Projected Future Changes in The 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-of-the-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 velocity at 250 hPa (U250), and consider the historical and end-of-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. The models project a moderate future increase in power for the MJO, a robust increase for Kelvin waves (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 intensification, and models with a more realistic KW in their control climate tend to simulate a weaker intensification.

Role of tropical lower stratosphere winds in quasi-biennial oscillation disruptions

“…As the Kelvin wave activity in the troposphere is expected to increase under higher CO 2 (fig. S16) ( 34 ), stronger wave forcing can be exerted on the shear zone.…”

“…Last, the strengthening of the westerly winds in the equatorial lowermost stratosphere is also important to understand a typical QBO in the future, as the enhanced westward flux entering the stratosphere due to the westerly acceleration ( 8 ) may contribute to the fast QBO phase progression rate. In a future climate, both wave sources in the troposphere and tropical upwelling in the stratosphere are expected to increase ( 28 , 34 ). Considering those factors along with the strengthened westerlies in the lowermost stratosphere will shed light on the future behavior of the QBO.…”