Abstract/Summary

The contrast between basin-integrated precipitation minus evaporation (P − E) of the Atlantic and Pacific Oceans is about 0.4 Sv (1 Sv ≡ 109 kg s−1 ). The Atlantic is net evaporative and the Pacific near neutral - a result consistent across datasets. This asymmetry has been linked to the higher sea surface salinity (SSS) in the Atlantic than in the Pacific (at all latitudes) and the absence of deep-water formation in the Pacific. This thesis focuses on the interbasin P − E contrast and the atmospheric moisture transports associated with it. Here, it is shown that the P −E asymmetry is primarily a result of greater Pacific precipitation per unit area south of 30◦N, with greater Atlantic evaporation per unit area north of 30◦N making a smaller contribution. Comparing the atmospheric moisture fluxes across the catchment boundaries of the ocean drainage basins (the integral of which is equal to basin-integrated P −E) shows that anomalous eastward fluxes across South-East Asia, in contrast to westward fluxes across other boundaries, dominate the annual mean P − E asymmetry, rather than the flux across Central America as is often stated. Using an airmass trajectory model it is shown that the moisture fluxes across catchment boundaries are dominated by contributions from the neighbouring basins. This also reveals that the atmosphere imports moisture to the Pacific more efficiently than to the Atlantic/Indian basins. Backward trajectories with Indian Ocean origin are associated with net precipitation of 0.43 Sv across the Pacific. The Somali low-level jet and Asian Summer Monsoon dominate the P − E asymmetry by diverting trajectories away from Africa into westerly flow across India towards the Pacific. This causes the anomalous eastward fluxes across South-East Asia and therefore the strong Pacific precipitation south of 30◦N. These results highlight the importance of continental geometry and steady large-scale circulation features in the mean state of the P − E asymmetry.