Changing behavior in the diurnal range of surface air temperatures over Mexico

Abstract

[1] The diurnal range in surface temperatures (DTR = maximum À minimum temperature) has been widely used as one indicator of potential climate change. On hemispheric space scales DTR trends over about the last half-century tend to be decreasing. This paper analyzes regional scale trends in DTR for Mexico (1940Mexico ( -2001. Our principal finding is that in recent decades (post-1970) DTR trends over Mexico are positive as maximum temperatures are warming at a significantly higher rate than minimum temperatures. Regional land use and land cover changes (LCCs) are identified as potential forcing mechanisms responsible for at least part of the observed DTR behavior. Background[2] The most recent report of the Intergovernmental Panel on Climate Change (IPCC) presents a consensual view of global warming [Houghton et al., 2001]. The IPCC reports that land surface air temperatures (LSAT) have increased significantly over the 20th century, that warming has been pronounced since the early-1970s, and that the 1990s was likely the warmest decade of the instrumental record. Easterling et al. [1997] demonstrate that much of this warming has involved a faster rise in daily minimum temperatures (T min ) than in maximum temperatures (T max ) and consequently, over large portions of the world, trends in diurnal temperature range (DTR) tend to be negative. In part, this behavior can be associated with long-term trends in cloudiness. Dai et al. [1999] find that cloud cover is the dominant factor modulating DTR -it directly influences the amount of incoming solar radiation and hence largely affects T max . They also find that DTR is weakly linked to changes in atmospheric water vapor (i.e., specific humidity [q]), suggesting that it influences both T max and T min so that DTR may be only marginally affected by changes in q.[3] This paper examines the historical (1940 -2001) variability in DTR and its components (T max and T min ) over Mexico. Our intent is to demonstrate how Mexico's DTR trends differ from the typically cited pattern of decreasing DTR, and then to briefly discuss some possible explanations for these differences. The analysis considers regional time series (Figure 1) for three seasons: cool (Oct-Feb); spring (Mar -May); and warm (Jun -Sep). The series are derived from a recently developed gridded (2.5°Â 2.5°lat-long) data set. Details on data set construction and regional definition are presented elsewhere [Englehart and Douglas, 2004]. Here we note three points. First, the series represent average LSAT anomalies (1961 -90 base period) taken over about 10 -30 stations. Second, prior to creating the regional averages, individual station data were evaluated for potential inhomogeneities using post-hoc quality control techniques (e.g., inspection of time series plots, spatial correlation fields). Suspect data points were removed prior to averaging. Third, efforts were made to minimize the urban heat island signal within the data set [e.g., Karl et al., 1988;Englehart and Douglas, 2003] by including only LSAT data f...

Temperature Trend Analysis in the North Pacific Watershed in Mexico

“…The probable effect of this process in adjacent regional extent deserves further research [42]. On the other hand, 43 stations developed increasing TR trends, from these, 26 had larger TMaxAvg slope values, a similar result described by [43] in Spain and for Mexico by [20] [21] [44]. The mechanisms of this process is described by [2] who posed that the conversion of soil cover increase TR due to the decrease of evaporative cooling, consequently increasing TMaxAvg in daytime, when evaporation and maximum temperature occur, a very evident process in semi-arid and seasonal regions.…”

“…Also [20], the direction of minimum and maximum temperature trends as the components of temperature range were explored [21]. Illustrated opposing temperature trends in different spatio-temporal magnitude.…”

Annual and seasonal surface air temperature trends in Mexico

“…For instance, mean air temperature trends in the Southern Hemisphere (Jones, 1994), and particularly in Australia (IPCC, 2007), seem to have been consistently positive since the 1950s; whereas the same trends over North America (IPCC, 2007), and particularly over Mexico (Englehart and Douglas, 2004), appear to have changed sign from negative in the 1940s to the 1970s period to positive in the 1970s to the 2000s period. In this last case even other related variables appear to show a similar behaviour; for example, Englehart and Douglas (2005) report that regional-scale trends in the diurnal range of T over Mexico present negative trends for the 1940-1970 period and positive trends for the 1971-2001 period. Therefore, for the case of Mexico, the statement that the country has warmed up in recent decades (or at least since 1971) seems to be well-justified; nevertheless questions remain regarding the nature of this warming, for example: Are there places within Mexico with negative (instead of positive) temperature trends during this period?…”

“…However, in the most recent period, the majority of the selected stations (∼60 to ∼80%, depending on the case) were located in non-urban settings (Table II) with 1376 stations; in both cases to be included in a particular period a station must have at least 4 years of data in it. These two periods are chosen because they are similar to those in Englehart and Douglas (2005), that is: 1940-1970 and 1971-2001, also 1970 corresponds to the inflection point of a second degree polynomial fitted to the average time series of all stations considered; and furthermore, it could be argued that global temperature anomalies began a positive tendency from the late 1960s to the early 1970s (Brohan et al, 2006). The above results in 26 720 T time series (two variables: T max and T min , times five cases: four seasons plus annual, times the sum of the number of selected stations from each period).…”

“…Or what is the role, if any, of the El Niño-Southern Oscillation (ENSO) phenomenon or of the Pacific Decadal Oscillation (PDO) on temperature trends over Mexico? Consequently, in this paper we revise all reliable T data from each available Mexican station, mainly because global warming could mean drier conditions for Mexico (Liverman and O'Brien, 1991); but also because we think that concealed within positive-(negative-) trend regional averages [see, for example, Englehart and Douglas (2005)], there are key regions where stations may exhibit trends which are either zero or statistically significant different than zero but negative (positive).…”