How will global warming affect climate systems like the ENSO or the MJO?

Question

How will global warming influence climate systems such as the Madden–Julian Oscillation (MJO) or the El Niño Southern Oscillation (ENSO)?

Answer 1

Still undetermined, though a number of papers on the subject suggest global warming will increase the variance in both of the climate systems you mention. They suggest more research and more refined models are needed to narrow down the uncertainties.

Some definitions:

The Madden-Julian Oscillation (MJO) is the major fluctuation in tropical weather on weekly to monthly timescales. The MJO can be characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days. (Australian Bureau of Meteorology)

The El Niño/ Southern Oscillation (ENSO) cycle refers to the coherent and sometimes very strong year-to-year variations in sea- surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. El Niño and La Niña represent opposite extremes in the ENSO cycle. (National Weather Service Climate Prediction Center)

El Niño is the "warm phase" of sea-surface temperatures in the east-central equatorial Pacific with a variety of effects on regional and global weather. La Niña is the "cold phase" of the oscillation.

One of the predictive challenges is that climate change is not monotonic. It's generally held that will be a "warmer-get-wetter" impact -- warmer oceans produce more evaporation and therefore more precipitation. But ocean states are also affected by highly complex relationships involving currents and convection.

Here are extracts from the abstracts of a couple of recent papers on the subject(s) (actual papers are paywalled):

In Intermodel Uncertainty in ENSO Amplitude Change Tied to Pacific Ocean Warming Pattern, Zheng, et.al. compare actual sea-surface temperature measurements to the outputs of various models and write:

To the extent that correcting model biases favors an El Niño–like mean warming pattern, this study suggests an increase in ENSO-related SST variance likely under global warming.

In The MJO and global warming: a study in CCSM4, Subramanian, et.al, write:

The RCP8.5 run exhibits increased variance in intraseasonal precipitation, larger-amplitude MJO events, stronger MJO rainfall in the central and eastern tropical Pacific, and a greater frequency of MJO occurrence for phases corresponding to enhanced rainfall in the Indian Ocean sector. These features are consistent with the concept of an increased magnitude for the hydrological cycle under greenhouse warming conditions.

In MJO Intensification with Warming in the Superparameterized CESM, Arnold, et.al. said their modeling runs showed "Intraseasonal precipitation increases at a rate of roughly 10% per 1 K of warming, and MJO events become 20%–30% more frequent," but conclude:

This work adds to existing evidence that MJO activity may increase in response to global warming. Evidence for an MJO dependence on SST has been identified in observations (Hendon et al. 1999; Slingo et al. 1999; Jones and Carvalho 2006; Oliver and Thompson 2012), and in some numerical models (Lee 1999; Caballero and Huber 2010; Arnold et al. 2013; Schubert et al. 2013). However, other models have shown negative or neutral trends with SST (Takahashi et al. 2011) or a strong dependence on the spatial pattern of warming (Maloney and Xie 2013), so any conclusions regarding future trends should be viewed as tentative.