Effects of El Niño Southern Oscillation on the space–time variability of Agricultural Reference Index for Drought in midlatitudes

Abstract

Agriculture is an economic activity directly affected by drought. With the constant increase of food demand and the need for high efficiency in food production, drought effects gain attention. The Agricultural Reference Index for Drought (ARID) was developed to quantify drought and better understand its effect on agriculture. ARID values range from 0 to 1 where zero is transpiration occurring at potential rate and one is full water deficit. El Niño Southern Oscillation (ENSO) is the main factor of climate variability around the world and affects the climate of two states located in midlatitudes, Florida, USA and Rio Grande do Sul, Brazil. An historical assessment of the relationship between ARID and ENSO will assist with establishing the role played by ENSO in the seasonal variation of soil moisture, which is critical to drought forecasting based on ENSO. The main objectives of this study were to investigate ARID's temporal and spatial variability in Florida and Rio Grande do Sul, and provide an assessment of ENSO-induced potential anomalies in ARID. Daily ARID values in Florida were calculated using data obtained from the National Weather Service COOP (Cooperative Observer Program) weather stations. In Rio Grande do Sul, ARID was calculated using daily weather data from the Instituto Nacional de Meteorologia (INMET) weather stations, Brazil. Daily ARID values were compiled into monthly averages and categorized according to ENSO phase based on the Multivariate ENSO Index (MEI). Results showed that typical ARID values for both regions vary throughout the year. The highest values were observed in the warm season due to increased evapotranspiration. During this period, ARID values varied from 0.3 to 0.8 in Florida and 0.2 to 0.7 in Rio Grande do Sul. In the cold period, South Florida had the highest ARID values, varying from 0.3 to 0.7, while for the rest of state and Rio Grande do Sul the values varied from 0 to 0.5. ENSO showed stronger influence from November to March in Florida, and November and May in Rio Grande do Sul. For both regions, El Niño caused ARID values below normal, indicating higher soil moisture while during La Niña the values were above normal. This study contributes to establish historical ARID records across the southeastern United States and southern Brazil, and can assist farmers to adjust management practices according to expected water stress conditions.