The abyssal origins of North Atlantic decadal predictability
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The abyssal origins of North Atlantic decadal predictability Stephen Yeager1 Received: 10 February 2020 / Accepted: 13 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The fundamental mechanisms that explain high subpolar North Atlantic (SPNA) decadal predictability within a particular modeling framework are described. The focus is on the Community Earth System Model (CESM), run in both a historical forced-ocean configuration as well as in a fully coupled configuration initialized from the former. The initialized prediction experiments comprise the CESM Decadal Prediction Large Ensemble (CESM-DPLE)—a 40-member set of retrospective hindcasts documented in Yeager et al. (Bull Am Meteorol Soc 99:1867–1886. https://doi.org/10.1175/bams-d-17-0098.1, 2018). Heat budget analysis confirms the driving role of advective heat convergence in skillful prediction of SPNA upper ocean heat content out to decadal lead times. The key ocean dynamics are topographically-coupled overturning/gyre fluctuations that are geographically centered over the mid-Atlantic ridge (MAR). Long-lasting predictive skill for ocean heat transport can be related to predictable barotropic gyre and sigma-coordinate AMOC circulations, but depth-coordinate AMOC is far less predictable except in the deepest layers. The foundation of ocean memory (and circulation predictive skill) in CESM-DPLE is Labrador Sea Water thickness, which propagates predictably through interior pathways towards the MAR where large anomalies accumulate and persist. Abyssal thickness anomalies drive predictable decadal changes in the gyre circulation, including changes in sea level gradient and near surface flow, that account for the high predictability of SPNA upper ocean heat content. Keywords Decadal prediction · North Atlantic · Subpolar gyre · AMOC
1 Introduction Recent studies offer compelling evidence that the subpolar North Atlantic Ocean is a region characterized by exceptionally high decadal predictability through the combined influence of external forcings and predictable internal variability related to large-scale ocean dynamics (Yeager and Robson 2017). Analyses of decadal hindcasts submitted to the 5th Coupled Model Intercomparison Project (CMIP5) consistently show that initialization (from observation-based state estimates) results in the largest positive impact on surface temperature skill in this region (e.g., Müller et al. 2012; Kirtman et al. 2013; Doblas-Reyes et al. 2013; Smith et al. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00382-020-05382-4) contains supplementary material, which is available to authorized users. * Stephen Yeager [email protected] 1
National Center for Atmospheric Research, Boulder, CO, USA
2019). The multidecadal variability of the Atlantic Meridional Overturning Circulation (AMOC) is commonly invoked as a key source of decadal ocean memory contributing to long-lasting prediction skill in the subpolar North Atlantic (SPNA), both for perfect model pot
