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Progress in Characterizing AMOC Structure and Variability from Observations

Progress in Characterizing AMOC Structure and Variability from Observations. Bill Johns RSMAS, University of Miami, Miami FL Outline : An AMOC tour from the subpolar gyre to the South Atlantic How to move forward in directly observing the AMOC What have we learned in the last ~5 years?.

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Progress in Characterizing AMOC Structure and Variability from Observations

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  1. Progress in Characterizing AMOC Structure and Variability from Observations • Bill Johns • RSMAS, University of Miami, Miami FL • Outline: • An AMOC tour from the subpolar gyre to the South Atlantic • How to move forward in directly observing the AMOC • What have we learned in the last ~5 years?

  2. AMOC Observational Network U.S. Programs International Programs

  3. Nordic Seas Overflows Quadfasel and Käse (2007) Based on Hansen et al. (2007), Macrander et al. (2005), and updates

  4. Nordic Seas Overflows (modeled) Faroe Bank Channel Olsen et al. (2008) Blue = obs.Red = model Total overflow 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). Faroe Bank

  5. Entrainment into the overflows LSW 4 Sv Dickson and Brown (1994)

  6. DWBC Measurements at Cape Farewell Mean transport = 8.6 Sv (σθ > 27.80) Bacon and Saunders (2010)

  7. Historical DWBC Measurements at Cape Farewell Baroclinic transport relative to 1000 db Mean: 5.5 Sv Bacon (1998)

  8. Historical DWBC Measurements at Cape Farewell (updated) Sarafanov et al. (2009) Baroclinic transport anomaly relative to mean value of 5.5 Sv

  9. Summary of LNADW (DSOW/ISOW) Transports -------9 Sv Haine at el. (2008)

  10. Labrador Sea Water Formation Rates Kieke et al. (2006) 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

  11. Labrador Sea Water Formation Rates (’97-’03) uLSW cLSW 97-99 99-01 01-03 Kieke et al. (2007)

  12. Export to the Subtropics (48ºN) Lumpkin et al. (2008) MOC: 16.2 Sv Mean UNADW: 7.1 Sv Mean LNADW: 9.1 Sv

  13. Summary for Subpolar region: 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%. 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. 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.

  14. RAPID/MOCHA 26.5°N Array Mid-ocean Array (Cunningham et al., 2007) Florida Current Monitoring by undersea cable (Baringer and Larson, 2001) Western Boundary array (Johns et al., 2008)

  15. 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

  16. AMOC Variability at 26.5ºN Contributions to northward flowing (upper ocean) part of AMOC cell MOC variability range: 5-30 Sv Kanzow et al. (2010)

  17. 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

  18. AMOC variability spectrum at 26.5ºN Contributions to northward flowing (upper ocean) part of AMOC cell Kanzow et al. (2010)

  19. AMOC seasonal cycle at 26.5ºN 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 Kanzow et al. (2010)

  20. 26.5ºN in perspective CCSP (2008): Abrupt Climate Change Bryden (2005) MOC values after application of seasonal correction Kanzow et al. (2010) Synthesis model ensemble

  21. South Atlantic “SAMOC” Program MOC and MHT estimates at 35ºS (AX18)

  22. South Atlantic (35ºS) Dong et al. (2009) 17 transects (2002-2007): Mean MOC: 17.9 ± 2.2 Sv Mean MHT: 0.55 ± 0.14 PW

  23. AMOC Pathways/Processes • 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 Lumpkin and Speer (2007) Schematic: R. Lumpkin NOAA/AOML

  24. NADW Transformation in S. Atlantic AMOC streamfunction in density coordinates Lumpkin and Speer (2007)

  25. AMOC Monitoring Strategy • 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) O-SNAP RAPID MOVE SAMOC Lumpkin and Speer (2007)

  26. Profiling Floats and AMOC Monitoring AMOC Variability at 41ºN from ARGO/altimetry Willis (2010)

  27. What have we learned in the past ~5 years? 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: 7.1Sv; 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?

  28. What have we learned in the past ~5 years? 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. 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? 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?

  29. Questions/Issues on Variability: 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?

  30. 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? HadCM3 (coupled) OCCAM (forced) Bingham et al. (2007)

  31. Questions/Issues on Variability: 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? HadCM3 l Dong and Sutton (2003) Johnson and Marshall (2002)

  32. Questions/Issues on Variability: HadCM3 4. Can we devise a long-term strategy for observing coherent modes of interannual/decadal variability? Forced Coupled Bentsen et al. (2004) Bingham et al. (2007)

  33. Questions/Issues on Variability: 5. Are we prepared to observe and document anthropogenically- forced AMOC changes? IPCC/AR4 A1-B scenario runs (Schmittner et al., 2005)

  34. Discussion…

  35. Atlantic MOC ECCO-50y ECCO-SIO ECCO-GODAE ECCO-JPL GFDL INGV SODA

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