Multiscale Air-Sea Interactions of the MJO and Onset of El Niño: A New Prospective of Precipitation and Density Current Coupling

Abstract

El Niño Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO) are two of the most significant climate phenomena, impacting global weather and climate patterns. Despite advancements in understanding ENSO dynamics, predicting El Niño onset remains challenging, especially considering potential changes due to global warming. The overall goal of this dissertation is to better understand the multiscale air-sea interactions of the MJO and onset of El Niño and implication for improving weather-climate prediction. The following science questions have been addressed in three publications:Q1. Does the MJO precipitation play a role in the warm pool eastward extension (WPEE)? Q2. What is(are) the physical mechanism by which the MJO precipitation promotes the WPEE? Q3. How often does the MJO precipitation and induced WPEE contribute to El Niño onset? Paper 1 (Chapter 2) departs from conventional studies on MJO-El Niño dynamics by focusing on MJO precipitation and surface winds as direct forcings to the ocean surface. Utilizing satellite observations spanning from 1998 to 2019 we quantify the MJO-induced sea- surface warming. We found that, for the first time, the MJO-induced warming predominantly occurs mostly after the MJO precipitation and westerly wind conditions. The post-MJO warming induces large-scale warm pool eastward extension (WPEE) that can persist for up to 30 days and even beyond. Notably, the strength and duration of MJO events, as well as the occurrence of multiple MJO events, is proportional to the magnitude of WPEE, particularly evident prior to the onset of El Niño events. Paper 2 (Chapter 3) provides the first quantitative analysis revealing key processes moving the warm pool eastward - via eastward density currents (EDC) driven by the MJO precipitation/freshwater. Our study elucidates how the extensive precipitation and fresh- water influx to the ocean lowers the west Pacific density, thereby increasing the east-west density gradient and generating an eastward zonal pressure gradient (ZPG) in the upper 30 m of the ocean. After the MJO precipitation and westerly wind conditions, this ZPG drives EDC that persist for weeks effectively counteracting and surpassing easterly wind forcing. These findings underscore the significant role of MJO-associated precipitation on ocean dynamics, offering a new processed-based discovery that differs from previous research on MJO wind-induced oceanic Kelvin waves. Paper 3 (Chapter 4) extends the key findings from Chapter 2 to examine eight El Niño onset events with 177 MJO events from 1993–2022, using ERA5 and ORAS5 reanalysis. Our results show that MJO-induced WPEE through EDC is closely linked to the characteristics of individual MJO events or the cumulative effect of multiple events. We emphasize the additional influence of MJO seasonal variability and the equatorial background oceanic conditions, further enhancing our understanding of the intricate interaction between MJO, WPEE, and El Niño onset dynamics. Key findings include: 1. In the equatorial western Pacific, 60-70% of MJO precipitation events are followed by WPEE lasting 15-30 days, causing warming anomalies above 0.5◦C. 2. Strong MJO forcing is characterized by horizontally extensive (4000 km), long-lasting (25-30 days), with significant precipitation/freshwater forcing (> 5 108 m3 or 8 mm day−1) and strong surface winds (> 4 m s−1) that project well onto the equatorial waveguide. 3. MJO precipitation events prior to El Niño, when the equatorial Pacific is transitioning from a neutral to El Niño conditions are statistical stronger. The strong precipitation/freshwater forcing reduces the salinity and increases east-west density gradient, resulting in a zonal pressure gradient that drives EDC for over 30 days post-MJO precipitation, causing WPEE. 4. Strong MJO events and EDCs in boreal winter and spring when the equatorial ocean is transitioning to El Niño, as shown by the warm pool that is extended (∼ 175◦E), small positive SSH (4-8 cm), and thermocline depth (6-8 m), enhances the ocean response to strong MJO forcing accelerating WPEE. 5. MJO events that occur when the equatorial western Pacific warm pool is greatly re- treated (∼ 160◦E), when easterlies in the central Pacific are anomalously strong, the thermocline is anomalously deep (> 15 m) and SSH is anomalously high (>8 cm) in the western Pacific (cold ENSO conditions), do not exhibit EDCs and the upper ocean response to MJO forcing is reduced. This dissertation emphasizes the role of the MJO-induced WPEE through precipitation/freshwater input. An extended warm pool can help a subsequent MJO event propagate further east than usual, resulting in a longer zonal fetch of surface westerlies and a more robust upper-ocean response. Our results complement the traditional understanding of Oceanic Kelvin waves and upper ocean stratification that can also strengthen MJO-induced WPEE. Successive MJO events often lead to weaker trade winds and a more flattened thermocline, all important factors to initiate the Bjerknes feedback at El Niño onset.