Abstract:

Dust emitted from desert surfaces on Earth impacts climate and weather by affecting cloud formation and interacting with solar and terrestrial radiation. It also carries important nutrients that feed land and ocean ecosystems. Therefore, it is important to accurately predict for how long these particles remain suspended in the atmosphere. However, current climate models underestimate this time and the associated horizontal transport. For example, it is still not well understood how coarse dust particles are able to move from the Sahara desert to the Americas. This transport happens in the so-called Saharan Air Layer (SAL), a nearly well-mixed dust-laden air layer that originates in the Saharan boundary layer over the African continent, which subsequently moves over the Atlantic Ocean in the upper atmosphere (above about 2 km from the ocean surface), reaching the Americas about 5 days later.

Using both a theoretical approach and Large-Eddy Simulations (LES), we show that vertical mixing caused by turbulence in the SAL can even double the airborne lifetime of dust particles, and is potentially one of the mechanisms responsible for the transatlantic transport of coarse dust particles. Informed by observations of potential temperature and wind speed, we developed a simple model for the mixing rates and the associated increase in dust airborne lifetime as a function of typically observed variables such as wind shear, buoyancy stratification and layer depth. Under conditions of strong stable stratification, mixing rates are reduced, and other mechanisms such as deep convection are expected to contribute more to the long-range transport of coarse particles.