The formation of “atmospheric rivers” – thin corridors that transport moisture away from the tropics – may, in some cases, be linked to coherent structures that occur in Earth’s large-scale wind patterns. That is the conclusion of a team of scientists in Spain and Switzerland. The researchers believe that their work could help to improve the identification and classification of atmospheric rivers, and better predict which ones will lead to extreme weather conditions.
The movement of water vapour from the Earth’s tropics to higher latitudes is an intermittent process, with 90% of moisture occurring in the form of atmospheric rivers. These are narrow corridors of moisture – each typically a few thousand kilometres long but only 400–600 km wide – that often carry more water than the Amazon river. There are usually four to five atmospheric rivers present in each hemisphere at any given time. A famous example is the “Pineapple Express”, which carries moisture from Hawaii to the west coast of North America.
When such a river makes landfall, the resulting precipitation can be extreme, with the largest causing sizeable floods. Despite both their impact on human activity and importance to Earth’s water and heat cycles, however, the formation of these structures is poorly understood.
Dynamic skeletons
Recently, the concept of Lagrangian coherent structures has emerged as a way of looking at transport within large-scale fluid flows. These structures are distinct surfaces of trajectories in a flow, and form the basic skeleton of the larger dynamic system. The structures last long enough to form separate areas of the fluid, each with distinct transport properties. Studying these structures has furthered our understanding of a variety of fluid flows, including clouds of volcanic ash, blooms of plankton and offshore oil spills.
The Lagrangian coherent structures serve as a kind of temporary scaffolding around which an atmospheric river can grow and lengthen
Vicente Pérez-Muñuzuri,University of Santiago de Compostela
“Given that atmospheric rivers over the Atlantic and Pacific oceans appear as coherent filaments of water vapour lasting for up to a week, and that Lagrangian coherent structures have turned out to explain the formation of other geophysical flows, we wondered whether Lagrangian coherent structures might somehow play a role in the formation of atmospheric rivers,” says team member Vicente Pérez-Muñuzuri, a physicist at the University of Santiago de Compostela.
To test this idea, Pérez-Muñuzuri and colleagues studied data on the wind-speed and water-vapour flux in atmospheric rivers running over the Atlantic from the Caribbean to the Iberian Peninsula. These data were compared with the predictions of a computer model that simulated the movement of thousands of air particles.
The results revealed a close similarity between the Lagrangian coherent structures formed in the simulations and the patterns of the atmospheric rivers that form over the Atlantic in the winter months. “The Lagrangian coherent structures serve as a kind of temporary scaffolding around which an atmospheric river can grow and lengthen,” says Pérez-Muñuzuri, with the wind field forming shear, filamentous jets that act as a transport barrier, separating regions of strong and weak horizontal moisture flow.
In contrast, strong Lagrangian coherent structures were not found to be associated with the shorter, less-defined and short-lived atmospheric rivers that typically form in the summer. The researchers suggest that in these rivers the water-vapour balance could be dominated by local sources. Team member Daniel Garaboa explains that the stronger and more persistent winds that occur in the winter lead to more homogeneous moisture-transport patterns.
“This new work has important implications for the definition and detection of atmospheric rivers,” comments Bin Guan, a climate physicist at NASA’s Jet Propulsion Laboratory in California who was not involved in the study. Guan explains that, as atmospheric rivers can have different shapes and moisture loads, it has previously proved challenging to develop universal criteria for defining these features. A better understanding of how the rivers form – and a classification of their types – may help to predict which types lead to extreme meteorological conditions.
The study is described in the journal Chaos.
