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 (M ov ) at 34°S in the Atlantic has often been used as an indicator for bi-stability, with a positive M ov suggesting a monostable AMOC and a negative M ov suggesting a bi-stable AMOC. Often studies have shown that the sign of the divergence of the M ov might be a good indicator of AMOC bi-stability. In this study we investigate how model bias affects the sign of M ov across all latitudes in the Atlantic basin, through a detailed analysis of the Coupled Model Inter-Comparison Project 5 (CMIP5) model ensemble. M ov , 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 M ov 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.

Decoupled Freshwater Transport and Meridional Overturning in the South Atlantic

“…These F gyre transports exhibit a marked antisymmetric pattern around the mean ITCZ location at~5°N, redistributing freshwater within a 0-300 m upper ocean layer in the subtropics of both hemispheres. However, applying the same reasoning as for F ov at 34°S (see section 1), one would conclude that all models are therefore unstable, as they systematically simulate a large negative F ov in the NASG (see Figure 1b and Mecking et al, 2017). Changes in evaporation minus precipitation induced by an AMOC collapse, such as an ITCZ shift, also support the large F gyre changes found by Mecking et al (2016) in the South Atlantic.…”

“…At 34°S, F gyre is consistently larger than F ov , so that the total transport is actually northward and compensates for the net evaporation in the subtropical South Atlantic. Yin and Stouffer (2007) and Mecking et al (2016) instead suggest that a better bistability indicator might be to measure F ov across the NASG where the salinity bias-corrected CMIP5 models show the largest correlations between F ov and AMOC strength (Mecking et al, 2017). In a freshwater hosing experiment with an eddy-permitting coupled model, Mecking et al (2016) showed that the dominant response of F gyre at 34°S is over twice as large as the changes in F ov , despite the total AMOC collapsed.…”

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

“…There may be several reasons for this diversity in model behavior. First, several studies suggest that the current generation of CGCMs may suffer from biases that hinder the models' ability to show AMOC multiple equilibria (Hawkins et al, ; Huisman et al, ; Liu et al, , ; Mecking et al, ; Weaver et al, ). Several of these studies point out that GCMs often have a fresh bias in the South Atlantic that affects the amplitude, if not the sign, of F ovS and hence the stability regime in which the modeled AMOC resides.…”

“…If, instead, the AMOC exports freshwater ( F ovS <0), haline forcing would oppose the circulation, and a reverse SA circulation would exist as an alternative equilibrium. Studies with GCMs found the condition F ovS =0 to be a remarkably accurate indicator of the boundary between the monostable and bistable regimes (Cimatoribus et al, ; De Vries & Weber, ; Drijfhout et al, ; Hawkins et al, ; Jackson, ; Mecking et al, ; Weber & Drijfhout, ). In fact, several studies with comprehensive GCMs showed that the stability characteristics of the AMOC can be altered by constraining F ovS to be positive or negative, as predicted by box models (Cimatoribus et al, ; De Vries & Weber, ; Jackson, ; Liu et al, ).…”

“…It has been well established that in many current generation coupled climate models F ovS tends to have a positive bias; hence, the AMOC is not exporting enough freshwater from the Atlantic (Cheng et al, ; Liu et al, , ; Mecking et al, ; Weaver et al, ). The upper 800 m of the South Atlantic, where the transport is northward, tends to be too fresh, while at depth, where the transport is predominantly southward, the ocean is too salty leading to a positive bias in F ovS .…”