Illustrating the Land-to-Ocean Evolution of African Easterly Waves using Moist Static Energy Budgets

African Easterly Waves (AEWs) play a crucial role as precursors to tropical cyclones in the Atlantic and Eastern Pacific Oceans, exhibiting remarkable longevity of up to 2-3 weeks as they traverse the Atlantic. Originating over the Sahel region of Africa, these disturbances encounter distinct atmospheric conditions, including a mid-level easterly jet and relatively strong (for the tropics) meridional temperature and potential vorticity gradients.

While hydrodynamic instability contributes to the development of AEWs over land, the ocean presents weaker background temperature and potential vorticity gradients and lacks the jet-induced hydrodynamic instability. This prompts essential questions: How does the transition from land to ocean influence AEWs, and what sustains their extended lifespan, particularly in reaching the eastern Pacific?

Our study addresses these questions by proposing that AEWs undergo fundamental structural changes during the land-to-ocean shift, transitioning from a quasigeostrophic-mode to a moisture mode. This hypothesis is examined through an analysis of dry and moist static energy (MSE) budgets, contrasting their behavior over land and ocean. Initial findings, based on ERA5 reanalysis data, revealed substantial structural differences between these two locations. Over land, changes in MSE were primarily influenced by background zonal advection while over ocean, wave-scale advections drove changes in MSE. The next phase involves using the Weather Research and Forecasting (WRF) model to replicate these budgets and evaluate the impact of varied atmospheric conditions.