East Pacific easterly waves in the CESM2 large ensemble: Present-day characteristics and projected future changes

This study investigates the characteristics of east Pacific (EPAC) easterly waves (EWs) in ten ensemble members of the Community Earth System Model version 2 Large Ensemble (CESM2-LENS) under the current climate, as well as their projected changes by the end of the 21st century under the Shared Socioeconomic Pathway 3–7.0 (SSP370) warming scenario. Under the current climate, the CESM2 ensemble mean produces realistic summer climatological mean precipitation, horizontal winds, and EW-filtered vorticity variability over the EPAC, although the amplitude of EW filtered precipitation variability is underestimated relative to observations. CESM2 also realistically simulates the distribution of EW tracks identified using 700-hPa curvature vorticity in the Caribbean and EPAC, along with the horizontal structure, amplitude, and propagation characteristics of EW disturbances. However, CESM2 EWs exhibit a more top-heavy vertical velocity profile and an eastward-tilted vertical structure relative to ERA5, suggesting some biases in vertical dynamical processes.

In the future warming scenario at the end of the 21st century, the mean precipitation over the EPAC strengthens and shifts southward, accompanied by intensified low-level mean easterly winds in the Papagayo jet region, although the increase in EW-filtered precipitation variability in this region is modest. EW track density and anomalous vorticity amplitude decrease along the Central American coast, while increasing within the southwestern ITCZ region. Changes in static stability with warming and weaker upper-level vertical velocity per unit precipitation explain the weakened midlevel vorticity in EWs, indicating that CESM2-simulated EPAC EWs have moisture mode characteristics. The weakened dynamical signal of the EWs may limit convective activity, thus resulting in only modest increases in EW precipitation amplitude.