Mechanisms Behind the Long-Distance Diurnal Offshore Precipitation Propagation in Northwestern South America

Northwestern South America (NWSA) is the rainiest region on Earth, with diurnal precipitation exhibiting extensive westward offshore propagation of up to about 1,200 km in boreal spring (March-May). The diurnal offshore precipitation propagation begins slowly (3–10 m s⁻¹) near the coast of NWSA (<200 km) but accelerates significantly (∼20 m s⁻¹) and shows an afternoon enhancement far from the coast (>400 km). However, the driving mechanisms behind this long-distance precipitation propagation remain unclear. Using a new cloud tracking and classification data set, we found that mesoscale convective systems (MCSs) are the dominant precipitation contributors in the offshore region of NWSA. Cloud tracking shows that the long-distance propagation and the afternoon enhancement of diurnal precipitation primarily originate from MCSs initiated in the early morning, either over open oceans or from the coast of Central America. Composite tendency analysis shows that MCSs initiated near the coast of Central America have significant upward cooling and moistening signals starting from the surface before initiation. Further analysis of surface diurnal perturbation fields indicates that the land breeze is the primary driving mechanism for MCS initiation. Conversely, for MCSs initiated over open oceans, a significant downward cooling signal from 400 hPa is observed ∼7 hr before initiation, corresponding to the passage of diurnal gravity waves emitted from the Andes. Additionally, our findings highlight the critical role of lower and mid-level moisture conditions in MCS initiation, alongside the influence of gravity waves.