Investigating the impacts of the North Atlantic Oscillation on global vegetation changes by a remotely sensed vegetation index

Large-scale climatic variability may have a critical impact on the vegetation growth at both local and global scales. In this study, a long time-series (1982-2006) monthly normalized difference vegetation index (NDVI) has been used as a proxy of vegetation vigour to investigate the global vegetation responses to the North Atlantic Oscillation (NAO, the dominant mode of atmospheric behaviour in the North Atlantic sector). The spatial distribution of the possible connections between NAO and global NDVI has been analysed by a cross-correlation method. The results reveal that the correlated regions between NAO and NDVI are concentrated in the middle-and high-latitude areas of the northern hemisphere around the N60 degrees. belt, the Africa zone around the N15 degrees. belt as well as the vast regions of the southern hemisphere around the S10 degrees-30 degrees. belt. As expected, owing to geographic proximity, NAO-related regions are spread globally forming five geographical west-eastward modes. Simultaneously, some correlated areas persist at one place over several months without geographic transfer. Our findings show that, besides the northern hemisphere, which has been the focus of previous studies, the vegetation responses to NAO are found across the southern hemisphere, with various time lags in different regions, sometimes even over one and a half years. This suggests the existence of a so far unrecognized mechanism that carries the NAO signal far to the southern hemisphere and even persists for multiple years. The lagged vegetation responses to NAO can provide potential for over one-year crop production prediction and agricultural water resource management in the NAO-related regions, as well as useful information for global terrestrial carbon cycle modelling.