Findings of a CPO-supported study published in the Journal of Advances in Modeling Earth Systems suggest researchers’ ability to use the Madden-Julian Oscillation (MJO) for medium-range weather forecasting may not increase under global warming.
Researchers of this study explained it is easy to assume global warming will lead to increased activity and thus improved mid-range weather forecasting. With an important influence in weather and climate, MJO’s behavior can be difficult to predict. This behavior (periods of high vs low activity) influences weather events at weekly and monthly scales.
This research was supported by the CPO Climate Variability and Predictability program and the Modeling, Analysis, Predictions, and Projections program.
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WTG balance is used to examine how changes in the moist thermodynamic structure of the tropics affect the MJO in two simulations of the Superparameterized Community Earth System Model (SP-CESM), one at preindustrial (PI) levels of and one where levels have been quadrupled (4×CO2). While MJO convective variability increases considerably in the 4×CO2 simulation, the dynamical response to this convective variability decreases. Increased MJO convective variability is shown to be a robust response to the steepening vertical moisture gradient, consistent with the findings of previous studies. The steepened vertical moisture gradient allows MJO convective heating to drive stronger variations in large-scale vertical moisture advection, supporting destabilization of the MJO. The decreased dynamical response to MJO convective variability is shown to be a consequence of increased static stability, which allows weaker variations in large-scale vertical velocity to produce sufficient adiabatic cooling to balance variations in MJO convective heating. This weakened dynamical response results in a considerable reduction of the MJO’s ability to influence the extratropics, which is closely tied to the strength of its associated divergence. A composite lifecycle of the MJO was used to show that northern hemisphere extratropical 525 hPa geopotential height anomalies decreased by 27% in the 4×CO2 simulation, despite a 22% increase in tropical convective heating associated with the MJO. Results of this study suggest that while MJO convective variability may increase in a warming climate, the MJO’s role in “bridging weather and climate” in the extratropics may not.