Geostrophic Closure of the Zonally Averaged Atlantic Meridional Overturning Circulation

Abstract

International audienceIt is typically assumed that the meridional density gradient in the North Atlantic is well and positively correlated with the Atlantic meridional overturning circulation (AMOC). In numerical “water-hosing” experiments, for example, imposing an anomalous freshwater flux in the Northern Hemisphere leads to a slowdown of the AMOC. However, on planetary scale, the first-order dynamics are linked to the geostrophic balance, relating the north–south pressure gradient to the zonal circulation. In this study, these two approaches are reconciled. At steady state and under geostrophic dynamics, an analytical expression is derived to relate the zonal and meridional pressure gradient. This solution is only valid where the meridional density gradient length scale is shorter than Earth’s curvature length scale, that is, north of 35°N. This theoretical expression links the north–south density gradient to the AMOC and can be used as a closure for zonally averaged ocean models. Assumptions and shortcomings of the approach are presented. Implications of these results for paleoclimate problems such as AMOC collapse and asymmetry in the meridional overturning circulation of the Atlantic and of the Pacific are discussed

AMOC sensitivity to surface buoyancy fluxes: Stronger ocean meridional heat transport with a weaker volume transport?

“…For further discussion on this realtionship we refer the reader to Sévellec and Huck (2015), and references therein. Integrating this relationship meridionally yields that the mean value of stream function is proportional to the north-south difference in baroclinic pressure.…”

AMOC Trends From 1850 to 2100 At Interannual To Multi-Decadal Time Scales Corroborated By Changes In Salinity Budget

“…Holliday et al (2020) also define a period between 2012 and 2016 when an extreme freshening occurred in the eastern SPR relative to 120 years of observations. We show that this unique event coincides with a period of high probability North Atlantic gyres affects the AMOC (Zhang, 2010;Robson et al, 2014;McCarthy et al, 2015;Yeager and Danabasoglu, 2014;Sévellec and Huck, 2016). The collective effect of these features happening in parallel could be the explanation for a shift in probability of an AMOC decline due to forced variations.…”

Temperature domination of AMOC weakening due to freshwater hosing in two GCMs

“…The theoretical arguments that underlay AMOC theory (e.g. Bryan 1987;Butler et al 2016;Sévellec and Huck 2016) indicate that density changes that are confined to the surface have a negligible effect on the AMOC, because thermohaline-led changes to the large scale circulation require perturbations to horizontal pressure gradients over a substantial depth range (Oliver et al 2005). Therefore it is necessary for the buoyant anomaly to propagate downwards through the water column; which is assumed to occur through the mixing down of freshwater.…”