aoml south atlantic moc related observations and plans

NOAA/CPO Climate Observation Division 6th Annual System Review, September 3-5, 2008

AOML South Atlantic MOC related Observations and Plans

Silvia L. Garzoli

• Molly O. Baringer (AOML)• Christopher S. Meinen (AOML)• Carlisle Thacker (AOML)• Shenfu Dong (CIMAS)

•Zulema Garraffo (RSMAS)•George Halliwell (RSMAS)•Ricardo Matano (OSU)

•Alberto Piola and Ariel Troisi (SHN)•Edmo Campos (USP)•Mauricio Mata (URGDS)•Sabrina Speich (LPO,UBO)


•Heat transport across 35°S•SAM: Moorings at 35°S•Model data experiments

Outline


Map of the South Atlantic and Southern Ocean, including the two principal choke point regions, the Drake Passage and south of South Africa, with the current and proposed locations of instrument deployments and the institutes leading the corresponding associated projects.

SAMOC Workshop, Estancia San Ceferino, Buenos Aires Argentina, May 8, 9, and 10, 2007


(Barreiro, 2007)

High density XBT 20 lines conducted by AOML since July 2002

AOML High density XBT AX18 line

Correlation of MOC and heat transport across 33°S (C.I.=0.2)


(Lumpkin and Garzoli, 2008)(Goni et al., 2008)

Mean surface circulation in the

region


Heat TransportMethodology(Baringer and Garzoli, 2007)

• XBT data is collected using Sippican T-7 probes typically to depths of about 800 meters• Data is extended to the ocean bottom using Levitus 0.25° data set.• Salinity is estimated for each XBT profile by using S (T, P, Lat, Long) derived from Argo

and CTD data (Thacker 2006).• Meridional Ekman transports are computed as the Ekman mass (My) and Ekman heat

(Hy) transport using NCEP daily reanalysis winds and interpolating the daily NCEP values to the time and location of the XBT observation.

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My = − τ x

ρfΔx

Hy = Myc pTEK

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V = v dx dz∫∫ Sv =106 m3 /s[ ]

€

M = ρ v dx∫∫ dz Kg /s[ ]

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H = ρ c p∫∫ θ v dx dz PW =1015Watts[ ]


Mean Heat transport (14 realizations CT-BA) = 0.53 PW Std = ±0.11 PW (Garzoli and Baringer, 2007)

Mean Heat transport (3 transect CT-R) = 0.54 PW Std= ± 0.10 PW.

Mean Heat transport (20 realizations) = 0.53 PW Std = ± 0.12 PW

Total, Ekman and Geostrophic components of the Heat transport across the AX18 lines.


Time series of the Ekman Heat transport integrated across the basin as a function of latitude. Dots indicate mean latitude of each cruise.


Annual cycle of the Ekman component, geostrophic component, and total heat transport across the AX18 20 realizations. Results from the 3 lines occupied from Cape Town to Rio are shown in a different color (yellow).


1990 1995 2000 2005

-0.4

0

0.4

0.8

1.2

35ºS Meridional Heat Flux (PW)

ORCA 30ºS

1990 1995 2000 2005

-0.4

0

0.4

0.8

1.2


ECCO 35ºS

ORCA 30ºS

1990 1995 2000 2005

-0.4

0

0.4

0.8

1.2


ECCO 35ºS

ORCA 30ºS

AX18 35ºS

Meridional heat flux – model comparisons

0.53 ±0.11 PW

(Garzoli and Baringer, 2007)

(Piola, 2007)

Comparison with other results


Mean model velocities at 1500 m depth

One of the largest uncertainties in the measured heat transport is the lack of direct measurements of the barotropic component of the flow, which is largest to the west of 47°W. This is particularly important because at the western boundary the Malvinas Current and the DWBC both flow in the same direction, creating a strong barotropic flow whose magnitude and variability are poorly known.

Model sections of the meridional velocity showing the DWBC at 30°S (left) and 34.5°S (right). Negative velocities indicate southward flow. (From POCM model, Tokmakian and Challenor, 1999).

Observing the DWBC


Proposed cruise track and tentative instrument locations for the new IES line in the South Atlantic. This program will be conducted in collaboration with scientists from Argentina and Brazil.

In conjunction with the CPIES deployed by Sabrina Speich (Fr)

Positions of the first stations of the BONUS-GOODHOPE transect. In green the two stations where the two C-PIES moorings have been deployed.

13 14 15 16 17 18 19

- 32

- 33

- 34

- 35

SAM


Can we do it?A combination of CTD with IES (using the GEM technique) and pressure gauges proved to be an adequate mean to monitor the transport of the Deep Western Boundary current in the North Atlantic across 26.5°N .

Comparison of the DWBC transport at 26.5°N estimated from three different techniques. All transports are integrated between 1200 and 4800 dbar. The same bottom pressure gauge data is used to provide the bottom absolute velocity reference for both the IES and the Dynamic height moorings. Note that the magenta and green lines switch to dotted in late January when the top of one mooring broke off and the remaining segment of the mooring slumped down due to the loss of buoyancy; current meter and dynamic height mooring data after this point should be viewed as having larger error bars.

(Meinen et al., 2004)


Pop up system

SAM first cruise October 08Deployed 4-5 yearsBiannual visits to recover the data


• Characterize the mean and time varying pathways of the AMOC• Evaluate the correlation between the AMOC strength and the meridional Heat Transport.• Defining the importance of variations in inter-ocean and inter-basin exchange and the connectivity of

the MOC.

AX22 1996 to presentAX18 2002 to presentAX25 2004 to present

AMOC variability in the South Atlantic Data analysis and a numerical model simulation

Comparison transports from XBT and altimeter


Global 1/12º HYCOM Climatological simulation.

Produced at NRL (J.Metzger, J. Shriver, A. Wallcraft, E. Chassignet, H. Hurlburt). ECMWF ERA40 forcing plus 6 hourly wind anomalies from a repeat year of NOGAPS winds.

A South and Tropical Atlantic regional model will be nested inside the Global model. In this way better accuracy can be achieved for:

• Trajectories of numerical drifters and floats

• Transports and diapycnal flux diagnostics.

Global SSH in the Regional model domain

(From Garraffo,2008)

daily boundary conditions from the global model


Particles will be launched in the regional model, in a 1x1 regular grid at several depths, following the model 3-D motion, plus at 0.25 deg in transects near the inflow into the domain and selected locations (yellow rectangles) e.g., DWBC, NADW.

Subsurface flow (mean model year 15, ~100m)

TG

STG

WG

NBCEU

MC

BCAC

Deep flow (mean of model year 15, ~3000m)

(From Garraffo, 2008)


Objective: To estimate the effectiveness of strategies for monitoring the overturning

circulation.

Technique: • Simulate observations and then evaluate how uncertainties associated with the

overturning circulation are reduced when they are assimilated into an ocean model.

• As accurate quantitative characterization of model errors and their correlations and of the noise characteristics of the simulated data strongly influence the results of an OSSE, these issues will receive considerable attention.

Methodology:• The system will be calibrated by simulating data similar to those from the

RAPID/MOCHA program and seeing whether they have an impact similar to the real data.

• The impact of similar data from other sections, e.g. near 30S, will be evaluated.• Once the system is working, the value of other types of observations can also be

considered, e.g., extending the depth range of Argo profiling floats.

"Observing System Simulation Experiments for the AMOC”

Carlisle Thacker (NOAA/AOML), George Halliwell (RSMAS/UM)


Thank you

aoml south atlantic moc related observations and plans

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