Ocean Density Currents Induced by MJO Precipitation: A Key Player in Warm Pool Eastward Extension During Onset of El Niño

Enhanced warm pool eastward extension (WPEE) over the western-central Pacific prior to the onset of El Niño has been observed following Madden-Julian Oscillation (MJO) precipitation events. However, the mechanism driving the WPEE after the MJO precipitation remains unclear. This study investigates how MJO precipitation affects WPEE by analyzing the zonal momentum budget of two coupled atmosphere-ocean model simulations with and without freshwater from Kerns and Chen (2021, https://doi.org/10.1016/j.ocemod.2021.101892). It is found that the zonal pressure gradient (ZPG) force created by decreasing salinity and density from MJO precipitation is a key driving force of the WPEE during the onset of El Niño in 2018. The ZPG forces eastward density currents, transporting warmer and fresher waters against persistent easterly winds. Eastward density currents can persist for up to 30 days post-MJO precipitation. This new mechanism works in concert with two other well-known MJO-induced processes in the upper ocean: (a) deepening of thermocline by oceanic Kelvin waves excited by the MJO's westerly winds, and (b) increased SST due to the barrier layer formation. The cumulative effects of these interactive processes during consecutive MJO events reduce the Pacific basin-scale east-west temperature gradient, enhancing the WPEE. These processes can contribute to flattening the east-west thermocline, decreasing in upwelling in the central Pacific, and warming in the eastern equatorial Pacific.