Projected future changes in equatorial wave spectrum in CMIP6

Abstract

The simulation of the Madden-Julian Oscillation (MJO) and convectively coupled equatorial waves (CCEWs) is considered in 13 state-ofthe-art models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). We use frequency-wavenumber power spectra of the models and observations for Outgoing Longwave Radiation (OLR) and zonal winds at 250 hPa (U250), and consider the historical simulations and end of 21st century projections for the SSP245 and SSP585 scenarios.The models simulate a spectrum quantitatively resembling that observed, though systematic biases exist. MJO and Kelvin waves (KW) are mostly underestimated, while equatorial Rossby waves (ER) are overestimated. Most models project a future increase in power spectra for the MJO, while nearly all project a robust increase for KW and weaker power values for most other wavenumber-frequency combinations, including higher wavenumber ER. In addition to strengthening, KW also shift toward higher phase speeds (or equivalent depths). Models with a more realistic MJO in their control climate tend to simulate a stronger future intensification.

Importance of Madden–Julian oscillation phase to the interannual variability of East African rainfall

“…Accordingly, the representation of the relationships detailed in this paper should be investigated in both forecast and climate models. For example, although modelling of both the MJO and EA rainfall has improved in CMIP6 models (MJO propagation especially), persistent biases such as underestimated MJO contributions to variability, and underestimated LR accumulations, remain (Ahn et al, 2020; Bartana et al, 2022; Le et al, 2021; Lyon, 2022). There are substantial uncertainties in climate change projections of the long rains (Rowell, 2019): ensuring that models represent the relationships observed here is an important step towards further reducing such uncertainties.…”

Tropical tropospheric warming pattern explained by shifts in convective heating in the Matsuno–Gill model

“…The static stability increases with warming, which makes $$$ \alpha $$$ around 18% larger based on calculations using values for the cold and warm PJ simulations. The strength of $$$ \alpha $$$ determines gravity wave speeds in the Gill model, and indeed there is evidence that gravity wave speeds and Kelvin wave speeds increase in CMIP6 models with warming (Bartana et al ., 2023). We acknowledge that this is an important consideration for the tropospheric temperature pattern; but given various other assumptions (see Section 2) and the strong variations in $$$ \epsilon $$$ throughout the experiments, the increase in static stability, and therefore $$$ \alpha $$$, is small in comparison.…”

Reproducibility of Equatorial Kelvin Waves in a Superparameterized MIROC: 1. Implementation and Verification of Blockwise‐Coupled SP‐MIROC

“…However, the ability of such schemes to diagnose cumulus activities is not perfect. In the tropics, where subgrid clouds have a particularly large influence on large-scale circulations, this imperfection leads to difficulties in reproducing prominent large-scale disturbances (Bartana et al, 2022;J.-L. Lin et al, 2006) such as the Madden-Julian Oscillation (MJO) and equatorial waves. Furthermore, most GCMs have a bias pattern in the surface precipitation commonly called "double-ITCZ," in which the intertropical convergence zone (ITCZ) in the south-eastern Pacific is excessively strong (J.-L. Lin, 2007).…”

“…However, the ability of such schemes to diagnose cumulus activities is not perfect. In the tropics, where subgrid clouds have a particularly large influence on large‐scale circulations, this imperfection leads to difficulties in reproducing prominent large‐scale disturbances (Bartana et al., 2022; J.‐L. Lin et al., 2006) such as the Madden‐Julian Oscillation (MJO) and equatorial waves.…”