*Result*: A Numerical Case Study of the Papagayo Jet Effect on East Pacific Easterly Wave Development.

*This study investigates the development of an east Pacific easterly wave (EW) event in August 2019 and the effect of the Papagayo jet on its evolution. Convective‐permitting numerical simulations were conducted. These included a control experiment and a Gap‐Filled experiment with a closed mountain gap near the Gulf of Papagayo, to test the effect of a weaker Papagayo jet on this EW event. The initial disturbance developed into a mesoscale convective system in the Panama Bight and transitioned into an EW in the Papagayo jet exit region, both of which are reasonably captured in the control experiment. The weakened Papagayo jet in the Gap‐Filled experiment reduces the meridional shear of the zonal wind and the associated barotropic energy extraction from the mean flow. This results in significantly weakened upward motion and vertical stretching near the center of the disturbance, thus weakening vorticity particularly at low levels below 700‐hPa in the jet region. The weaker Papagayo jet and diminished low‐level vorticity in the Gap‐Filled experiment can further limit the subsequent EW vorticity intensity as it travels northwest along the Mexican coast. However, the Papagayo jet appears to have a limited impact on the horizontal vorticity structure and propagation of the EW disturbance. These results suggest that the broad‐scale monsoonal westerlies south of the Papagayo jet may help maintain the low‐level horizontal wind shear, even when the Papagayo jet is weakened, thereby contributing to the formation of the EW's tilted vorticity structure. Plain Language Summary: Easterly waves (EWs) are prominent tropical convective systems in the east Pacific that can lead to heavy rainfall and sometimes trigger tropical cyclones. This study investigates an EW event that developed in the east Pacific Ocean in August 2019. We explore the role of a strong low‐level easterly wind known as the Papagayo jet, which crosses a mountain gap in Central America and has an important effect on the development of EWs. We conduct a series of numerical simulations to test what happens when this jet is weakened by blocking the mountain gap in the jet region. We find that the weakened Papagayo jet makes the EW disturbance less intense, with the weakening especially evident in the lower troposphere. However, the Papagayo jet has limited impact on the horizontal vorticity structure and propagation of the EW disturbance. This research helps improve our understanding of how regional background wind patterns influence tropical weather systems. Key Points: A weaker Papagayo jet reduces meridional shear of the zonal wind and inhibits low‐level interactions with the easterly wave in the modelA weaker Papagayo jet and reduced low‐level vorticity further limit the easterly wave's intensity along the Mexican coast in the modelThe Papagayo jet has a limited impact on the horizontal vorticity structure and propagation of the easterly wave [ABSTRACT FROM AUTHOR]

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