Exploring the effects of the land surface on tropical precipitation

Projected precipitation changes over tropical land tend to be enhanced by vegetation responses to CO₂ forcing in Earth System Models. Projected decreases in rainfall over the Amazon basin and increases over the Maritime Continent are both stronger when plant physiological changes are modelled than if these changes are neglected, but the reasons for this amplification remain unclear. The responses of vegetation to increasing CO₂ levels are complex and uncertain, but changes in stomatal conductance likely dominate the evapotranspiration response in Earth System Models.

We investigate why vegetation changes cause precipitation to increase more strongly over the Maritime Continent while decreasing more strongly over the Amazon basin. We employ an idealized Atmospheric General Circulation Model with a simplified vegetation scheme that captures CO₂-driven stomatal closure.

We find that – counter-intuitively – rainfall is enhanced over a narrow rectangular island when terrestrial evaporation falls to zero with high CO₂. Strong heating and ascent over the island trigger moisture advection from the surrounding ocean. In contrast, over larger continents rainfall depends on continental moisture recycling.

Simulations with two large rectangular continents representing South America and Africa reveal that the stronger decrease in rainfall over the Amazon basin is due to a combination of local and remote effects:

Finally, we investigate the impact of land-surface hydrology on continental rainfall on seasonal timescales. Using our idealized model and realistic continents, we study the strength of the South East Asian monsoon for different continental evaporation schemes. Surprisingly, when terrestrial evapotranspiration is unlimited (i.e. does not depend on soil moisture availability), monsoon precipitation is much weaker than when terrestrial evapotranspiration is limited by soil moisture. In order to explain this behavior, we compare the atmospheric energy budgets and circulation between the simulations.

Our results show that the land-surface hydrology plays an important role in modifying tropical precipitation and atmospheric dynamics on seasonal timescales and in the long-term under climate change, and that further investigation into the topic is called for.