The role of the Pacific decadal oscillation (PDO) in El Niño–Southern Oscillation (ENSO)-related Mexican climate anomalies during winter and summer is investigated. The precipitation and mean temperature data of approximately 1000 stations throughout Mexico are considered. After sorting ENSO events by warm phase (El Niño) and cold phase (La Niña) and prevailing PDO phase: warm or high (HiPDO) and cold or low (LoPDO), the authors found the following: 1) For precipitation, El Niño favors wet conditions during summers of LoPDO and during winters of HiPDO. 2) For mean temperature, cooler conditions are favored during La Niña summers and during El Niño winters, regardless of the PDO phase; however, warmer conditions are favored by the HiPDO during El Niño summers.

Maximum and minimum surface air temperatures (T max , T min ) throughout Mexico were analysed to look for a regional sign of climate change. Temperature (T ) records were divided into two periods: early (1940-1969) and recent (1970-2004); and the analysis was performed for the four seasons plus the annual average. For these 20 cases, and for each of ∼1400 selected stations, time series were constructed and their linear trends (m) were obtained. The statistical significance of m was tested by posing the null hypothesis m = 0, i.e. that there was no trend. The length of the time series (n) considered for this test was the n-effective (n eff ) that takes into account the fact that consecutive values of T have non-zero correlation. The null hypothesis was rejected in less than 25% of the stations in all cases. The principal findings are: (1) Mexico warmed up during the recent period, and this warming was more generalized in T max than in T min and in summer than in the other seasons; (2) Mexico cooled down during the early period, and this cooling was more generalized in winter than in the other seasons; (3) In neither of these two cases El Niño-Southern Oscillation (ENSO) seems to play any direct role; (4) In contrast to ENSO, the trends and phases of the Pacific Decadal Oscillation (PDO) are consequent in both cases: a warming trend at the beginning of the 1970s and a warm PDO phase prevailing during the recent warming period; as well as a cooling trend at the beginning of the 1940s and a cold PDO phase prevailing during the early cooling period; and (5) north-western and central Mexico temperature trends often contrast with those of the rest of the country.