The influence of ENSO during spring over northwestern Mexico

Abstract

This study focuses on better understanding how El Niño‐Southern Oscillation (ENSO) influences the occurrence of particularly dry conditions (warm temperature and dry precipitation anomalies) in northwestern Mexico during boreal spring (March–May), a season that is characterized by the driest regional conditions, and by a high socioeconomic sensitivity to the interannual variations in them. For that, surface observations (CRU TS4.05 and ERSSTv5) and low‐tropospheric circulation reanalysis (ERA5) were analysed using linear regressions, contingency tables, and composites over two periods, 1901–2020 and 1959–2020. The results confirm that the positive phase of ENSO (El Niño) tends to reduce the probabilities of a drier than normal spring and that its negative phase (La Niña) increases them, but suggest that such signal is relatively weak. In particular, the drying effects associated to La Niña are generally small, as they explain less than 10% of the interannual variability in precipitation, and most of the driest springs occurred during neutral years. The results also shed light into the causes of the driest springs: internal atmospheric variability. On average, during the driest springs, sea surface temperature anomalies are statistically non existent over the eastern Pacific, but these events are characterized by an expansion of the southeastern (SE) side of North Pacific Subtropical High (NPSH). It is found that during spring, a strengthened SE side of the NPSH weakens the moisture flux from the Pacific Ocean into northwestern Mexico. These results suggest that the seasonal predictability of spring for northwestern Mexico might not only depend on the strength and phase of ENSO but also on the processes controlling the variability of ridges over the extratropical eastern Pacific.