The El Niño/Southern Oscillation and Precipitation Variability in Baja California, Mexico

Abstract

This study evaluates precipitation variability in Baja California in relation to the El Niño/Southern Oscillation (ENSO) using the Southern Oscillation Index (SOI). To evaluate precipitation climatology, data for 102 weather stations were analyzed. Data were directly averaged for stations with records longer than 30 years, but normalized for stations with shorter records. To test for uniformity of precipitation departures, the SOI was compared with average precipitation departures for long-term stations (established before 1960), in eight subregions of Baja California. The results revealed that, unlike California, the interannual variability of both annual and monthly precipitation is strongly linked to SOI. During El Niño events, above-normal precipitation occurs largely in February and March; but precipitation amounts are subnormal during La Niña events, and mostly limited to December and January. Gradients of precipitation departure tend to be uniform across Baja California during individual years. The variability of precipitation is attributed to the interannual dislocation by ENSO of the polar-front jet stream along the Pacific coast, as described in other studies. In El Niño events, the circulation acquires a positive-phase Pacific-North America (PNA) pattern, with an enhanced Tropical Northern Hemisphere (TNH) teleconnection mode expressed in a southern-branch jet stream across Baja California. There is strong on-shore advection of moist surface westerlies on the West coast of North America. In La Niña events, the composite circulation acquires a negative-phase PNA pattern, and the jet stream is mostly poleward of Baja California and California. This results in weak on-shore advection by moist surface westerlies. Seasonal shifts in precipitation anomalies may be related to geographic shifts in diabatic forcing from the equatorial warm pool, and consequent teleconnections into the extra-tropical wave-train in the Pacific-North American region. The annual uniformity of precipitation departures across the eight subregions of Baja California suggests that frontal storms do not produce anomalous orographic precipitation gradients during El Niños. Timely, long-term precipitation forecasts could help accommodate the region's landuse to its cyclical pattern of drought and flood.