Precipitation enhancement over tropical land through the lens of the moisture–precipitation relationship

Tropical precipitation P has been found to be related to column relative humidity r by a simple relationship known as the moisture–precipitation relationship P(r). Based on one decade of daily ERA5 reanalysis data, we test whether P(r) is able to reproduce the tropical land–ocean precipitation contrast measured by 𝜒, the ratio between mean precipitation over land and ocean. We find that
captures the mean seasonal cycle of 𝜒 as long as we account for the fact that P(r) is distinct over land and ocean, and that it varies seasonally. Typical values of 𝜒 above 0.86 imply that precipitation is enhanced over land, relative to the ocean. We therefore investigate next whether this enhancement is due to the differences in P(r) and/or in the humidity distribution between land and ocean. We show that, rather than enhancing precipitation, the presence of land modifies P(r) in such a way that precipitation over land is disfavored compared to over ocean. Precipitation enhancement over land is instead explained by the modified terrestrial humidity distribution that features a more pronounced tail towards high r values compared to the one over ocean. All results rest on an accurate construction of P(r) from the underlying data. Simple fit models such as an exponential function that were proposed by previous studies are unable to capture the seasonal cycle of 𝜒 and fail to explain land–ocean differences in precipitation.