Temporal Scales of Mid-Tropospheric CO2, Precipitation, and Vegetation Phenology in the US Great Plains

Natural and anthropogenic sources of CO₂ around the globe contribute to mid-tropospheric concentrations, and it remains unknown how these relate to regional ecosystem dynamics. CO₂ concentrations in the mid-troposphere from 2002 to 2010, retrieved by NASA's Atmospheric Infrared Sounder (AIRS), were examined in relation to precipitation and vegetation phenology across the Great Plains. Lagged correlations, wavelet multi-resolution analysis, and the information theory metric of relative entropy were applied to assess regional relationships between mid-tropospheric CO₂, normalized difference vegetation index (NDVI), and precipitation (PPT). Correlations from lagged time series and wavelet multi-resolution analysis indicate time scales of exchanges and contributions between the surface and the mid-troposphere, while the relative entropy metric reveals how representative one measured variable is of another. Results show that PPT and NDVI contribute to mid-tropospheric CO₂ at the 18-month time scale, while spatial patterns seen at this time scale for PPT and mid-tropospheric CO₂ are reflective of the influence of PPT on NDVI at the annual scale. PPT showed positive correlations with NDVI and mid-tropospheric CO₂ in the northwest of the region and negative correlations in the southeast. Interesting patterns of negative and positive correlations emerge at the three-year scale and may be indicative of influences of land-use/cover change and intensive agriculture on carbon dynamics. In the Great Plains, AIRS observations of mid-tropospheric CO₂ agree with regional PPT and NDVI at seasonal, annual, and longer scales. Future studies may consider these dominant temporal scales in the examination of processes that actively transport and distribute CO₂ at the surface and in mid-troposphere, and in the assessment of source/sink dynamics in this region.