A Lagrangian Perspective on the Atlantic and Pacific Precipitation‐Evaporation Asymmetry

Total precipitation minus evaporation (P − E) for the Atlantic is negative while it is approximately neutral for the Pacific. This has frequently been attributed to westward Atlantic‐to‐Pacific moisture flux across Central America. However, this Eulerian perspective has limited scope as it does not consider the origins of the water crossing ocean drainage basin boundaries and the possibility that it has remote sources. By using an airmass trajectory model, we take a Lagrangian approach to investigate the origin of the moisture contributing to fluxes, Qn, across these boundaries. Qn is partitioned into contributions from each basin, the stratosphere and trajectories not assigned an origin. The total Qn across each basin boundary are mainly composed of contributions from the two adjacent basins while remote or stratospheric origins make small contributions. Partitioning Qn shows that the atmosphere exports ∼1 Sv water vapor from the Atlantic, Indian and Pacific basins with a similar quantity imported to the Pacific. However, Atlantic and Indian atmospheric imports are ∼0.5–0.6 Sv. Normalizing by drainage basin perimeters reveals that the import to these basins is half as efficient as Pacific import. Partitioning P − E into contributions from other basins shows that Pacific moisture import is dominated by trajectories with Indian basin origin (∼38%, or 0.43 Sv, of total Pacific import). The import is greatest in boreal summer due to the Asian monsoon flow and stronger westward flux across Central America. These anomalies dominate the difference in annual imports between the Pacific, Atlantic and Indian basins. Plain Language Summary Precipitation and evaporation are approximately balanced across the Pacific Ocean drainage basin while the Atlantic and Indian basins have surpluses of evaporation. This has previously been explained by strong westward atmospheric moisture transport from the Atlantic across Central America to the Pacific in the trade winds. However, this hypothesis does not account for moisture that is transported from remote sources. We have used an airmass trajectory model to calculate pathways of moisture for 14 days prior to arrival at the ocean drainage basin catchment boundaries and determine the origins of moisture crossing the boundaries then partition the cross‐boundary moisture flux into contributions from each ocean basin. We find that most of the cross‐boundary flux is explained by contributions from the two basins directly adjacent to