A Coupled Initialization Strategy to Represent Historical Atlantic Meridional Overturning Circulation Decadal Variation and Prediction
Abstract
Decadal variations of the Atlantic meridional overturning circulation (AMOC) play a key role in decadal climate predictions. Previous studies suggest that forced ocean — sea-ice (FOSI) model simulations, which can represent historical decadal variation of AMOC under the observation-based atmospheric forcing, can be used to initialize decadal climate predictions with high skill in the North Atlantic. However, the methodology of the FOSI initialization can generate large ocean initialization shocks, potentially lowering model predictive skill based on seasonal climate prediction studies. This study aims to address the initialization shock issue by developing and evaluating an alternative initialization strategy in which historical AMOC decadal variation is simulated in a fully coupled predictive model. It is shown that a simple initialization strategy that restores model sea-surface temperature (SST) and sea-surface salinity (SSS) to those of FOSI can effectively reproduce historical AMOC decadal variation and the associated dense water propagation in FOSI. This approach can be viewed as an extension of the SST-restoring technique widely used in the seasonal prediction community with a key emphasis on the inclusion of SSS restoring. Extensive model sensitivity experiments, including using observed SST and SSS as restoring target, are conducted to further investigate the role of SSS restoring in simulating historical AMOC decadal variability. The results suggest that restoring coupled model SSS to observed climatological SSS while restoring SST to full observed values can reproduce historical AMOC decadal variation in FOSI, indicating that the role of SSS restoring is primarily to correct coupled model salinity bias, ensuring a realistic surface density distribution that is essential for a realistic simulation of AMOC decadal variation. Preliminary decadal prediction experiments using this simple coupled initialization strategy show that the historical AMOC decadal variation replicated by SST- and SSS- restoring in a coupled predictive model can indeed lead to improved model prediction skill in the North Atlantic compared to FOSI initialization. This new initialization strategy offers a potential improvement to decadal climate predictions by reducing initialization shock using only observed SST and SSS that are more readily available.
Subject
decadal variabilitysurface assimilation
decadal climate prediction
ocean initialization shock
Citation
Zhang, Qiuying (2022). A Coupled Initialization Strategy to Represent Historical Atlantic Meridional Overturning Circulation Decadal Variation and Prediction. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /197290.