Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model

M Mengel,C.-F Schleussner,A Levermann,A Born

doi:10.5194/esd-3-189-2012

Abstract

Abstract. Direct observations, satellite measurements and paleo records reveal strong variability in the Atlantic subpolar gyre on various time scales. Here we show that variations of comparable amplitude can only be simulated in a coupled climate model in the proximity of a dynamical threshold. The threshold and the associated dynamic response is due to a positive feedback involving increased salt transport in the subpolar gyre and enhanced deep convection in its centre. A series of sensitivity experiments is performed with a coarse resolution ocean general circulation model coupled to a statistical-dynamical atmosphere model which in itself does not produce atmospheric variability. To simulate the impact of atmospheric variability, the model system is perturbed with freshwater forcing of varying, but small amplitude and multi-decadal to centennial periodicities and observational variations in wind stress. While both freshwater and wind-stress-forcing have a small direct effect on the strength of the subpolar gyre, the magnitude of the gyre's response is strongly increased in the vicinity of the threshold. Our results indicate that baroclinic self-amplification in the North Atlantic ocean can play an important role in presently observed SPG variability and thereby North Atlantic climate variability on multi-decadal scales.

Highlights

Most northern deep water formation occurs in the Nordic Seas in combination with strong overflows over the Greenland-Scotland ridge (GSR) and associated entrainment of surrounding water masses (Hansen et al, 2004)
Surface wind stress has a strong influence on the strength and variability of the subpolar gyre (SPG) and the gyre strength has been linked to the northern hemisphere main mode of wind stress variability (Hurrell, 1995), the North Atlantic Oscillation (NAO) (Curry et al, 1998; Boning et al, 2006) and the Atlantic Multidecadal Oscillation (AMO) (Hakkinen et al, 2011a,b)
In order to investigate the role of the threshold behaviour for SPG variability, we introduced temporal variations in surface freshwater and wind stress forcing (Fig. 4)

Summary

Introduction

Most northern deep water formation occurs in the Nordic Seas in combination with strong overflows over the Greenland-Scotland ridge (GSR) and associated entrainment of surrounding water masses (Hansen et al, 2004). Simulations with a high resolution ocean model suggest that the Atlantic inflow into the Nordic Seas is modulated significantly by the strength of the subpolar gyre (SPG) south of the GSR (Hatun et al, 2005). The coarse-resolution climate model CLIMBER3α, which we apply in this study, exhibits a threshold behaviour of the Atlantic SPG with respect to surface freshwater forcing (Fig. 1) that has been related to baroclinic feedbacks in the region (Levermann and Born, 2007; Born and Mignot, 2011). We outline the feedback mechanism that potentially leads to the threshold behaviour through self-amplification in our model We propose that this selfamplification plays a crucial role in multi-decadal gyre variability, as illustrated by the SPG response to time-dependent variations in surface freshwater flux and wind-stress as a function of the ocean state’s distance from the threshold

Model and experiments

SPG threshold behaviour and mechanism of transition

Role of threshold for SPG variability

Variance

Findings

Conclusion and discussion