Intercomparison of interannual changes in NDVI from PAL and GIMMS in relation to evapotranspiration over northern Asia

Vegetation over an extensive area influences actual evapotranspiration (ET) from the land to the atmosphere mainly through transpiration activity. The authors' previous study (Suzuki and Masuda, 2004. J Meteor Soc Japan, 82, 1233 -- 1241) found an interannual covariability between ET and the Normalized Difference Vegetation Index (NDVI) over a continental-scale land surface. This result suggested that vegetation controls interannual variation in ET, and therefore vegetation change must be considered to predict future climate. In this prior study, NDVI data from the Pathfinder AVHRR Land (PAL) dataset were analyzed. However, studies of NDVI interannual change are subject to uncertainty, because NDVI data often contain errors associated with sensor- and atmosphere-related effects. This study is aimed toward reducing this uncertainty by employing another major NDVI dataset, from the Global Inventory Monitoring and Modeling Studies (GIMMS) group, in addition to PAL. GIMMS-NDVI data were produced with a calibration method that differs from the one employed for PAL-NDVI data. An intercomparison of the PAL-NDVI and GIMMS-NDVI datasets provide an effective basis for further analysis of the covariability of NDVI and ET interannual changes. The analysis was carried out for the northern Asia region from 1982 to 2000. 19-year interannual changes (monthly anomalies) in the PAL-NDVI and GIMMS-NDVI values were compared. The correlation coefficient (R) in summer months exhibits high positive values (over 0.8 in June). This result indicates that PAL-NDVI and GIMMS-NDVI display similar interannual variation for active growing season months. Interannual change in PAL-NDVI and GIMMS-NDVI were both compared with interannual change in model-assimilated ET. Although the R between GIMMS-NDVI and ET is slightly less than for PAL-NDVI and ET, for both NDVI datasets the annual maximum correlation with ET occurs in June, which is near the central period of the growing season. A positive correlation between GIMMS-NDVI and ET was observed over most of the vegetated land area in June, and a similar result was obtained with PAL-NDVI. These results reinforce the authors' prior research that indicates the control of interannual change in ET is dominated by interannual change in vegetation activity.