This paper explores the role that tropical storms (TS) of the eastern North Pacific play in the rainfall climatology of western Mexico. It uses an 18-station rainfall grid, along with a data base of storm tracks (1949 -1997). TS rainfall is defined by a distance threshold, so that for each station in the grid, daily rainfall recorded when a TS is located within 550 km of the station is considered as TS-derived; rainfall recorded in the absence of a TS or when a storm is beyond 550 km from a station is by default 'non-TS' rainfall. A variety of statistics are presented to demonstrate the static associations that exist between TS activity and regional rainfall patterns. In sum, they clearly suggest that improved seasonal forecasts of TS activity in the eastern North Pacific will be critical to advancing seasonal climate prediction for Mexico. This study also addresses the interannual variability in TS and non-TS rainfall. Principal component analysis is used to define the region's primary modes of rainfall variation. Considering the temporal behaviour of the rainfall modes, analyses reveal no statistically significant trends in TS rainfall. In contrast, non-TS rainfall modes exhibit significant trends. This study provides support for the idea that these trends are partly tied to long-period fluctuations in the ocean-atmosphere system. The results also indicate that while TS rainfall is not subject to trend behaviour, its year-to-year fluctuations, apparently, are linked to large-scale processes like El Niñ o-Southern Oscillation (ENSO) and the state of North Pacific sea-surface temperatures (SSTs). Copyright

[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...

No abstract

Monthly and seasonal variations in surface air temperature (SAT) over Mexico have not received much research attention, a situation partly reflecting the lack of a coherent historical data set. As a step toward rectifying the data gap, this study outlines the development of a gridded monthly (2.5°× 2.5°lat.-long.) SAT data set for Mexico. Using the data set, we investigate several basic dimensions of SAT variability. Our analysis demonstrates that much of the variability can be compactly expressed in terms of four regions which are physically plausible with respect to the country's climatology. Not surprisingly, persistence is an important component of regional SAT variability. Evaluated month to month, persistence tends to be greatest during the warm season, whereas across seasons there is evidence for persistence of warm season anomalies into the following cool season, behaviour that is consistent with positive feedback relationships between SAT, rainfall and land surface conditions. The regional time series display longer period variability that is partially linked to the state of the large-scale, slowly evolving climate modes of the Atlantic multidecadal oscillation and the Pacific decadal oscillation. Analyses are also presented to describe teleconnections between SAT and the El Niño-southern oscillation phenomena, and SAT and other large-scale atmospheric modes, such as the Pacific North American pattern and the North Atlantic oscillation.

This study provides an empirical description of intraseasonal rainfall variability within the North American monsoon (NAM) region. Applying particular definitions to historical daily rainfall observations, it demonstrates that distinct intraseasonal rainfall modes exist and that these modes differ considerably from the monsoon core region in northwest Sonora (SON), California, to its northward extension in southeast Arizona (AZ). To characterize intraseasonal rainfall variability (ISV), separate P-mode principal component (PC) analyses were performed for SON and AZ. The results indicate that in each area, much of the ISV in rainfall can be described by three orthogonal modes. The correlations between ISV modes and total seasonal rainfall reinforce the notion of differing behaviors between the monsoon's core and extension. For SON all three ISV modes exhibit significant correlation with seasonal rainfall, with the strongest relationship in evidence for the ISV mode, which is related to rainfall intensity. For AZ, total rainfall exhibits the strongest correlation with the ISV mode, which emphasizes season length and rainfall consistency. Examination of longer-period behavior in the ISV modes indicates that, for SON, there is a positive linear trend in intensity, but a countervailing trend toward a shorter monsoon season along with less consistent rainfall in the form of shorter wet spells. For AZ, the evidence for trend in the ISV modes is not nearly as compelling, though one of the modes appears to exhibit distinct multidecadal variability. This study also evaluates teleconnectivity between ENSO, the Pacific decadal oscillation (PDO), and the NAM's intraseasonal rainfall variability. Results indicate that part of the intraseasonal rainfall variability in both SON and AZ is connected to ENSO while only SON exhibits a teleconnection with the long-period fluctuations of the PDO.