A strong and persistent warm pool eastward extension (WPEE) over the western-central Pacific is observed has been observed prior to the onset of El Niño from January-June 2023. The WPEE is companied by three Madden-Julian Oscillation (MJO) events. It is consistent with a recent coupled atmosphere-ocean modeling study by Kerns and Chen (2021, OM) showing that the WPEE results from freshwater from the MJO rainfall led to forming barrier layers and increased SST. Using a zonal momentum budget analysis from the coupled model simulations, this study shows that the pressure gradient force (PGF) induced by decreasing salinity and density from the freshwater is a driving force for WPEE. A strong zonal density front and a broad region of eastward currents driven by the MJO-induced large-scale PGF transport warmer and fresher waters against the persistent equatorial trade winds. They can persist for up to 40 days beyond the active phase of the MJO convection. This new mechanism identified by this study is supported by two other important processes in the upper ocean: 1) depending on the thermocline by oceanic Kelvin wave forced by the westerly wind bursts from the MJO and 2) SST warming due to freshwater-induced barrier layer formation. Over time, the cumulative effects of these interactive processes lead to a reduction in the basin-scale east-west Pacific temperature gradient, ultimately leading to the WPEE. The consequences of these processes include a flattened west-east thermocline, decreased upwelling in the central Pacific, and weakened equatorial trade winds, all of which are crucial factors for initiating the Bjerknes feedback and potentially serve as one of the triggering mechanisms for the onset of El Niño.