The North American monsoon macrosystem: A case study in the exploration of new ecological scales

Background/Question/Methods

We are studying the internal and external forcings associated with the North American Monsoon System, which provides annually-recurring climatological and ecological coherency to a large region in northwestern Mexico and the southwestern US.  The spatial and temporal distribution of summer rainfall synchronizes ecological processes within this ‘macrosystem’.  Climate and ecological processes of the macrosystem are covered by three NEON Observatory sites.  In order to study interactions among climate and ecology in this system we are developing high-resolution, regional land-atmosphere models, ecophysiological models, and an extensive past climate chronology using tree-ring analyses.  In terms of climate drivers, we have focused on model interactions and feedbacks among surface greening, decreased albedo, increased evapotranspiration and the regional redistribution of precipitation.  In terms of ecological responses to those drivers we have focused on topographic redistribution of water at the watershed scale, with associated influences on the spread of summer-active, invasive C4 grasses.  In terms of validating regional climate model predictions and extension of the predictions into past time, we have focused on the use of early- and late-wood portions of the annual tree ring record in Ponderosa pine, including resolution of signals in the stable isotope composition of cellulose.

Results/Conclusions

Low level moisture from the Gulf of California feeds the monsoon macrosystem as it develops annually and moves northward from Mexico, while upper level moisture is provided from the Gulf of Mexico.  Oceanic origins appear to dominate early monsoon precipitation, while the recycling of moisture from land surface evapotranspiration is more important as the system develops.  Studies to date with dynamic vegetation in a regional climate model have revealed a positive feedback on monsoon activity – enhancing precipitation recycling and driving the seasonal northward development of the macrosystem.  We are validating these model projections using a suite of satellite land surface products to provide insight into temporal and spatial covariance in surface greening and monsoon activity, and to delineate interannual variation in macrosystem boundaries at high spatial resolution.  Deployment of the ecophysiological/demographic model ECOTONE and the soil hydrology model CATHY, at the watershed scale have shown that the local distribution of monsoon moisture has the potential to positively influence the spread of summer-active invasive grasses.  Tree ring analyses have shown that the late-wood signal from Ponderosa pine is capable of validating the spatiotemporal distribution of monsoon precipitation, and thus macrosystem boundaries, during the past 200-300 years.