1. Dongxiao Zhang and Mike McPhaden
Observed Multidecadal Variability of the Atlantic Meridional Overturning Circulation (AMOC)
Dongxiao Zhang and Mike McPhaden
October 2008

2. Maier-Reimer modified after Broecker
Thermohaline
Circulation or
Global
Conveyor
Belt
Maier-Reimer modified after Broecker
North Atlantic SST
Rahmstorf (2002)

3. Schematic of AMOC
Lumpkin and Speer (2003)

4. Model Observations Atlantic Multidecadal Oscillation (AMO)
Upper 800m current during stronger AMOC
U.S. major hurricane landings
Goldenberg
et al. (2001)
Vellinga and Wu (2004)
Summer precipitation
Sutton and Hodson
(2005)
SST regression to AMOC

5. Bryden et al. (2005)
MOC Uncertainties in Climate Models, Observations and Data Assimilation Products
from ECMWF meeting 2006
by Armin Koehl
Schneider et al.
J. Climate 2007

6. Large Amplitude of High Frequency Variability in RAPID Section
Cunningham et al. 2007

7. Compute North Brazil Current/North Brazil Undercurrent (NBC/NBUC)as a measure the AMOC strength at 6°S.
Geostrophic Transport Calculation: Historical hydrographic data off the Brazil coast
Upper limb of the AMOC, superimposed on salinity at 100m

8. Total of 138,584 profiles that have both T and S measurements, reaching 1200m or deeper
hydrocasts between 4.5°-11.5°S and Brazil coast to 25°W deeper than 1200m,
every 9 years ( )

9. Multi-Decadal Variation of the Atlantic Meridional Overturning Circulation (AMOC) at 6° S Error bars include uncertainties from reference level, aliasing due to incompletely resolved off-shore recirculation‏ and seasonal cycle.
Compared to Schott et al. 2005:
shipboard ADCP mean at 5°S, Sv (with southward recirculation)
shipboard ADCP mean at 11°S, 21.7Sv (with southward recirculation)
moored current measurement mean at 11°S, Sv (with partial recirculation)

10. Thickness of Labrador Sea water between
34.62 and σ1500,
a measure of the vigorous of deep convection

11. T and S at 100m

12. Salinity anomalies at 100m

13. Correlation map of de-trended SST and AMOC transport time series at 6°S, inferred from the NBC/NBUC.

14. tropical North Atlantic SST
Sutton and Hodson (2007)
modeling experiment
winter SLP response to
tropical North Atlantic SST

15. Observed AMOC multidecadal variability Atlantic hurricane activity,
vs.
AMO
Atlantic hurricane activity,
de-trended Sahel
summer time rainfall

16. Summary NBC/NBUC transport offers a measure of the AMOC strength.
The observed multidecadal transport variability is well above error bar even when the largest amplitude of seasonal cycle is included in the error assessment.
Consistent variabilities in T and S along the upper limb of AMOC, in the Labrador Sea Deep Convection and AMO in SST suggest the observed multidecadal signal of the AMOC is real.
The quick response of the northern North Atlantic SST to the AMOC at 6°S is consistent with the forcing from the atmospheric anomalies, that is forced by the tropical Atlantic anomalies associated with the AMOC through atmospheric teleconnection as suggested by numerical model studies.
The connection between the AMOC and the Atlantic hurricane activity and Sahel rainfall could be a part of the atmospheric response in the tropical Atlantic.
The results will provide important constraints on climate models and initial conditions for decadal climate prediction using these models.

18. Apparent Multi-Decadal Variation of the Atlantic Meridional Overturning Circulation (AMOC) at 6° S (Zhang and McPhaden, 2008)‏
NBC/NBUC, a surface and intermediate water return flow of AMOC, is used to measure the AMOC strength at 6°S.
Geostrophic Transport Calculation: Historical hydrographic data off the Brazil coast
The recent enson cycle I referred to is the 2005 cooling and the El nino. This forecast plume from IRI display all the dynamical model performance, jun 05 – mar 07. the last set of prediction start from march 2007, shown in this figure. Observation is the black line.
Allmost all the models missed the prediction of 2005 cooling. Many models predicted to the opposite direction. The NCEP CFS, indicated by the blue diamonds, though not successful gettign the magnitude right, it has some limited success in the cooling dirction. But it has relatively poor performance in predicting thhe el noni, evevn the forecst starts from july and august.
Compared to Schott et al. 2005:
shipboard ADCP mean at 5°S, Sv (with southward recirculation)
shipboard ADCP mean at 11°S, 21.7Sv (with southward recirculation)
moored current measurement mean at 11°S, Sv (with partial recirculation)

19. Schematic of AMOC based on WOCE sections
Lumpkin and Speer (2003)
Mean current structure off the coast the Brazil at 10-11S