CR 25:229-243 (2004)  -  doi:10.3354/cr025229

ABSTRACT: We investigated the variability of 1 d precipitation extremes (top 10%) in Tijuana (Mexico) during 1950-2000. Interannual rainfall variability is associated with El Niño/Southern Oscillation (ENSO), which explains 30% of rainfall and 36% of 1 d extreme precipitation variance. Interannual precipitation exhibits a large change, with a relatively dry period and less variability during 1950-1976, followed by a relatively wet period and more variability during 1976-2000. All extremely wet years (>1 SD) and the largest frequency of 1 d extremes occurred after 1976-1977, with 6 out of 8 extremely wet years characterized by El Niño episodes and 2 by neutral conditions. However, more than half of the 1 d extremes during 1950-2000 occurred in non-ENSO years, providing evidence for the fact that neutral conditions contribute significantly to extreme climatic variability in the region. Extreme events that occur in neutral (strong El Niño) conditions are associated with a Œpineapple express¹ and a Œneutral Pacific/North America (PNA)¹ (negative tropical/northern hemisphere [TNH]) teleconnection pattern that links an anomalous tropical convective forcing west (east) of the date line with a strong subtropical jet over the study area. The neutral PNA is a variant of the typical ±PNA teleconnection pattern; it is shifted northwestward of the +PNA (typical of El Niño) and southeastward of the -PNA (typical of La Niña) and is characterized by a strong ridge over the Gulf of Alaska and a deep low over California. We believe that the trough in the jet, typical of extreme events during neutral and strong El Niño years, is further intensified by thermal interaction with regional anomalies of a warm California Current off Baja California, low-level moisture advection from the subtropical warm sea-surface region, and intense convective activity over the study area.

KEY WORDS: Climatic variability · Extreme precipitation · ENSO · Non-ENSO · Tijuana · Mexico