Climate-Driven Shifts in Rainy and Dry Season Timing in the Tropical Andes Using Harmonic Analysis

The advance and delay of the rainy season is among the most frequently cited effects of climate change by Ecuadorian farmers. However, its assessment is not feasible using the conventional indicators recommended by the standardized indices of the Expert Team on Climate Change Detection and Indices (ETCCDI). This study aims to analyze such advances and delays through harmonic analysis in Tungurahua, a predominantly agricultural province in the Tropical Central Andes, where in-situ data are scarce. Daily in-situ data from five meteorological stations were used, including precipitation, maximum, and minimum temperature records spanning 39 to 68 years. The study involved an analysis of the region’s climatology, climate change indices, and harmonic analysis using Cross Wavelet Transform (XWT) and Wavelet Coherence Transform (WCT) to identify seasonal patterns and their variability (advance or delay) by comparing historical and recent time series, and Kriggin for regionalization. The year 2000 was used as a breakpoint for comparing past and present trends. Results show a generalized increase in both minimum and maximum temperatures. In the case of extreme rainfall events, no significant changes were detected. Harmonic analysis was found to be sensitive to missing data. Furthermore, the observed advances and delays in seasonality were not statistically significant and appeared to be more closely related to the geographic location of the stations than to temporal shifts.