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Coupled processes and associated subsurface dynamics near the eastern edge of the Indo/western Pacific (WP) Warm Pool are important for air‐sea interactions involved in tropical Pacific dynamics. We seek to shed light on the physical mechanisms governing air‐sea interactions in the region and the impacts of their biases in models. In this study, we use the Ocean ReAnalysis System 5 (ORAS5) to identify mean‐state biases in the National Center for Atmospheric Research Community Earth System Model version 2 (CESM2) with a particular focus on upper ocean properties and air‐sea interaction processes. We show that the CESM2 has warm and fresh surface biases in the tropical Pacific Ocean, a barrier layer that is too thin in the WP, and an isothermal layer depth (ILD) that is too deep in the eastern Pacific (EP). These biases impact air‐sea interaction processes involved in El Niño development. We compare the strong El Niño events in ORAS5 and CESM2 and show that biases in barrier layer thickness in the WP and in ILD in the EP are significant before the onset of the El Niño events. These biases then influence vertical mixing and entrainment processes, resulting in mixed layer cooling biases. Biases in the sea surface temperature seasonal cycle in the CESM2 also influence the development of the El Niño. We emphasize how the El Niño progression in models can be influenced by its sensitivity to the mean state biases in both subsurface ocean structure and seasonal cycle through local as well as the large‐scale physical processes. Plain Language Summary: The western tropical Pacific has warm ocean surface temperatures that extend to over 100 m depth due to the prevailing trade winds. In contrast, the eastern tropical Pacific has cold ocean surface temperatures due to the upwelling of cold water from below. Changes to the trade winds can lead to changes in these temperatures and the whole ocean‐atmosphere coupled system. We shed light on the important physical processes that govern the ocean‐atmosphere interactions in the western tropical Pacific and how models have limitations in representing some of the processes, which potentially leads to errors in the simulation of important climate events such as El Niño. We show that the Community Earth System Model, a widely used global climate model, has warmer and fresher ocean surface waters compared to observations in this region which then influences the ocean‐atmosphere interactions adversely in this region. We also show that the so‐called "barrier" layer, which restricts the cold, deep water from reaching the surface when mixing occurs, is too thin in the climate model, which significantly hinders the model from reproducing the observed evolution of large El Niño events. Key Points: Coupled processes and subsurface dynamics near the eastern edge of the western Pacific (WP) Warm Pool are important for air‐sea interactionsThe CESM2 has warm and fresh surface biases in the tropical Pacific Ocean, a too thin barrier layer bias in WPBarrier layer thickness biases in the west Pacific and isothermal layer depth biases in east influence the development of El Niño [ABSTRACT FROM AUTHOR]

A data-assimilating ⅓° regional dynamical ocean model is evaluated on its ability to synthesize components of the Tropical Pacific Ocean Observing System. The four-dimensional variational data assimilation (4DVAR) method adjusts initial conditions and atmospheric forcing for overlapping 4-month model runs, or hindcasts, that are then combined to give an ocean state estimate for the period 2010-13. Consistency within uncertainty with satellite SSH and Argo profiles is achieved. Comparison to independent observations from Tropical Atmosphere Ocean (TAO) moorings shows that for time scales shorter than 100 days, the state estimate improves estimates of TAO temperature relative to an optimally interpolated Argo product. The improvement is greater at time scales shorter than 20 days, although unpredicted variability in the TAO temperatures implies that TAO observations provide significant information in that band. Larger discrepancies between the state estimate and independent observations from Spray gliders deployed near the Galápagos, Palau, and Solomon Islands are attributed to insufficient model resolution to capture the dynamics in strong current regions and near coasts. The sea surface height forecast skill of the model is assessed. Model forecasts using climatological forcing and boundary conditions are more skillful than climatology out to 50 days compared to persistence, which is a more skillful forecast than climatology out to approximately 20 days. Hindcasts using reanalysis products for atmospheric forcing and open boundary conditions are more skillful than climatology for approximately 120 days or longer, with the exact time scale depending on the accuracy of the state estimate used for initializing and on the reanalysis forcing. Estimating the model representational error is a goal of these experiments. [ABSTRACT FROM AUTHOR]