Modeling the Effect of Land Surface Evaporation Variability on Precipitation Variability. Part II: Time- and Space-Scale structure

Abstract

This is the second of a two-part article investigating the impact of variations of land surface evaporability on the interannual variability of precipitation. The first goal of this part is to analyze the relationship between the atmospheric internal variability and the evaporative forcings. The hypothesis that the sum of ocean- and atmosphere-induced variabilities can be linearly amplified by the land variability is critically revisited and generally found not applicable to the climate model used and the numerical experiments conducted. A set of parameters to evaluate the departure from the linear behavior is defined, quantifying the impact of the different forcings over the total variability. Some areas of the world (e.g., the monsoon region, the continental United States, and southeastern Africa), where the impact of internal atmospheric dynamics on precipitation variability is small compared to the impact of the evaporative forcings, are localized. Over these areas, the variability of precipitation might be more predictable, given a good knowledge of the surface boundary forcings. In the second half of this article the time structure of the land forcing is analyzed, to quantify the contributions of the interannual variations, diurnal cycle, and high-frequency (i.e., synoptic scale) variations and compare them with the contribution of the oceanic forcing. The general conclusion is that interannual variability of both sea surface temperature and land evaporability is very important to the overall variability of precipitation over the Tropics. Over land in the subtropics and midlatitudes equatorward of the polar front there are also substantial feedbacks at the interannual scale. The impact of synoptic-scale variations of land evaporability is generally smaller, except for some areas in the midlatitudes near the polar front, particularly continental Eurasia and parts of North America. Finally, there is no general, widespread evidence showing the importance of the diurnal cycle of evaporability to the interannual variability of precipitation. However, strong regional differences are detected, and some tropical areas, like the Congo basin, where the diurnal cycle does contribute to the interannual variability of precipitation are outlined.