Trend and ENSO-based analysis of last spring frost and chilling in Iran

The frost and chilling phenomena are the important climatic and environmental hazards that cause a lot of damage to the agricultural sector every year. In this respect, the present study intends to highlight the possibility of attributed occurrence to large-scale atmospheric indices. In this study, the probability of last spring frost (LSF) and chilling (LSC) at a critical temperature of + 4.4 to - 3.3 degrees C and different phases of El Nino Southern Oscillation index (ENSO) were extracted using Weibull Distribution Function (WDF) at 18 synoptic stations in a period of 59 years from 1961 to 2019. Then, the anomaly of LSF and LSC occurrence date was investigated in different ENSO phases (EP) and intensities (EI) and finally the LSF and LSC occurrence trend and the factors affecting this trend were analyzed using Mann-Kendall method and Sen's slope estimator. Different EP and EI were classified using Oceanic Nino Index (ONI) in Nino 3.4 region. The results showed that the occurrence date and probability of the LSF and LSC with different base and critical temperatures in EP are associated with precedence and latency over the whole period and the anomaly of LSF and LSC occurrence date fluctuates between + 17 to - 12 and + 15 to - 17 days, respectively. In addition, the frequency and anomaly of the LSF and LSC at different base temperatures in different EI have been intensified compared to its different EP; therefore, the occurrence date of LSF and LSC can be predicted based on the EP and EI. On the other hand, the trend analysis of the date of the LSF and LSC indicates its retreat towards early spring or last winter at the main studied stations, which is due to global warming. According to the results obtained in some of the studied areas, the delay of LSF and LSC is also observed, which is due to local topographic characteristics, altitude and proximity to moisture sources.