EGU24-15819, updated on 14 Mar 2024
EGU General Assembly 2024
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

Influence of Subtropical Jets on the Equatorial Spectrum: implications for future changes in Kelvin waves and MJO variance

Hagar Bartana1, Chaim Garfinkel1, Chen Schwartz1, Ofer Shamir2, and Jian Rao3
Hagar Bartana et al.
  • 1Hebrew University of Jerusalem, Institute of Earth Sciences, Atmospheric Science, Israel
  • 2Courant Institute of Mathematical Sciences, Center for Atmosphere-Ocean Science, New York, USA
  • 3Nanjing University of Information Science and Technology, Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environment Change (ILCEC)/Collaborative Innovation Center on Forecast
Projected changes of tropical Convectively Coupled Equatorial Waves (CCEWs)  due to increased greenhouse gases, and the dynamical mechanism that forces those changes, are investigated in 13 state-of-the-art models from phase 6 of the Coupled Model Intercomparison Project (CMIP6).   The equatorial wave spectrum in the historical simulation from most of these models quantitatively (and even quantitatively) resembles that observed, a remarkable improvement from CMIP5. Most models project a future increase in power spectra for the Madden-Julian Oscillation  (MJO), while nearly all project a robust increase for Kelvin Waves (KW). In contrast, the power spectrum weakens for most other wavenumber-frequency combinations, including higher wavenumber Equatorial Rossby waves (ER). In addition to strengthening, KW also shift toward higher phase speeds (or equivalent depths). These changes are even more pronounced in models with smaller biases in their historical simulations.

The qualitatively different projected response of the different CCEWs (e.g., KW strengthening vs. ER weakening) suggest that dynamical forcings have an important role in the physical mechanism of the changes. This hypothesis is tested using targeted simulations of the Model of an Idealised Moist Atmosphere (MiMA) in which we impose perturbations in upper-troposphere zonal winds mimicking projected end-of-century changes in wind. These simulations demonstrate that future changes in KW and the MJO strongly depend on changes in the South Pacific subtropical jet. A similar dependence is also evident in the CMIP6 models. However, the winds in the south Pacific subtropical jets in the historical simulation of these CMIP6  models  are highly biased, and models with a stronger change in the jet tend to project a stronger intensification of both the MJO and KW. It is therefore necessary to account for this bias in the subtropical winds in order to provide more reliable projections of the KW and the MJO.

How to cite: Bartana, H., Garfinkel, C., Schwartz, C., Shamir, O., and Rao, J.: Influence of Subtropical Jets on the Equatorial Spectrum: implications for future changes in Kelvin waves and MJO variance, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15819,, 2024.