Land sources supply most rain on the northern Tibetan Plateau through a moisture relay

Why it matters

Where Tibetan Plateau rain comes from has been debated, and many views stress ocean moisture. This study suggests that how upstream land splits moisture between rain and re-evaporation controls inland rainfall. The authors say climate projections should represent upstream land evaporation better.

What they did

The authors ran the CESM1 climate model with water tracers that tag moisture by where it evaporated. They split the globe into 17 source regions. The model was nudged to MERRA-2 weather fields and used observed sea surface temperatures for 2003–2021. They checked the results against satellite water-vapor isotope data, Lagrangian moisture-flow data and 7-day backward trajectories. They also compared wet and dry years in the southern and northern plateau.

Key findings

• Land sources give 78.9 ± 12.1% of summer and 68.4 ± 10.0% of winter rain over the northern plateau. Western Europe, Central Asia, Northern Africa and the plateau’s own evapotranspiration are the main ones.
• Southern plateau summer rain gets 33.7 ± 1.8% from the Indian Ocean, but land sources still supply 59.7 ± 6.4%.
• Ocean moisture, such as from the North Atlantic, often falls as rain over Western Europe, Central Asia and Northern Africa first. It then evaporates again and travels on to the plateau. The authors call this a terrestrial moisture relay.
• In wet northern summers, stronger advection and convection of Western Europe–Central Asia moisture raise convective rain. In wet northern winters, Northern Africa moisture is advected more strongly and condenses into more large-scale rain.
• Lagged correlations between upstream evapotranspiration and plateau rain support the relay idea.

Limitations

• Results depend on a nudged model with prescribed sea surface temperatures, which partly suppresses ocean–atmosphere feedback. Biases remain over complex terrain and monsoon areas.
• The broad source regions may affect how much is credited to each one. The analysis does not resolve runoff, infiltration or groundwater, and the model is too coarse to resolve cloud-scale processes.
• Moisture that re-evaporates is credited to the land region, not its ocean origin. Whether the relay applies to other inland regions is untested.

Glossary

• Eulerian water tracer: A model method that tags water by where it evaporated and follows it through vapor, cloud and rain.
• Moisture relay: Ocean moisture falls as rain over land upwind, evaporates again, and moves on downstream.
• Precipitation efficiency: The share of tagged water vapor in a column that turns into surface rain.
• δD: The deuterium isotope ratio in water vapor, used as a clue to where moisture came from.