Atmospheric moisture recycling: Role of advection and local evaporation

Abstract

An approximate formulation of how much moisture that precipitates out comes from local evaporation versus horizontal transport, referred to as "recycling," has allowed new estimates of recycling to be mapped globally as a function of length scale. The recycling is formulated in terms of the "intensity of the hydrological cycle" I, which is alternatively referred to as a "precipitation efficiency" as it denotes the fraction of moisture flowing through a region that is precipitated out, and a "moistening efficiency," M, which is defined as the fraction of moisture evaporated from a region to that flowing through. While datasets of the pertinent quantities have improved, they still contain uncertainties. Results show that often the intensity is not greatest at times of greatest precipitation because moisture transport into the region is also a maximum, especially in the monsoonal regions. The annual cycle variations of I are fairly small over North America and Europe while large seasonal variations in M occur in most places. Seasonal mean maps of precipitation, evaporation (E), and atmospheric moisture transport are presented and discussed along with the seasonal and annual means of derived precipitation and moisture efficiencies and the recycling fraction. The recycling results depend greatly on the scale of the domain under consideration and global maps of the recycling for seasonal and annual means are produced for 500- and 1000-km scales that therefore allow the heterogeneity of the fields across river basins to be captured. Global annual mean recycling for 500-km scales is 9.6%, consisting of 8.9% over land and 9.9% over the oceans. Even for 1000-km scales, less than 20% of the annual precipitation typically comes from evaporation within that domain. Over the Amazon, strong advection of moisture dominates the supply of atmospheric moisture over much of the river basin but local evaporation is much more prominent over the southern parts, and, for the annual cycle as a whole, about 34% of the moisture is recycled. Over the Mississippi Basin, the recycling is about 21%. The smaller number mostly reflects the smaller domain size. Relatively high annual values of recycling (>20%) occur in the subtropical highs, where E is high and the advective moisture flux is small, and in convergence zones where, again, the advective moisture flux is small. Low annual values occur over the southern oceans, the North Pacific, and the eastern equatorial Pacific, where the moisture flux is at a maximum.