Annual and seasonal surface air temperature trends in Mexico

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. Copyright © 2008 Royal Meteorological Society