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

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

Schematic of AMOC Lumpkin and Speer (2003)

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

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

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

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

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 (1955-1995)

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 1990-2004 at 5°S, 22.1 Sv (with southward recirculation) shipboard ADCP mean 2000-2004 at 11°S, 21.7Sv (with southward recirculation) moored current measurement mean 2000-2004 at 11°S, 23.3 Sv (with partial recirculation)

Thickness of Labrador Sea water between 34.62 and 34.72 σ1500, a measure of the vigorous of deep convection

T and S at 100m

Salinity anomalies at 100m

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

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

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

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.

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 2006-7 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 2006-7 el noni, evevn the forecst starts from july and august. Compared to Schott et al. 2005: shipboard ADCP mean 1990-2004 at 5°S, 22.1 Sv (with southward recirculation) shipboard ADCP mean 2000-2004 at 11°S, 21.7Sv (with southward recirculation) moored current measurement mean 2000-2004 at 11°S, 23.3 Sv (with partial recirculation)

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