Linkage between the North Atlantic atmospheric patterns and the long-term changes of winter precipitation in the Iberian Peninsula

Resumen

The most advanced climate projections indicate, with high confidence, an increase in the frequency, persistence, and severity of dry periods in southern Europe by the end of the 21st century. This intensification of droughts is explained by a decrease in precipitation, and especially the rise temperatures and evapotranspiration rates. In terms of precipitation changes, the largest reduction is projected for winter. The expansion of the subtropical high-pressure belt is projected, along with the northward movement of storms into northern Europe, where wetter conditions are projected by the end of the 21st century. This pattern of expected changes in Europe appears to have been observed over the last few decades. Focusing on the Iberian Peninsula, a higher frequency and intensity of drought periods since 1980 corroborate the trend projected by simulations. However, this signal seems less robust when examining longer periods of the Instrumental Era. Additionally, precipitation projections continue to exhibit high uncertainty among models and their simulations. Therefore, there is a need to gain a comprehensive understanding of the long-term observed climatic variability, as well as the factors contributing to the uncertainty in climate projections. With this primary objective in mind, this doctoral thesis aims to enhance knowledge about observed precipitation variability in Europe, primarily in the Iberian Peninsula, from the beginning of available records. It seeks to connect this behavior with the dynamics of the major climate variability modes in the region, such as the North Atlantic Oscillation (NAO). The goal is to evaluate whether climate projections can effectively interpret all the climatic variability that results in decadal and centennial precipitation cycles. The results confirm a notably unique behavior in numerous climate indicators since 1980. In the Iberian Peninsula, extensive regions experience a significant decline in precipitation, which propagates, with even greater intensity, in hydrological series. This trend is primarily linked to the intensification of NAO into positive phases, resulting in a higher frequency of high-pressure systems over the Azores. At least during the Industrial Era, our results highlight its anomalous behavior since the 1980s based on several indicators, such as its more dominant role, explaining more variance than in previous decades, being more intense, something unprecedented, and showing a spatial pattern with certain particularities. Although other factors appear to play a significant role, our findings show with high confidence that the decrease in winter precipitation is the primary trigger for the intensification of droughts since 1980 in most of the territory of the Iberian Peninsula, and likely in other regions of the Western Mediterranean. Given that climate projections anticipate the persistence of NAO+ intensification (e.g., expansion of the Hadley Cell, intensification of the subtropical belt), these drier conditions could extend over the next decades, and even worsen, constituting a key environmental threat to water resource availability. However, the magnitude of the expected changes differs considerably between models capable of reproducing long-term variations in large-scale circulation and those models that are too rigid. This opens up an exciting horizon in which improving the ability of simulations to capture fully the observed natural variability of the climatic system is a priority. We will never be able to accurately predict what the climate holds at a regional scale, and its impacts on living beings in those territories, if climate models continue to exhibit inherent biases in capturing such fundamental indicators as synoptic conditions in the North Atlantic.