Abstract

Abstract This study broadly characterizes and compares the key processes governing internal Atlantic multidecadal variability (AMV) in two resolutions of HadGEM3-GC3.1: N216ORCA025, corresponding to ∼60 km in the atmosphere and 0.25° in the ocean, and N96ORCA1 (∼135 km in the atmosphere and 1° in the ocean). Both models simulate AMV with a time scale of 60–80 years, which is related to low-frequency ocean and atmosphere circulation changes. In both models, ocean heat transport convergence dominates polar and subpolar AMV, whereas surface heat fluxes associated with cloud changes drive subtropical AMV. However, details of the ocean circulation changes differ between the models. In N216 subpolar subsurface density anomalies propagate into the subtropics along the western boundary, consistent with the more coherent circulation changes and widespread development of SST anomalies. In contrast, N96 subsurface density anomalies persist in the subpolar latitudes for longer, so circulation anomalies and the development of SST anomalies are more centered there. The drivers of subsurface density anomalies also differ between models. In N216, the NAO is the dominant driver, while upper-ocean salinity-controlled density anomalies that originate from the Arctic appear to be the dominant driver in N96. These results further highlight that internal AMV mechanisms are model dependent and motivate further work to better understand and constrain the differences.

Highlights

  • The observational record of North Atlantic sea surface temperature (SST) shows pronounced multidecadal variability, with the whole basin going through decades of warm and cool phases relative to global-mean temperatures (Sutton et al 2018)

  • To highlight the point that the upper-ocean warming associated with internal Atlantic multidecadal variability (AMV) is more than just an ocean advective process, we investigate the mechanism behind the surfacedriven warming before exploring the details of ocean circulation changes in more details in later sections

  • This study presents an analysis of the Atlantic multidecadal variability (AMV) in 500-yr preindustrial control simulations with the HadGEM3-GC3.1 model at both N96ORCA1 (∼135 km in the atmosphere, 18 in the ocean) and N216ORCA025 (∼60 km in the atmosphere, 0.258 in the ocean) resolutions

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Summary

15 FEBRUARY 2022

DUNSTONEc a Department of Meteorology, University of Reading, Reading, United Kingdom b National Centre for Atmospheric Science, Department of Meteorology, University of Reading, Reading, United Kingdom c Met Office Hadley Centre, Exeter, United Kingdom (Manuscript received 9 April 2021, in final form 8 September 2021)

Introduction
Characteristics of the AMV
Processes governing regional OHC changes
Drivers of North Atlantic density anomalies
Discussions
Findings
Summary and conclusions
Full Text
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