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

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.