Interannual variability in terrestrial net primary production: Explorationof trends and controls on regional to global scales

Climate and biophysical regulation of terrestrial plant production and inte rannual responses to anomalous events were investigated using the NASA Ames model version of CASA (Carnegie-Ames-Stanford Approach) in a transient sim ulation mode. This ecosystem model has been calibrated for simulations driv en by satellite vegetation index data from the National Oceanic and Atmosph eric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) over the mid-1980s. Relatively large net source fluxes of carbon were esti mated from terrestrial vegetation about 6 months to 1 year following El Nin o events of 1983 and 1987, whereas the years 1984 and 1988 showed a drop in net primary production (NPP) of 1-2 Pg (10(15) g) C from their respective previous years. Zonal discrimination of model results implies that the nort hern hemisphere low latitudes could account for almost the entire 2 Pg C de crease in global terrestrial NPP predicted from 1983 to 1984. Model estimat es further suggest that from 1985 to 1988, the northern middle-latitude zon e (between 30 degrees and 60 degrees N) was the principal region driving pr ogressive increases in NPP, mainly by an expanded growing season moving tow ard the zonal latitude extremes. Comparative regional analysis of model con trols on NPP reveals that although Normalized Difference Vegetation Index " greenness" can alone account for 30%-90% of the variation in NPP interannua l anomalies, temperature or radiation loading can have a fairly significant I-year lag effect on annual NPP at middle- to high-latitude zones, whereas rainfall amount and temperature drying effects may carry over with at leas t a 2-year lag time to influence NPP in semiarid tropical zones.