Easterly wave contributions to seasonal rainfall over the tropical Americas in observations and a regional climate model

Easterly waves (EWs) are important moisture carriers and their variability can impact the total May-November rainfall, defined as seasonal precipitation, over the Tropical Americas. The contribution of EWs to the seasonal precipitation is explored over the tropical Americas using rain gauge stations, reanalysis data and a regional model ensemble during the 1980-2013 period. In the present study, EWs are found to produce up to 50% of seasonal rainfall mainly over the north of South America and contribute substantially to interannual regional rainfall variability. An observational analysis shows that the El Nino Southern Oscillation (ENSO) affects EW frequency and therefore, their contribution to seasonal rainfall. In recent years, tropical cyclone (TC) activity over the Main Development Region (MDR) of the tropical North Atlantic has a negative impact on regional seasonal precipitation over northern South America. High TC activity over MDR corresponds to below-normal precipitation because it reduces the EW activity reaching northern South America through the recurving of TC tracks. Recurving TC tracks redirect moisture away from the tropical belt and into the mid-latitudes. However, this relationship only holds under neutral ENSO conditions and the positive phase of the Atlantic Multidecadal Oscillation. A 10-member regional model multi-physics ensemble simulation for the period 1990-2000 was analyzed to show the relationships are robust to different representations of physical processes. This new understanding of seasonal rainfall over the tropical Americas may support improved regional seasonal and climate outlooks.