Progress in Characterizing AMOC Structure and Variability from Observations Bill Johns RSMAS,...

Progress in Characterizing AMOC Progress in Characterizing AMOC Structure and Variability from Structure and Variability from

ObservationsObservations

Bill Johns Bill Johns

RSMAS, University of Miami, Miami FL

OutlineOutline: :

1.1. An AMOC tour from the subpolar gyre to the South An AMOC tour from the subpolar gyre to the South Atlantic Atlantic

2.2. How to move forward in directly observing the AMOCHow to move forward in directly observing the AMOC

3.3. What have we learned in the last ~5 years?What have we learned in the last ~5 years?

AMOC Observational Network

International ProgramsU.S. Programs

Nordic Seas Overflows

Quadfasel and Käse (2007)

Based on Hansen et al. (2007), Macrander et al. (2005), and updates

Nordic Seas Overflows (modeled)

Olsen et al. (2008)

Blue = obs. Red = model Total overflow

Faroe Bank

Faroe Bank Channel

Model hindcast of Faroe Bank overflow during the observational record (top), and for the last 50 years (right). Total Nordic Sea overflow shown in green (right).

Entrainment into the overflows

Dickson and Brown (1994)

LSW 4 Sv

DWBC Measurements at Cape Farewell

Bacon and Saunders (2010)

Mean transport = 8.6 Sv (σθ > 27.80)

Bacon (1998)

Historical DWBC Measurements at Cape Farewell

Baroclinic transport relative to 1000 db

Mean: 5.5 Sv

Historical DWBC Measurements at Cape Farewell (updated)

Sarafanov et al. (2009)

Baroclinic transport anomaly relative to mean value of 5.5 Sv

Summary of LNADW (DSOW/ISOW) Transports

Haine at el. (2008)

------- 9 Sv

Kieke et al. (2006)

Labrador Sea Water Formation Rates

LSW Production estimates: 2-10+ Sv (Haine et al., 2008)

CFC 11/12 Tracer Inventories (1970-1997):

Average cLSW production: 4.4 – 5.6 Sv Average uLSW production: 3.2 – 3.3 Sv Average total LSW production: 7.6 – 8.9 Sv

Kieke et al. (2007)

Labrador Sea Water Formation Rates (’97-’03)

97-99 99-01 01-03

uLSW cLSW

Lumpkin et al. (2008)

MOC: 16.2 Sv Mean UNADW: 7.1 Sv Mean LNADW: 9.1 Sv

Export to the Subtropics (48ºN)

1. Increasing evidence that overflows are stable (over the modern record, last 50 yrs)

2. DWBC at Cape Farewell (overflows+entrainment) is variable on decadal timescales; mean transport of ISOW/DSOW ~9 Sv (not 13 Sv). Varies by ±30%.

3. LSW production constrained by tracer inventories. Recent biennial surveys resolve temporal variability of formation. Mean LSW production 7.6-8.9 Sv (1970-97), Highly variable. Cycling between cLSW/uLSW, linked to NAO forcing.

4. Export to subtropics in MOC (at 48N): 16.2 ± 2.0 Sv (1993-2000). Relatively stable (recently). LSW: 7.1± 1.4 Sv; LNADW: 9.1± 1.7 Sv.

Summary for Subpolar region:

RAPID/MOCHA 26.5°N Array

Florida Current Monitoring by undersea cable (Baringer and Larson, 2001)

Western Boundary array (Johns et al., 2008)

Mid-ocean Array

(Cunningham et al., 2007)

MOC streamfunction and layer transports (Rapid Array 26.5ºN)

MOC: 18.5 Sv

UNADW: 8.2 Sv LNADW: 12.3 Sv

uLSW 2.8 Sv

cLSW 5.4 Sv

ISOW 4.8 Sv

DSOW 7.5 Sv

AABW 2.1 Sv

Contributions to northward flowing (upper ocean) part of AMOC cell

MOC variability range: 5-30 Sv

AMOC Variability at 26.5ºN

Kanzow et al. (2010)

MOC and Heat Transport Variability

3.5 year mean MOC: 18.5 ± 4.9 (3.8*) Sv (σerr = 2.1 Sv)

mean MHT: 1.33 ± 0.40 (0.24*) PW (σerr = 0.12 PW) *with contribution by Ekman transport variability removed

Contributions to northward flowing (upper ocean) part of AMOC cell

AMOC variability spectrum at 26.5ºN

Kanzow et al. (2010)

AMOC seasonal cycle and seasonal contributions to upper ocean part of AMOC cell

The interior transport (TUMO) cycle can be explained by linear, forced Rossby wave response to wind stress curl, contained mostly in eastern basin

AMOC seasonal cycle at 26.5ºN

Kanzow et al. (2010)

Bryden (2005) MOC values after application of seasonal correction

Kanzow et al. (2010)

26.5ºN in perspective

CCSP (2008): Abrupt Climate Change

Synthesis model ensembleSynthesis model ensemble

“SAMOC” Program

MOC and MHT estimates at 35ºS (AX18)

