The ENSO-induced South Pacific Meridional Mode

Abstract

Previous studies have investigated the role of the Pacific meridional mode (PMM), a climate mode of the mid-latitudes in the Northern and Southern Hemisphere, in favoring the development of the El Niño Southern Oscillation (ENSO). However little is known on how ENSO can influence the development of the PMM. Here we investigate the relationship between ENSO and the South Pacific Meridional Mode (SPMM) focusing on strong SPMM events that follows strong El Niño events. This type of events represents more than 60% of such events in the observational record and the historical simulations of the CESM Large ensemble (CESM-LE). It is first shown that such a relationship is rather stationary in both observations and the CESM-LE. Our analyses further reveal that strong SPMM events are associated with a coastal warming off northern central Chile peaking in Austral winter resulting from the propagation of waves forced at the equator during the development of El Niño events. The time delay between the ENSO peak (Boreal winter) and this coastal warming (Austral winter) can be understood in terms of the differential contribution of the equatorially-forced propagating baroclinic waves to the warming along the coast. In particular, the difference in phase speeds of the waves (the high-order mode the wave the slower) implies that they do not overlap along their propagation south of 20°S. This contributes to the persistence of warm coastal SST anomalies off Central Chile until the Austral summer following the concurrent El Niño event. This coastal warming is favorable to the development of strong SPMM events as the South Pacific Oscillation become active during that season. The analysis of the simulations of the Coupled Intercomparison Project phases 5 and 6 (CMIP5/6) indicates that very few models realistically simulate this ENSO/SPMM relationship and associated oceanic teleconnection.

On the interpretation of changes in the subtropical oxygen minimum zone volume off Chile during two La Niña events (2001 and 2007)

“…The weak ENSO amplitude over 2000-2008 can be understood in terms of the sole occurrence of CP El Niño and La Niña, or mixed-type events (2006) that have a much weaker amplitude than extreme EP El Niño events of 1982-1983 and 1997-1998. The CP events are associated with a weaker oceanic teleconnection than the strong EP El Niño (Dewitte et al, 2012) but can modulate significantly the upwelling favorable winds off Chile south of 30°S through atmospheric teleconnection during the development of ENSO (Cai et al, 2020;Dewitte et al, 2023). This explains the overall large correlation between the C index and the OMZ metrics and the direction of the phase-lagged relationship (C index ahead OMZ metrics) (see Table 3).…”

Extreme coastal El Niño events are tightly linked to the development of the Pacific Meridional Modes

“…Events of type-II are not associated with the usual dynamics of basin-wide events. They are more closely connected to the development of the North and South Pacific Meridional Modes [22,41,42]. These modes are thought to be primarily thermally driven by the wind-evaporation-SST (WES) feedback [43] -a process in which SSTdriven wind anomalies reduce evaporation, modifying SSTs which then feedback onto the wind response.…”

“…Applying the same procedure in the region (40S-0S, 140W-90W; as in Ref. [42]) yields the SPMM index. The regression patterns of these indices are shown in Figure 6a and 6b.…”

Oxidative conditions along the continental shelf of the Southeast Pacific during the last two millennia: a multiproxy interpretation of the oxygen minimum zone variability from sedimentary records

“…The bottom oxygenation suggested by Muñoz et al (2020) could have been associated with changes in equatorial variability favoring the atmospheric teleconnection off Central Chile. In particular there is a transition zone around 30°S for the ENSO atmospheric teleconnection: to the South of 30°S, upwelling favorable winds tend to reduce during El Niño (Dewitte et al, 2023).…”