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.
History:
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,
- NEW! A. Biastoch and J. Durgadoo: Net Surface Heat Flux time series used to force AG01, along
30E,
35E,
40E,
45E,
50E,
55E,
- NEW! Regional map of the mean net surface heat flux (using OAFlux product) (locations 30E,38.5S and 37.5E,36.5S are shown as grey squares)
- Global maps of the
mean net surface heat flux (using OAFlux product),
mean CO2 flux (using Takahashi et al. 2009), and
mean MJJAS Tskin - Tair and winds(NCEP1).
- Mean OAFLUX latent and sensible heat flux maps for
2001,
2002,
2003,
2004,
2005.
The locations (30E,38.5S; 35E,38S; and 55E,42S) are shown as grey squares.
- OAFLUX Qnet timeseries along:
29.5E,
30.5E,
34.5E,
36.5E,
37.5E, and
54.5E.
- OAFLUX Qlat timeseries along:
29.5E,
30.5E,
34.5E,
36.5E,
37.5E, and
54.5E.
- OAFLUX Qsen timeseries along:
29.5E,
30.5E,
34.5E,
36.5E,
37.5E, and
54.5E.
- M. Rouault:
HOAPS3 mean latent heat flux 1988-2005 with mean 1993-2006 geostrophic current,
mean AVHRR SST, and
AVHRR SST 1985-2007 decadal trend and mean 1993-2007 geostrophic currents.
- SAMW is formed quite a bit further east. See Sallee et al Oc. Dyn 2006:
Map of Wintertime MLD from Argo, and
Longitudinal distribution of Heat Budget terms between STF and SAF.
- Lutjeharms and Ansorge J. Mar. Sys. 2001:
Location of ARC and Subtropical Front from Discovery 164 cruise section (~40E) and
Volume transport of ARC along its full length.
- S. Speich: Southern Ocean
Mean wind stress,
Mean wind speed,
Intensity of mean surface geostrophic velocity, and
Intensity of the instantaneous (29 11 2006) surface geostrophic velocity.
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),
- NEW! A. Biastoch and J. Durgadoo: Maximum current from AG01 (30-years of 5-day averages), along
30E,
32E,
34E,
36E,
38E,
40E,
55E.
- NEW! T. Tozuka: Maximum currents from
OFES 3-day output
- NEW! T. Tozuka: Mean currents from
OFES (1980-2006) at:
2.5m, 527m, 1041m, and
2119m, 3114m, and 4287m
- NEW! T. Tozuka: Maximum currents from
OFES (1980-2006) at:
2.5m, 527m, 1041m, and
2119m, 3114m, and 4287m
- NEW! T. Tozuka: current histograms from
OFES 1980-2006 3-day output, along 30E,
32E,
34E,
36E,
38E, and
40E.
- NEW! T. Tozuka: intercomparison with WHOI mooring current meters at 38S, 15E from WOCE current meter database:
- NEW! HYCOM surface and bottom currents at:
38E 35S,
37.5E 36.5S,
38E 38S.
- NEW! HYCOM Very strong mid-depth northward jet or eddy
at 37.3E 36.5S
- NEW! HYCOM surface currents intercomparison with
ARC. Note that ARC started hopping on 10 Jan 2011.
- Smith and Sandwell v8.2
bottom topography. In region near:
30E,
35E and
55E.
NEW! Smith and Sandwell v8.2 details near 37.5E 36.5S,
38E 35S, and
38E 38S
Smith and Sandwell v8.2 details near 30E: +/- 1 degree,
+/- 0.5 degree.
- M. Rouault:
mean 1993-2006 geostrophic currents. Note that these altimetric surface currents are a factor of 2 too weak. Thus the maximum scale should be 1.6 m/s, rather than 0.8 m/s.
- M. Rouault: SAR radial currents on
10-Sep-09 21:15UTC,
25-Aug-09 7:23UTC,
22-Aug-09 7:17UTC,
22-Aug-09 7:17UTC,
3-Aug-09 7:15UTC,
- CLS Soprano SAR Ocean Products Demonstration: Mean radial surface currents from 329 ascending Envisat ASAR WSM acquisitions.
- Lutjeharms and Ansorge J. Mar. Sys. 2001:
geographic velocities and transport across sections
central and terminal regions of ARC
Volume transport of ARC along its full length.
- Park et al. JGR 1993: "Water mass and circulation in the Crozet Basin":
- BoebeletalDSRII 2003: "Path and variability of Agulhas Return Current":
- Durgadoo et al. Antarctic Science 2010:
RMS sea level anomaly map.
- Sallee et al. OcDyn 2006:
Map of Average velocity at 400m depth deduced from ARGO and PALACE floats.
- K. Speer: LADCP profile at
30E 38.5W on 12Feb08
- I6S LADCP
LDEO website
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.