Beating of the Amazon: The Diurnal Cycle of Amazonian Hydroclimate

Abstract:

Between 20 and 40% of the precipitation that falls over the Amazon basin comes from transpiration from the dense canopy and evaporation from soils within the forest itself. This is traditionally known as recycled precipitation. Water vapor tracers embedded within the Weather Research and Forecasting (WRF) atmospheric model allow us to “tag” the moisture that originates from the Amazonian basin and track it in space and time as it moves through the atmosphere and eventually rains out of the atmospheric column. This tool allows us to analyze detailed processes that occur at much smaller spatial and temporal scales than previously studied.

Analysis of the results led us to the discovery of a characteristic “beating” of the atmospheric moisture of Amazonian origin. Water vapor originating from Amazonian ET ebbs and flows with a characteristic diurnal pattern. The characteristic timescale of total water vapor is dominated by the annual cycle, while the tracer water vapor (the water vapor of Amazonian origin) doesn’t show a predominant annual signal but shows a very important diurnal cycle. The characteristic timescales of evapotranspiration, precipitation and moisture transport by winds drive the variability of tracer water vapor. As expected, evapotranspiration shows the strongest diurnal cycle following the atmospheric evaporative demand and photosynthetic activity of plants. Precipitation also shows a strong diurnal cycle as convection peaks in the late afternoon. Finally, advection of moisture in the column by winds (or integrated vapor transport) also shows a strong diurnal cycle because the winds peak at night. Easterly winds from the Atlantic Ocean traverse the Amazon basin until they encounter the Andes mountains and veer south, the winds are strongest at night when the boundary layer collapses. Conceptually, tracer water in the air column increases as evapotranspired moisture accumulates during the day, then some of this water is rained out through convective precipitation and another part of it is “swept away” by strong nocturnal winds. The cycle then repeats the following day. These processes come together to form a “beating” pattern that characterizes atmospheric moisture of the Amazon forest.