The influence of land-atmosphere moisture coupling on net ecosystem exchange and fire responses to forcing by ENSO (Invited)

Monday, December 9, 2019 - 13:40
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For the full duration of the Scripps Mauna Loa atmospheric CO2 record, El Niño events have created consistent, massive perturbations to the global carbon cycle. Shifts in the Walker Circulation reduce precipitation over multiple tropical continents, increasing drought stress and the mortality of tropical forest trees. Simultaneously, tropical land surface air temperatures increase, reducing the efficiency of photosynthesis and increasing respiratory costs for autotrophs and heterotrophs. The relative importance of drying and warming during different El Niño Southern Oscillation (ENSO) phases as drivers of carbon cycle variability has received considerable attention in the literature, yet significant uncertainties remain. Here we describe mechanisms by which land-atmosphere coupling intensifies El Niño-driven warming, with implications for the magnitude of net ecosystem exchange (NEE) and fire responses. Decreases in precipitation during the wet season in many tropical forests during El Niño reduce the build-up of soil moisture, which in turn limits evapotranspiration (ET) during the following dry season. In addition to direct effects on plant stress, lower ET increases surface air temperature and lowers surface relative humidity. Within the E3SM Earth system model, these regional-scale meteorological changes in the Amazon intensify and prolong a positive NEE CO2 anomaly in response to El Niño forcing. A broad suite of satellite remote sensing data streams provide further evidence for a concurrent decline in fuel moisture, yielding larger and more intense wildfires. Given that land-atmosphere moisture coupling shapes the temperature environment experienced by the terrestrial biosphere, separating temperature and moisture controls on the carbon cycle response to ENSO becomes challenging. We argue that drought stress exacerbates positive air temperature anomalies, and that past work assuming temperature and precipitation are exogenous variables may underestimate the importance of soil moisture as a mechanism contributing to temperature increases and therefore atmospheric CO2 variability during ENSO.

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