WHOI PO

Title: Wave Goodbye to Stability: How the Madden-Julian Oscillation, Warmer Waters, and Easterly Waves Shake Up Climate

Abstract: Easterly waves (EWs) are one of the precursors to tropical storms, which are among the deadliest and costliest extreme weather events due to their coastal hazards and flood risks. Other disturbances, such as the Madden-Julian Oscillation (MJO), make conditions favorable for tropical cyclogenesis by creating periods of anomalous westerly winds that are associated with enhanced cyclonic flow, moisture, precipitation, and EW activity. Our previous work has shown that the MJO is expected to become stronger in a warmer environment, which could cause its modulation of EWs to become stronger with increased effects on vulnerable communities. Here, we use the Community Earth System Model (CESM2) Large Ensemble (LENS) to examine the frequency and intensity of African EW (AEWs) and their modulation by the MJO during the boreal summer, including future projections to 2081-2100. While results indicate a general decrease in EW activity change across the tropics in a warming climate, they do not reveal how the MJO modulates AEWs even though MJO activity increases. This pattern becomes clearer with the use of the composite method, which reveals how these stronger MJO amplitudes in a warmer climate could influence modulation of AEWs anomalies. The MJO shifts toward the central Pacific, with more time spent in either the inactive or strong phase, at the expense of time spent in moderate MJO amplitudes. Using the compositing method, we show that the modulation of AEWs by the MJO appears to strengthen in a warmer climate by enhancing positive AEW anomalies during phases 2–4 and intensifying negative anomalies during phases 6–8, which has potentially important consequences for future subseasonal prediction of tropical cyclone activity. As future climate change can amplify the risk to life and property for vulnerable communities, this research advances the knowledge and projections required to address these emerging risks in a warming climate.