Data

s

AMOC variations and the Atlantic Dipole

# Model Name Years1 ACCESS1-0 5002 ACCESS1-3 5003 bcc-csm1-1 5004 BNU-ESM 5595 CanESM2 9966 CCSM4 5017 CNRM-CM5 8508 CSIRO-Mk3-6-0 5009 EC-EARTH 452

10 FGOALS-g2 70011 FGOALS-s2 50012 FIO-ESM 80013 GFDL-CM3 50014 GFDL-ESM2G 50015 GFDL-ESM2M 50016 GISS-E2-R 52517 HadGEM2-ES 55018 Inmcm4 50019 IPSL-CM5A-LR 100020 MIROC-ESM 53121 MIROC5 57022 MPI-ESM-LR 100023 MPI-ESM-P 115624 MPI-ESM-MR 100025 MRI-CGCM3 50026 NorESM1-M 501

Table 1. Model Details.

Conclusions

Figure 6: Lagged regression of SST onto the AMOC at 30oN. SST changes are in oC for a 1 Sv change in the AMOC. Numbers in the title of each panel indicate the model number (Table 1) and the lag of the NH SST (years) relatively to the AMOC

• The observations of the AMOC are not long enough to determine the link between the AMOC and SST dipole. (Fig. 2a)

• The Atlantic Dipole SST Index has varying behavior from model to model. Some models exhibit strong oscillations in both the Atlantic Dipole and the AMOC Index (Fig. 2b). While other models show little to no relationship between the two (Fig. 2c,d).

• In many models there is a strong link between the AMOC and SST (Fig. 5).

•The NH SST alone is a better estimate of the AMOC. The addition of SH SSTs reduces the correlation by as much as 30%.

• Models with a weak relationship between AMOC and NHSST improve the most when using the Atlantic Dipole.

Relationship between the Atlantic Dipole and AMOC.

• A peak in the AMOC is followed by a peak in the NH SST. The strength of this relationship varies greatly across the models (Fig.4).

• There is no consistency across the models on the relationship between the AMOC and the SH SST. The few models that do exhibit a hemispheric SH SST cooling do not agree on the timing with respect to the AMOC maximum. Cooling of the SH SST occurs before the AMOC peak in some models and after in others (CNRM-CM5, FGOALS-s2).

• The relationship between the Atlantic Dipole and the AMOC is dominated by the NHSST and the SHSST influence makes the relationship worse in most models.

Figure 1: Location of the NH SST (blue) And the SH SST (black) boxes.

• Dominant periods vary from model to model, from 8 years to greater than 80 years (Fig. 4).

• Dominant NH SST periods are often associated with dominant AMOC periods.

• Few models show peaks in the SH SST at the same periods as the AMOC.

Power spectra: the AMOC, NH SST, SH SST

Figure 3: Power spectra of the AMOC Index (red), NH SST (blue) and the SH SST (black). Dominant periods are often shared between AMOC and NH SST, but not the SH SST.

Les.muir@yale.edu

References & AcknowledgementsLatif, M., C. Böning, J. Willebrand, A. Biastoch, J. Dengg, N. Keenlyside, U. Schweckendiek, G. Madec, 2006: Is the Thermohaline Circulation Changing?. J. Climate, 19, 4631–4637.Taylor, K.E., R.J. Stouffer, G. A. Meehl, 2012: An Overview of CMIP5 and the Experiment Design. Bull. Amer. Meteor. Soc., 93, 485-498.

Funding from: DOE Grant DESC0007037 “A Generalized Stability Analysis of the AMOC in Earth System Models: Implication for Decadal Variability and Abrupt Climate Change”and the Packard Foundation

IntroductionVariations of the Atlantic Meridional Overturning Circulation (AMOC) are believed to be an important driver of decadal to multi-decadal climate variability. In particular a number of observational and modeling studies investigating the AMOC have linked an inter-hemispheric sea surface temperature (SST) dipole to fluctuation in the overturning circulation. In the absence of direct measurements of the AMOC extending beyond the past decade, modeling and observational studies have used changes in a hemispheric temperature difference, the Atlantic Dipole Index, as a proxy for changes in the AMOC.

Here we use the CMIP5 (Coupled Model Intercomparison Project Phase 5) to determine if there is a strong connection between AMOC variations and the hemisphere SST dipole at multi-decadal timescales and whether one could indeed use the Atlantic SST dipole as an index for AMOC variability.

• The data considered in this study comes from the CMIP5 data set [Taylor et. al. (2012)]. Only models with preindustrial control (piControl) simulations extending for more than 450 years are considered, resulting in 26 models (Table 1).

• The AMOC Index is defined as the maximum of the annual mean zonally integrated Atlantic overturning stream function between 500m and 2000m at 30°N.

• The Northern Hemisphere SST (NH SST) is defined as the mean SST within the box 60 –10°W, 40°–60°N and the Southern Hemisphere SST (SH SST) is the mean SST within the box 50°W–0°W, 40°–60°S (Fig. 1). The Atlantic Dipole is defined as the difference between the NH SST and the SH SST. [Latif et. al. (2006)]

• Band-pass filtering is performed between 10 and 100 years to isolate multi-decadal changes.

How the AMOC affects climate on decadal to centennial timescales: the North Atlantic versus an interhemispheric seesaw.

Les Muir and Alexey V. Fedorov Dept. Geology and Geophysics, Yale University

Figure 4: Lag correlations of NH SST (blue), SH SST (black) and the Atlantic Dipole (red) against the AMOC index. Positive correlations at negative lag indicate AMOC changes lead SST changes. Most models show a strong link to NH SST lagging AMOC changes, with a weak link to SHSST.

Figure 2: a) Observed Atlantic Dipole from HadISST (black line) and Observed AMOC (red) from RAPID-MOC. b), c), d) are model examples of the Atlantic Dipole (black) and AMOC (red) showing a range from strong agreement (b) to no agreement (d). Thick lines are decadal running means while thin lines are annual means.

Relationship between the Atlantic Dipole and AMOC. (cont.)

Figure 5: Maximum lag correlations of the AMOC Index against the Atlantic Dipole (ordinate) and against the NH SST (abscissa). Points below the diagonal indicate that the NH SST is a better approximation to the AMOC than the Atlantic Dipole.

• The spatial structure associated with changes in the AMOC Index are dominated by increases in Northern Atlantic SSTs (Fig. 6).

• While some models show cooling in the Southern Atlantic Ocean others show warming. This leads to no agreement across the models on changes to Southern Atlantic SSTs associated with multi-decadal changes in the AMOC.

• On decadal to centennial timescales, AMOC variability affects mainly the Northern Atlantic. In most of the CMIP5 models the effect on the Southern Atlantic is minor. Consequently, the NH SST is a much better indicator of AMOC changes than the Atlantic SST Dipole.

• The AMOC explains up to 70% of the variance in the NH SST. A strengthening of the AMOC typically leads to a warming of the North Atlantic after a several-year lag.

• A few models show cooling in the Southern Hemisphere following the AMOC strengthening but the signal is not robust across the models.