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Despite the well‐recognized initial value nature of the subseasonal forecasts, the role of subsurface ocean initialization in subseasonal forecasts remains underexplored. Using observing system experiments, this study investigates the impact of ocean in situ data assimilation on the propagation of Madden–Julian Oscillation (MJO) events across the Maritime Continent in the European Centre for Medium‐Range Weather Forecasts (ECMWF) subseasonal forecast system. Two sets of twin experiments are analyzed, which only differ on the use or not of in situ ocean observations in the initial conditions. Besides using the Real‐time Multivariate MJO Index (RMMI) to evaluate the forecast performance, we also develop a new MJO tracking method based on outgoing longwave radiation anomalies (OLRa) for forecast evaluation. We find that the ocean initialization with in situ data assimilation, though having an impact on the forecasted ocean mean state, does not improve the relatively low MJO forecast skill across the Maritime Continent. Moist static energy budget analysis further suggests that a significant underestimation in the meridional moisture advection in the model forecast may hinder the potential role played by the ocean state differences associated with data assimilation. Bias of the intraseasonal meridional winds in the model is a more important factor for such underestimation than the mean state moisture biases. This finding suggests that atmospheric model biases dominate the forecast error growth, and the atmospheric circulation bias is one of the major sources of the MJO prediction error and should be a target for improving the ECMWF subseasonal forecast model.

Idealized simulations show that the approximate colocation between the ITCZ and the energy flux equator (EFE), which holds on the annual and zonal average, breaks down on subseasonal timescales, as the Hadley cell develops a shallow return flow and negative gross moist stability (GMS). Here, we explore if similar mechanisms are seen in reanalysis data. In the zonal mean, a temporal offset exists between the ITCZ and the EFE as the ITCZ is retreating from the Northern to Southern Hemisphere and the Hadley cell transports energy northward across the equator despite a northward‐shifted ITCZ. At these times, the southern cell has a bottom‐heavy structure, with a distorted cell boundary and northward energy transport. In the Eastern Pacific, while bottom‐heavy structures exist throughout the year, the bottom heaviness is stronger in boreal fall, when GMS is negative, and SSTs are weak while their Laplacian is large and negative below the ITCZ.