Preprints
https://doi.org/10.5194/gmd-2024-183
https://doi.org/10.5194/gmd-2024-183
Submitted as: development and technical paper
 | 
18 Oct 2024
Submitted as: development and technical paper |  | 18 Oct 2024
Status: this preprint is currently under review for the journal GMD.

Development and evaluation of a new 4DEnVar-based weakly coupled ocean data assimilation system in E3SMv2

Pengfei Shi, L. Ruby Leung, and Bin Wang

Abstract. The development, implementation, and evaluation of a new weakly coupled ocean data assimilation (WCODA) system for the fully coupled Energy Exascale Earth System Model version 2 (E3SMv2) utilizing the four-dimensional ensemble variational (4DEnVar) method are presented in this study. The 4DEnVar method, based on the dimension-reduced projection four-dimensional variational (DRP-4DVar) approach, replaces the adjoint model with the ensemble technique, thereby reducing computational demands. Monthly mean ocean temperature and salinity data from the EN4.2.1 reanalysis are integrated into the ocean component of E3SMv2 from 1950 to 2021, with the goal of providing realistic initial conditions for decadal predictions and predictability studies. The performance of the WCODA system is assessed using various metrics, including cost function reduction, root mean square error (RMSE) differences, correlation differences, and model biases. Results indicate that the WCODA system effectively assimilates the reanalysis data into the climate model, achieving consistently negative cost function reductions and notable improvements in RMSE and correlation across various ocean layers and regions. Significant enhancements are observed in the majority of global ocean regions, particularly in the North Atlantic, North Pacific and Indian Ocean. Model biases in sea surface temperature and salinity are also substantially reduced. Furthermore, analysis of the connections between the ocean states and the regional climate over the US shows that the WCODA system improves the simulation of interannual precipitation and temperature variability over the southern US. The ultimate goal of the WCODA system is to advance the predictive capabilities of E3SM for subseasonal-to-decadal climate predictions, thereby supporting research on strategic energy-sector policies and planning.

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Pengfei Shi, L. Ruby Leung, and Bin Wang

Status: open (until 18 Dec 2024)

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Pengfei Shi, L. Ruby Leung, and Bin Wang
Pengfei Shi, L. Ruby Leung, and Bin Wang

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Short summary
Improving climate predictions has significant socio-economic impacts. In this study, we developed and applied a weakly coupled ocean data assimilation (WCODA) system to a coupled climate model. The WCODA system improves simulations of ocean temperature and salinity across many global regions. It also enhances the simulation of interannual precipitation and temperature variability over the southern US. This system is to support future predictability studies.