North Atlantic Oscillation Signal in the Interannual Variability of Early Austral Summer Vegetation over East Africa

Vegetation in East Africa is highly vulnerable to climate variability. While local factors have been extensively studied, the influence of large-scale extratropical circulation patterns, particularly the North Atlantic Oscillation (NAO), on vegetation dynamics remains poorly understood. This study investigates the impact of the NAO on the interannual variability of early austral summer (December) vegetation over coastal southern East Africa and quantifies its contribution relative to the El Niño–Southern Oscillation (ENSO). We used satellite-derived leaf area index (LAI) and normalized difference vegetation index (NDVI) from GIMMS, precipitation from CRU, soil moisture/temperature and surface solar radiation from ERA5-Land, and atmospheric reanalysis from NCEP/NCAR for 1982–2015. Empirical orthogonal function (EOF) analysis, partial correlation, composite analysis, singular value decomposition (SVD), and cross-validated regression were employed to examine vegetation variability, isolate independent local drivers, and elucidate the mechanisms linking NAO to vegetation. It is found that the strongest interannual vegetation variability occurs in December over coastal southern East Africa. Partial correlation reveals that LAI is significantly and independently positively correlated with soil moisture, and negatively with soil temperature, air temperature, and surface solar radiation; rainfall exhibits no significant independent influence. A positive NAO phase is significantly correlated (r = 0.51) with increased LAI. The positive NAO enhances vegetation through two pathways: (1) at low levels, an anticyclonic anomaly over the North Atlantic strengthens dry northeasterlies toward the Congo basin; these winds turn eastward, acquire moisture, and converge over the study region, increasing soil moisture to promote vegetation growth; and (2) at upper levels, a Rossby wave train propagates to the Arabian region, strengthening the tropical easterly jet and inducing anomalous ascent, further favoring soil moisture and vegetation growth over the coastal southern East Africa. Combined NAO and ENSO indices explain ~41% of LAI variance, with NAO alone accounting for approximately 26%, exceeding the contribution of ENSO. It is thus concluded that the NAO is a key, previously underappreciated driver of early summer vegetation variability in coastal southern East Africa, acting primarily through soil moisture modulation and with a greater contribution than ENSO. These findings advance the understanding of extratropical–tropical interactions and offer a new predictive avenue for vegetation forecasts in the region.