SMOC Brazil 2010

Monitoring the Formation Rate of NADW Components Using Tracer Inventories

Rana A. FineRosenstiel School, University of

Miami

Studies show little to no meridional coherence in MOC across gyre boundaries (Dengler et al., Nature, 2004; Bingham et al. GRL, 2007). South Atlantic is a majorregion for water mass transformation for the compensating MOC flows. Proposing to use CFCs & SF6 to look at coherence for AAIW and NADW between their source regions and the southwestern Atlantic.

We are now routinely measuring both CFCs and SF6.

Even though CFCs decreasing in atm, presently get excellent ratio age using SF6:CFC.Data from ALE/GAGE network

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1950 1960 1970 1980 1990 2000C

FC

-11:

CF

C-1

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SF

6 :CF

C-11, S

F6 :C

FC

-12 [x 100]

Year

CFC-11:CFC-12

SF6:CFC-11

SF6:CFC-12

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1950 1960 1970 1980 1990 2000

CF

C-1

1, C

FC

-12

[ppt

]

SF

6 [ppt]

Year

CFC-12

CFC-11

SF6

Atmospheric time histories of CFCs, SF6

Total amount of CFCs and SF6 present in subsurface water mass is directly related to the

rate of its conversion from surface water.

Highest CFC inv (and CO2) due to MOC: downstream of NADW formation,NADW is ~50% of Atlantic inventory to 10°S; other high (40-60°S) is related to formation of Mode and Antarctic Intermediate WaterWilley, Fine, et al., GRL, 2004

NADW1994

AAIW

CFC-11 inventory in AAIW from data: Maxima in SE Pacific and SW Atlantic

(Lachkar et al. DSR 2009)

1997-98

DWBC

24.5N

60N

NADW component of MOC

Smethie et al., AGU Monogr., 2007

Maximum CFC-11 in DWBC. Build up over time showsCLSW spreads from its source to subtropics in ~10 years,after spending time in recirculation gyres and eddies.Updated from Molinari, Fine et al., GRL, 1998

NADW:

ULSWCLSW

MNADW/ISOW

LNADW/DSOW

CFC ages: giving information about storage andeffective spreading rates of MOC componentsFine et al., GRL, 2002

UNADW LNADW

Brazil Basin shows highest CFCs in NADWalong western boundary (& Bottom Water), alsopatchiness of elevated CFCs in interior. (No CFCs available from 2003 section.)

Proposing to measure CFCs & SF6every other year on CTD cruises servicingIES etc over western boundary andwhen possible entire Brazil Basin. Is the DWBC re-constituted again at 30S?What is the relationship of western boundary NADW in Brazil Basin to upstream?

1993 Section along 30°S

Monitoring using Monitoring using CFCs & SF6:: • Independent tracers that are sensitive indicators to be used to

quantify formation rates and variations in the NADW and SAMW/AAIW components.

• Provide information on coherence between high latitude source regions and regions downstream- via western boundary and interior pathways.

• Though the total rate of formation of NADW may not have changed in past few decades, there maybe changes in its individual components.

Decreases in production of CLSW may be offset by increases in ULSW, vice versa. In contrast, percentage contributions to DSOW change, but no evidence for a long term trend in DSOW transport.

Important to monitor more than the total NADW/AAIW rate- need to monitor individual water mass components to understand system dynamics- can impact heat, freshwater, CO2 fluxes.

Fine and Smethie