Spatial-Temporal Variability of Frost Occurrence in Ethiopia during the ONDJ Season: Roles of ENSO and AO

Abstract

Frost events pose a critical climatic risk to agriculture and ecosystems in the Ethiopian Highlands. Yet, a quantitative understanding of the spatiotemporal variability and underlying climatic drivers remains elusive. In Ethiopia, frost occurs most frequently during the October-January (ONDJ) season, particularly in the central, southern, and northeastern highlands, where it exhibits high variability. This study investigates the spatiotemporal characteristics and climatic drivers of ONDJ frost days in Ethiopia. Trend analysis suggests a statistically significant decline in frost-day frequency across much of the highlands, consistent with the observed rise in minimum temperatures. EOF analysis identifies two leading modes of variability: a dominant, spatially uniform mode (EOF1, 64.1% of variance) and a secondary northwest-southeast dipole mode (EOF2, 11.98% of variance). The corresponding principal component time series (PC1 and PC2) both exhibit prominent interannual to near-decadal (4 - 10 years) variability. Further analysis indicates that while the PC1 is significantly positively correlated with ENSO (R = 0.41), the PC2 is strongly related to the extratropical Arctic Oscillation (AO) with a significant positive correlation coefficient (R = 0.36). El Niño induces an anomalous anticyclone over East Africa subjecting the Ethiopian highlands to dry, subsident conditions. The associated reduction in total cloud cover (TCC) enhances nocturnal radiative cooling, thereby increasing frost occurrence. In contrast, a positive phase of AO is associated with an anomalous cyclone over the northeastern Africa and Arabian Peninsula. This circulation favors subsidence in the northwest and ascent in the southeast, modulating TCC in opposing directions and giving rise to a dipole pattern in frost-day frequency. Therefore, effectively integrating ENSO and AO signals into early warning and seasonal forecasting systems is critical for developing proactive adaptation strategies and improving frost-risk management in Ethiopia.