Multiscale Processes in MJO-induced Warm Pool Eastward Extension prior to the Onset of 2018 El Niño

Abstract:

The El Niño–Southern Oscillation (ENSO) prediction has been a great challenge, especially the timing of the onset of El Niño. The zonal movement of the western equatorial warm pool is fundamental to ENSO dynamics. Studies showed a warm pool eastward extension (WPEE) often associated with strong and/or consecutive Madden-Julian Oscillation (MJO) events, which lead to believe that the MJO can serve as “trigger” to the El Niño onset. However, the mechanism(s) responsible for the MJO-ENSO interaction is not well understood. This study aims to better understand the physical processes by which the MJO induces WPEE that may lead to the onset of El Niño.

In this study, we use satellite and in situ observations to show MJO events occurred in January-May before the onset of 2018 El Niño. A relatively high-resolution coupled atmosphere-ocean model simulation is used to better understand the physical processes associated with the MJO-induced WPEE. Observations include TRMM-GPM rainfall, CCMP surface winds, OISST, SMAP surface salinity, and in situ measurements from moorings and ARGO floats. The coupled model consists of WRF with 12-km of horizontal resolution and 45 vertical levels, and HYCOM with 1/12o and 42 levels. The simulation reproduces the observed MJO events and WPEE from January-May 2018 and provides some important physical insights into multiscale processes associated with the MJO-induced WPEE. Besides, the simulation shows that freshwater from precipitation during the MJO(s) resulted in changes in salinity, leading to the formation and maintenance of barrier layers. We examine the mechanisms including 1) oceanic Kelvin waves deepening the thermocline, 2) upper ocean stratification building heat up, and their role in the equatorial zonal SST gradient relaxation leading to the onset of El Niño. Heat and momentum budgets in the ocean mixed layer are used to quantitatively identify physical mechanism at a high temporal and spatial resolution that explains most of the WPEE along the equator, north of the equator close to the ITCZ, and south of the equator in the SPCZ, within the tropical band. Results show that processes from the diurnal cycle to intraseasonal timescales contributed to the barrier layer formation and maintenance, which allow maximum SST warming by the shortwave radiation, especially in the late afternoon.