South Atlantic

17 transects (2002-2007): Mean MOC: 17.9 ± 2.2 Sv Mean MHT: 0.55 ± 0.14 PW

South Atlantic (35ºS)

Dong et al. (2009)

Complex AMOC structure in South Atlantic (relative to N. Atlantic subtropics):

• Eddy transport processes important in both upper and lower limbs

• Contributions of warm/salty (Agulhas) and cool/fresh (AAIW/SAMW) to the upper limb

• Interaction of DWBC with equator; interior NADW pathways; deep water mass transformation

AMOC Pathways/Processes

Lumpkin and Speer (2007)

Schematic: R. Lumpkin NOAA/AOML

AMOC streamfunction in density coordinates

NADW Transformation in S. Atlantic

Lumpkin and Speer (2007)

Establish discrete set of trans-basin arrays (moorings + autonomous profiling) for continuous AMOC estimates

Value:

• Accurate multi-year mean AMOC estimates, for comparison with future (and past) AMOC states

• Understanding of processes underlying short-term (intraseasonal to annual) variability

• Benchmarks for evaluation of modeled AMOC variability (GCMs, data synthesis models)

AMOC Monitoring Strategy

Lumpkin and Speer (2007)

O-SNAPO-SNAP

RAPIDRAPID

MOVEMOVE

SAMOCSAMOC

AMOC Variability at 41ºN from ARGO/altimetry

Profiling Floats and AMOC Monitoring

Willis (2010)

1. Increasing evidence that overflows are stable (over the modern record, last 50 yrs). -> Denmark Strait and Iceland-Faroes Ridge monitoring remains challenging.

2. Mean transport of ISOW/DSOW at Cape Farewell appears to be ~9 Sv (not 13 Sv). Varies by ±30% on decadal timescales. -> Entrainment variability? LSW “blocking” at Gibbs?

LSW production can be temporally monitored by transient tracers. Mean LSW production 7.6-8.9 Sv (1970-97). Cycling between cLSW/uLSW, w/ link to NAO forcing. -> How to monitor going forward (SF6 )? Pathways of export to the subtropics?

LSW makes up nearly half of the deep limb of the AMOC. 48ºN: LSW: 7.1 Sv; DSOW/ISOW: 9.1 Sv. 26ºN: LSW: 8.2 Sv; DSOW/ISOW (minus AABW): 10.2 Sv. -> How are variations in LSW production reflected in export to subtropics? Modulating/buffering processes?

What have we learned in the past ~5 years?

1. Large short-term (intraseasonal to annual) MOC variability in subtropics. Ekman forcing dominates at intraseasonal; geostropic variability dominates on longer time scales (annual+). Annual MOC cycle documented and its fundamental mechanism explained.

2. AMOC snapshots derived from single hydrographic sections can be subject to considerable aliasing. The interior baroclinic flow cannot be assumed steady. The Bryden (2005) “trend” can be largely explained by seasonal aliasing.

3. MOC strength is fairly uniform throughout the basin. (16-18 Sv). Minor “internal” closure. -> How does the partitioning of internal components vary? uLSW/CLSW? Agulhas leakage vs. AAIW?

4. Complex NADW transformation processes in the S. Atlantic. DWBC eddies; interior pathways -> eastern boundary “DWBC”. Significant upward shift in mean density of NADW limb. -> Equatorial mixing/deep jets?

What have we learned in the past ~5 years?

1. Response of MOC to variation in deep water formation rates (convective + overflow)? - LSW formation rates have large interannual variation (and still disagreement on the “mean” formation rate). How are these variations connected to export, and MOC variability? - New results (Bacon and Saunders, 2010) call into question accepted magnitude of DSOW/ISOW entrainment and DWBC flux at Cape Farewell – while sill overflows appear ~steady. Does this imply a reduction in LNADW limb of AMOC? What is the “transfer function” between overflows and LNADW part of MOC? Is it stable/robust?

Questions/Issues on Variability:

2. Inter-gyre AMOC coherence/connectivity? - Models suggest “breakpoints” in AMOC coherence at certain latitudes. Where and for what time scales? Mechanisms?

Questions/Issues on Variability:

Bingham et al. (2007)

HadCM3 (coupled) OCCAM (forced)

3. Inter-hemispheric (and global) AMOC coherence/connectivity? - Role of “equatorial buffer” in AMOC meridional coherence? - Influences arising from S. Atlantic (e.g. change of partitioning between warm/cold routes)? Changes external to Atlantic?

Questions/Issues on Variability:

Johnson and Marshall (2002)

HadCM3

l

Dong and Sutton (2003)

4. Can we devise a long-term strategy for observing coherent modes of interannual/decadal variability?

Questions/Issues on Variability:

Bingham et al. (2007)

HadCM3

Forced

Bentsen et al. (2004)

Coupled

5. Are we prepared to observe and document anthropogenically- forced AMOC changes?

Questions/Issues on Variability:

IPCC/AR4 A1-B

scenario runs

(Schmittner et al., 2005)

Discussion…

Atlantic MOCECCO-SIO

ECCO-50y

ECCO-GODAE

ECCO-JPL

INGV

SODA

GFDL