A Lagrangian Analysis of the Atmospheric Branch of the Global Water Cycle. Part II: Moisture Transports between Earth’s Ocean Basins and River Catchments

Abstract A diagnostic Lagrangian method to trace the budgets of freshwater fluxes, first described in Part I of this article, is used here to establish source–sink relationships of moisture between earth’s ocean basins and river catchments. Using the Lagrangian particle dispersion model FLEXPART, driven with meteorological analyses, 1.1 million particles, representing the mass of the atmosphere, were tracked over a period of 4 yr. Via diagnosis of the changes of specific humidity along the trajectories, budgets of evaporation minus precipitation (E − P) were determined. For validation purposes, E − P budgets were calculated for 39 river catchments and compared with climatological streamflow data for these rivers. Good agreement (explained variance 87%) was found between the two quantities. The E − P budgets were then tracked forward from all of earth’s ocean basins and backward from the 39 major river catchments for a period of 10 days. As much previous work was done for the Mississippi basin, this basin was chosen for a detailed analysis. Moisture recycling over the continent and moisture transport from the Gulf of Mexico were identified as the major sources for precipitation over the Mississippi basin, in quantitative agreement with previous studies. In the remainder of the paper, global statistics for source–sink relationships of moisture between the ocean basins and river catchments are presented. They show, for instance, the evaporative capacity of monsoonal flows for precipitation over the Ganges and Niger catchments, and the transport of moisture from both hemispheres to supply the Amazon’s precipitation. In contrast, precipitation in northern Eurasia draws its moisture mainly via recycling over the continent. The atmospheric transport of moisture between different ocean basins was also investigated. It was found that transport of air from the North Pacific produces net evaporation over the North Atlantic, but not vice versa. This helps to explain why the sea surface salinity is higher in the North Atlantic than in the North Pacific, a difference thought to be an important driver of the oceans’ thermohaline circulation. Finally, limitations of the method are discussed and possible future developments are outlined.