The effect of model bias on Atlantic freshwater transport and implications for AMOC bi-stability

Abstract

Evidence from paleo-proxy records suggests that the Atlantic Meridional Overturning Circulation (AMOC) can be in both an AMOC on state, the AMOC as we observe it today, and an AMOC off state, where the AMOC becomes extremely weak or even collapses. The freshwater transport due to the AMOC () at 34°S in the Atlantic has often been used as an indicator for bi-stability, with a positive suggesting a monostable AMOC and a negative suggesting a bi-stable AMOC. Often studies have shown that the sign of the divergence of the might be a good indicator of AMOC bi-stability. In this study we investigate how model bias affects the sign of across all latitudes in the Atlantic basin, through a detailed analysis of the Coupled Model Inter-Comparison Project 5 (CMIP5) model ensemble. , in the CMIP5 models is generally too positive in the southern Atlantic due to a salinity bias, while in the subtropical North Atlantic the values of are influenced by a combination of velocity and salinity biases. We compare these results to observations, reanalysis products and Hadley Centre Global Environmental Model version 3 global configuration version 2, a current generation coupled model which exhibits a stable AMOC off state, and discuss the differences that can lead to the possibility of a bi-stable AMOC as opposed to a monostable AMOC.

Keywords:

• AMOC
• AMOC collapse
• abrupt climate change
• CGCM
• CMIP5

Acknowledgements

We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups (listed in Table 1 of this paper) for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. Some of the computations were preformed on JASMIN, a super-data-cluster which we are thankful to have been allowed to use (Lawrence et al., 2013). We would also like to thank Drew Peterson for allowing us to use the GloSea5 data in our analysis. We would like to acknowledge Jeff Blundell and Adam Blaker for their technical help and useful conversations. And finally, we acknowledge Jan Sedáček and Michael Eby for discussions about the computations of at 34°S in Weaver et al. (2012).

Notes

No potential conflict of interest was reported by the authors.