Role of Sea‐Surface Salinity in Simulating Historical Decadal Variations of Atlantic Meridional Overturning Circulation in a Coupled Climate Model

• Published in: Geophysical research letters Vol. 49; no. 4
• Main Authors: Zhang, Qiuying, Chang, Ping, Yeager, Stephen G., Danabasoglu, Gokhan, Zhang, Shaoqing
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.02.2022; American Geophysical Union (AGU)
• Subjects: Atlantic Meridional Overturning Circulation (AMOC); Atmosphere; Atmospheric models; Climate models; Climate prediction; decadal climate prediction; decadal variability; Decadal variations; Enthalpy; ENVIRONMENTAL SCIENCES; Feasibility studies; Heat content; Long-term changes; Modelling; ocean initialization shock; Oceans; Salinity; Salinity effects; Sea surface; Sea surface temperature; Simulation; surface assimilation; Surface salinity; Surface temperature; Upper ocean; Variability
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL096922

Extending climate prediction from seasonal to decadal timescales requires realistic initialization of not only upper ocean heat content but also the Atlantic meridional overturning circulation (AMOC). However, it remains a major challenge to realistically initialize AMOC in a coupled system while also maintaining a balanced atmosphere‐ocean initial state. This study demonstrates the feasibility of generating fully coupled historical states with realistic AMOC variability. Employing a forced ocean—sea‐ice (FOSI) model simulation as the “truth,” we show reproducibility of key features of historical AMOC decadal variability in a fully coupled model by restoring sea‐surface salinity, in addition to sea‐surface temperature restoring widely used in seasonal prediction. The atmospheric state of the restored coupled model solution is much closer to that of the free coupled simulation than to the observations used in FOSI, pointing to potential advantages of using this approach for initializing decadal predictions with reduced ocean initialization shock. Plain Language Summary Realistic simulations of the Atlantic meridional overturning circulation (AMOC) at decadal time scales in coupled climate models are vital to decadal climate predictions. Because of the sparseness of deep ocean observations, forced ocean—sea‐ice (FOSI) model simulations have been used to initialize decadal climate predictions in a fully coupled model. However, such an approach suffers from imbalance between atmosphere and ocean states because of systematic differences in the atmospheric state used for FOSI and that of the coupled climate model, causing ocean initial shocks that may lower prediction skill. Here, using a FOSI simulation as synthetic observations, we demonstrate that key features of historical AMOC decadal variability can be realistically replicated in a fully coupled simulation by nudging both sea‐surface temperature (SST) and salinity toward those of FOSI. Because the atmospheric state in the restored simulation is generated by the same coupled model, it is much closer to that in the free coupled simulation than in FOSI. This approach may be considered as an extension of the SST‐restoring practice widely used for seasonal climate prediction, opening an alternative way to initialize decadal climate prediction to reduce the imbalance and initialization shocks, thereby improving prediction skill. Key Points Realistic Atlantic meridional overturning circulation (AMOC) decadal variations can be simulated in a coupled climate model by sea‐surface temperature (SST) and sea‐surface salinity (SSS) restoring Sea‐surface salinity restoring plays a critical role in simulating realistic AMOC decadal variations Simultaneously restoring SST and SSS in coupled model simulations may reduce initialization shock in initialized decadal climate predictions