Tropical land-to-ocean precipitation differences explained in the framework of the moisture-precipitation relationship

The highly nonlinear moisture-precipitation relationship P(r) is a simple statistical model for tropical precipitation P that takes column relative humidity r as its only input variable. Its simplicity and robustness make the relationship useful for conceptual models and as a tool for model diagnostics. We use 10 years of daily ERA5 Reanalysis data to test if P(r) can reproduce the tropical land-ocean contrast of precipitation which we quantify by the tropical precipitation ratio χ(t), defined as the ratio between the spatiotemporal mean rain rate over land and ocean. We find that P(r) can adequately reproduce both magnitude and phase of the average seasonal cycle of χ(t) as long as we take into account that P(r) gets modified by the presence of land and that the relationship varies seasonally over both land and ocean. Since the values of χ(t) indicate that precipitation is enhanced over tropical land, we investigate in a second step whether this enhancement is explained by the distinct P(r) relationships over land and ocean, i.e. whether it rains more over land than over ocean for a given value of r, or by distinct land and ocean humidity distributions, or both. Our results show that the influence of the land surface on P(r) has the effect of disfavoring precipitation over land rather than enhancing it. Precipitation enhancement over land, thus, stems from the distinct humidity distributions over land and ocean with the land distribution exhibiting a more pronounced tail towards high r values compared to the ocean distribution.