Causes of Interannual–Decadal Variability in the Meridional Overturning Circulation of the Midlatitude North Atlantic Ocean

Abstract

The causes and characteristics of interannual-decadal variability of the meridional overturning circulation (MOC) in the North Atlantic are investigated with a suite of basin-scale ocean models [the Family of Linked Atlantic Model Experiments (FLAME)] and global ocean-ice models (ORCA), varying in resolution from medium to eddy resolving ( 1 ⁄2°-1 ⁄12°), using various forcing configurations built on bulk formulations invoking atmospheric reanalysis products. Comparison of the model hindcasts indicates similar MOC variability characteristics on time scales up to a decade; both model architectures also simulate an upward trend in MOC strength between the early 1970s and mid-1990s. The causes of the MOC changes are examined by perturbation experiments aimed selectively at the response to individual forcing components. The solutions emphasize an inherently linear character of the midlatitude MOC variability by demonstrating that the anomalies of a (non-eddy resolving) hindcast simulation can be understood as a superposition of decadal and longer-term signals originating from thermohaline forcing variability, and a higher-frequency wind-driven variability. The thermohaline MOC signal is linked to the variability in subarctic deep-water formation, and rapidly progressing to the tropical Atlantic. However, throughout the subtropical and midlatitude North Atlantic, this signal is effectively masked by stronger MOC variability related to wind forcing and, especially north of 30°-35°N, by internally induced (eddy) fluctuations.

Interior pathways of the North Atlantic meridional overturning circulation

“…For the results discussed in this chapter, output from the highest-resolution member of the Family of Linked Atlantic Modeling Experiments (FLAME) ocean general cir-5 culation model is used [3] [7] to simulate Lagrangian particles. Information about this eddy-resolving ocean general circulation model is provided in Section 2.2.1.…”

“…The third and final model included in this analysis is ORCA05, a lower resolution configuration of the NEMO package similar to ORCA025 [3] [25], except that its resolution at the equator is 1/2 o . The two ORCA models used in this analysis were designed to be as similar as possible within the logistical and technical constraints of different resolutions.…”

Spatio-temporal characteristics of Agulhas leakage: a model inter-comparison study

“…In this study, the integration domain is controlled by the isohalines rather than the isotherms, as expected: as the waters move through the Cape Basin they experience cooling through air-sea interaction and mixing, leaving salinity as the dominant signal. AL is thus better defined by its salinity signature than by temperature (van Sebille et al 2010a;Biastoch et al 2008a). Yet, some models have Indian ocean that is fresher than South Atlantic (Weijer and Sebille 2014), in which case any Eulerian threshold method that assumes a local maxima in salinity in AL water masses 1 3…”

The Present and Future System for Measuring the Atlantic Meridional Overturning Circulation and Heat Transport

“…Models have shown that the freshwater flux through the South Atlantic may be a precursor to changes in the AMOC further north (e.g. [49]). Other theories for forcing variability in the AMOC include the role of wind stress variability in the Southern Ocean (e.g.…”

“…Reference [57] and [49] show that in a range of models the AMOC has quite different timescales for its meridional coherence in the subpolar compared to the subtropical North Atlantic. Northward of 40°N the variability has a strong coherent decadal variability, while in the subtropical gyre higher frequencies dominate.…”