ETCCDI INDICES IN THE ODESSA REGION BASED ON ERA5-LAND REANALYSIS AND STATION OBSERVATIONS: TRENDS AND VALIDATION FOR 1991–2020

Abstract

Changes in the precipitation extremes regime in the North-Western Black Sea region are investigated based on ten ETCCDI (Expert Team on Climate Change Detection and Indices) indices derived from the ERA5-Land reanalysis (spatial resolution 0,1°) for the standard climate normal period 1991–2020. Temporal trends are assessed using the non-parametric Mann-Kendall test with trend magnitude estimated by Sen's slope estimator. Spatial analysis is performed for the ERA5-Land grid within the domain 46,0–47,5°N, 30,0–31,5°E; seasonal analysis is conducted separately for the cold (November–March) and warm (April–October) seasons. Validation of the А. Б. Семергей-Чумаченко reanalysis results is carried out by comparison with observational data from the Odessa meteorological station (ECA&D, STAID: 400) for the same period. Annual trend analysis reveals a statistically insignificant at the standard level of p<0,05, but pronounced decreasing tendency in total wet-day precipitation (PRCPTOT: −3,48 mm/yr, p=0,064). Seasonal trend decomposition demonstrates that all statistically significant trends are confined to the warm season: a decrease in PRCPTOT (−2,60 mm/yr, p=0,032), a reduction in the frequency of days with precipitation ≥10 mm (R10MM: −0,088 days/yr, p=0,012), and a statistically insignificant at p<0,05, but pronounced increase in the length of dry spells (CDD: +0,208 days/yr, p=0,094). No significant trends are detected during the cold season. Spatial trend analysis using the Mann-Kendall test reveals the predominance of significant negative signals for PRCPTOT and R10MM across most of the study domain, with the most pronounced decrease in the northern and central parts of the domain. The spatial distribution of CDD trends is characterised by heterogeneity: significant positive trends are recorded in the eastern part of the region, while the coastal zone exhibits insignificant negative or neutral values, potentially indicating a modifying influence of the Black Sea. For RX1DAY, spatial analysis reveals significant negative trends in the northern part of the region, which contrasts with the increasing trend recorded at the Odessa meteorological station (+0,90 mm/yr, p=0,010), indicating limitations of the reanalysis in capturing point-scale precipitation extremes. Validation against ECA&D data reveals satisfactory agreement for accumulated precipitation indices (PRCPTOT: R=0,45, p=0,012; R10MM: R=0,51, p=0,004). The results may indicate increasing warm-season dryness in the North-Western Black Sea region during 1991–2020, consistent with regional tendencies of summer precipitation decline characteristic of the Black Sea and Mediterranean sectors. The identified trends have practical implications for water resource assessment and agroclimatic conditions in southern Ukraine.