporal variability at this location and potentially very large spatial variability (which can be difficult to decipher with a single mooring). The stability of the front may enable a very localized ecosystem to develop in this trough (any supporting studies of this speculation?). It should be noted that while the oceanography may have less temporal variability, the atmosphere will be dominated by synoptic storms with significant temporal variability.

  • NEW!34E, 36S (Perhaps a better site? -- No)
    Pros: Maximum deep currents are relatively weak at this site according to the AG01 30-year model run and the OFES models. Net surface heat flux is out of the ocean during winter (although into the ocean during summer) and weakly out of the water in the mean. Bathymetry is relatively shallow ~3500m. It is in the ASCLME region.
    Cons: HYCOM shows very strong bottom currents for this region that we would not be able to survive. It is on somewhat of a plateau, so that if it walks, it may go into deeper water. The heat fluxes are into the water during summer (though out of water during winter). Further work is needed to assess interannual variability in the deep currents.

  • NEW!37.5E, 36.5S (Perhaps a better site?)
    Pros: Maximum deep currents are weak at this site according to the INALT01 30-year model run and the OFES model. Net surface heat flux is out of the ocean during winter (although into the ocean during summer) and weakly out of the water in the mean. Bathymetry is relatively flat. It is in the still in ASCLME region? Heat fluxes are out of ocean in the mean.
    Cons: HYCOM shows strong bottom and mid-depth currents for this region that might be difficult to survive if combined with waves. It is on somewhat deep (~5000m). Starting to get far from shore? Are we still in ASCLME? Would Algoa be able to serve this site?

    Links to groups and projects working in this region:

  • ASCLME project: Agulhas and Somali Current Large Marine Ecosystems project.
  • MAHYVA project: Multi-disciplinary Analysis of Hydro-climatic Variability at the catchment scale project.

    Please send Meghan Cronin figures illustrating these conditions (signal, survivability, partnerships, logistics, etc.) and other links for posting on this website.

    Contributors: Meghan Cronin, Mathieu Rouault, Lisa Beal, Kevin Speer, Johann Lutjeharms, David Vousden, Sabrina Speich, Jonathan Durgadoo, Arne Biastoch, Tomoki Tozuka, Will deRuijter.

    Meghan F. Cronin
    Pacific Marine Environmental Laboratory
    7600 Sand Point Way NE
    Seattle, WA 98115 USA
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