Unravelling the origin of the atmospheric moisture deficit that leads to droughts

Drought is the main natural hazard at the planetary scale and although this is a very complex phenomenon that involves many aspects of the hydrological cycle, there is always a deficit of precipitation compared to usual, understanding usual as climatological. This deficit can occur essentially for three reasons, either because there is less moisture available in the air column or because there is less atmospheric instability that forces air to rise, or for both reasons simultaneously. As the existing local humidity in the air column is mostly insufficient to justify precipitation, less humidity available for precipitation implies a deficit in the moisture which reaches the site in question. Therefore, on a global scale, moisture transport deficits lead to the occurrence of droughts.

In a first approximation, this humidity can have two origins, or comes directly from the ocean, or is subsequently recycled from the continents themselves. The processes that control the evaporation over oceans or the continents as well as the moisture transport are very different, and there is a variable relationship between the oceanic and terrestrial origin of precipitation both globally and regionally. In a second approximation, the main sources of humidity at a global level are those regions where evaporation greatly exceeds precipitation over them, which mainly occurs in the subtropical oceans, some inland seas, and the two continental areas known as green oceans, the Amazon and the Congo basins.

It is known where the humidity coming from the whole ocean or the whole continent precipitates, as well as the sinks of the humidity that comes from these large individual sources. It has also been studied how anomalous moisture transport affects droughts in specific regions, but the probability of occurrence of droughts at a planetary scale on continental areas given a deficit of the moisture transported from the global oceanic area, the global continental area, and each of these major sources has not been fully evaluated. Here we make use of a Lagrangian approach widely used and checked, which consists of estimating how much precipitation comes from the humidity arriving from a specific moisture source and enables to reveal the origin of the atmospheric moisture deficit underlying the occurrence of droughts.