Observed Multidecadal Variability of the Atlantic Meridional Overturning

Circulation(AMOC)

Dongxiao Zhang and Mike McPhaden

October 2008

ThermohalineCirculation orGlobal ConveyorBelt

Maier-Reimer modified after Broecker

North Atlantic SST

Rahmstorf (2002)

Schematic of AMOC

Lumpkin and Speer (2003)

Goldenberg et al. (2001)

Sutton and Hodson (2005)

Vellinga and Wu (2004)

Observations Model

U.S. major hurricane landings

Atlantic Multidecadal Oscillation (AMO)

Summer precipitation

SST regression to AMOC

Upper 800m current during stronger AMOC

MOC Uncertainties in Climate Models,

Observations and Data Assimilation

Products

Bryden et al. (2005)

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.

winter SLP response

to

tropical North Atlantic SST

Sutton and Hodson (2007) modeling experiment

Observed AMOC multidecadal variability

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

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