Modulation of the Madden-Julian Oscillation by ENSO: Inferences from Observations and GCM Simulations

Abstract

The impact of the El Niñ o-Southern Oscillation (ENSO) on the Madden-Julian Oscillation (MJO) is studied, based on reanalysis data and output from an ensemble general circulation model (GCM) experiment. Observed monthly sea surface temperature variations over the period of 1950-99 are imposed in the deep tropical eastern/central Pacific in the course of the SST experiment. Both GCM, and reanalysis data, indicate that intraseasonal activity of the low-level zonal wind is enhanced (reduced) over the central (western) Pacific during El Niñ o events. The propagation and growth/decay characterisitcis of the MJO in different phases of ENSO is also examined, based on a lag correlation technique. During warm events there is an eastward shift in the locations of strong growth and decay, and the propagation of the MJO becomes slower in the warm ENSO phase. These changes are reversed during La Niñ a epsiodes.Using output from the GCM experiment, the effects of ENSO on the circulation and convection during the MJO lifecycle are studied in detail. Further eastward penetration of MJO-related convection is simulated during warm events over the central Pacific. An instability index related to the vertical gradient of the moist static energy is found to be useful for depicting the onset of MJO convection along the equator. During warm events, the stronger magnitudes of this index over the central Pacific are conducive to more eastward penetration of convective anomalies in the region. These changes are mainly due to the intensified moisture accumulation at low levels. Analysis of the moisture budget suggests that the stronger moisture accumulation can be related to the increased low-level humidity over the central Pacific during warm events.

Impacts of two types of El Niño on the MJO during boreal winter

“…By contrast, during CP El Niño winters (Fig. 5), the MJO emerges over the western Indian Ocean, and can maintain or even enhance its intensity over the region west of 120 • E, which is in agreement with previous studies (Kessler, 2001;Tam and Lau, 2005). However, the propagation tends to be concentrated to the west of the dateline, and convection anomalies become greatly reduced to the east of 180 • .…”

“…These previous studies indicate a two-way interaction between enhanced MJO activity and the development of El Niño (Kessler and Kleeman, 2000;Marshall et al, 2016). Besides, the MJO extends further east to the central Pacific during El Niño events, which is associated with the warmer SST beneath (Kessler, 2001;Tam and Lau, 2005). Further study suggests that the lifetime of the MJO is also dependent on the state of ENSO: the MJO propagates faster through the Maritime Continent and western Pacific during El Niño (Pohl and Matthews, 2007).…”

Contrasting Madden–Julian Oscillation activity during various stages of EP and CP El Niños

“…Both ENSO and MJO are involved in the shifts of the large-scale convection and atmospheric circulation in the tropics. The connection between the two modes, however, is complicated and ambiguous (Hendon et al, 1999(Hendon et al, , 2007Slingo et al, 1999;Tam and Lau, 2005). Slingo et al (1999) and Hendon et al (1999) reported that there is no obvious correlation between the year-to-year MJO amplitude and ENSO.…”

“…On the one hand, pronounced MJO events produce westerly wind passage in the western Pacific, a key factor for the onset and development of ENSO (Harrison and Schopf, 1984;Kessler et al, 1995;Belamari et al, 2003;Wang et al, 2013). On the other hand, ENSO may impact the evolution of the MJO in space and time through changing the background convection in the tropical Pacific (Hendon et al, 1998;Takayabu et al, 1999;Tam and Lau, 2005;Pohl and Matthews, 2007;Roundy and Kravitz, 2009). …”

Analysis of Intraseasonal and Interannual Variability of the Asian Summer Monsoon Using a Hidden Markov Model

“…Tam and Lau (2005) found that the phase speed of wintertime ISO (MJO) is modulated by the ENSO. Yun et al (2009) discussed the influence of East Asian summer ISO on the ENSO in a quasi-biennial time scale.…”