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The Atlantic-to-Pacific atmospheric freshwater transport was calculated using Lagrangian water mass trajectories. These were decomposed into eastward and westward moving classes, carrying water over Afro-Eurasia and over America, respectively. The results reveal that the midlatitude westerlies are contributing to midlatitude precipitation in the Pacific Ocean through transporting water mass from the midlatitude Atlantic Ocean over Afro-Eurasia. In addition, precipitation in the Eastern Tropical Pacific Ocean is found to be associated with the easterly winds carrying water mass from the tropical Atlantic Ocean. A quantitative analysis of the atmospheric freshwater transport furthermore shows that annually, the westerlies carry 0.40 Sv, approximately twice as much water as the easterly trade winds (0.26 Sv) to the Pacific Ocean, but with a strong seasonality. The Atlantic Ocean exports more freshwater across Afro-Eurasia than across America, except during the June-August periods. The average residence time of this atmospheric water transport is roughly twice as long when it crosses Afro-Eurasia (54 days) rather than America (24 days).

Plain Language Summary The Pacific Ocean is less saline than the Atlantic Ocean. This salinity difference feeds the Conveyor Belt, which transfers warm water from the Pacific Ocean to the Atlantic as a shallow current and returns cold water from the Atlantic to the Pacific as a deep current that flows further south. One explanation of this salinity difference is that the evaporation dominates over precipitation in the Atlantic and vice versa for the Pacific. This salinity asymmetry has often been believed to be due to the westward directed winds carrying the moisture over America. However, in the present study we show that the moisture transport by the eastward directed winds over Afro-Eurasia is also important. The present study reveals that the moisture export from the midlatitude Atlantic to the midlatitude Pacific Ocean over Afro-Eurasia by the eastward directed winds is approximately twice as large as the moisture export from the tropical Atlantic Ocean to the Eastern Tropical Pacific Ocean across America by the westward directed winds, except during the June-August periods. The moisture carried over Afro-Eurasia by the eastward directed winds from the Atlantic to the Pacific Ocean tends to take place at higher altitude and remains longer in the atmosphere than the moisture carried by the westward directed trade winds.