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 along the coast of 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 the Global Inventory Modelling and Mapping Studies (GIMMS), precipitation from the Climatic Research Unit (CRU), soil moisture/temperature and surface solar radiation from ERA5-Land (the high-resolution land-surface component of the fifth-generation ECMWF atmospheric reanalysis), and atmospheric reanalysis from NCEP/NCAR for the period 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 the NAO to vegetation. This study reveals that December exhibits the strongest interannual vegetation variability along the coast of 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 with increased LAI, with a Pearson correlation coefficient of 0.51. 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. After turning eastward, these winds acquire moisture and converge over the study region, increasing soil moisture and thus favoring vegetation growth; and (2) at upper levels, a Rossby wave train propagates toward the Arabian region, strengthening the tropical easterly jet and inducing anomalous ascent that further increases soil moisture and favors vegetation growth along the coast of southern East Africa. Combined NAO and ENSO indices explain approximately 41% of LAI variance, with the NAO alone accounting for approximately 26% of this total variance, exceeding the contribution of ENSO. It is thus concluded that the NAO is a key, previously underappreciated driver of early-austral-summer vegetation variability along the coast of southern East Africa, which operates primarily via soil-moisture modulation and makes a larger contribution than ENSO. These findings improve our understanding of extratropical–tropical interactions and offer a new pathway for vegetation forecasts in the region.