César O. Monzón

Seasonal Variation of Surface Temperature–Modulating Factors in the Sonoran Desert in Northwestern Mexico

The authors explore a new approach to monitoring of desertification that is based on use of results on the relation between albedo and surface temperature for the Sonoran Desert in northwestern Mexico. The criteria of predominance of radiation by using the threshold value of Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index (NDVI) were determined. The radiation mechanism for regulating the temperature of the surface and the definition of threshold values for AVHRR and MODIS NDVI have an objective justification for the energy budget, which is based on the dominance of radiation surface temperature regulation in relationtoevapotranspiration.Changesin the extentofaridregionswith AVHRRNDVIof ,0.08 andMODIS NDVI of ,0.10 can be considered to be a characteristic in the evolution of desertification in the Sonoran Desert region. This is true because, in a certain year, the time span of the period when radiation factor predominates is important for the desertification process.

Hydro-meteorology of Lake Chapala

The purpose of this work is to examine fluctuations of the hydrometeorological characteristics of Lake Chapala, such as air temperature, precipitation, evaporation, atmospheric pressure, wind, water level in the lake and volume of flow of the rivers in the Lake Chapala region. Linear and nonlinear (maximum likelihood) methods of spectral analysis were used to investigate these phenomena. The main periods were identified in the range from hours to decades, obtaining estimates of the mean square amplitudes. The thermodynamic mode of Lake Chapala was also investigated, particularly the vertical-horizontal distribution of temperature of the water and currents. For this purpose, measurements of the distributions of currents and temperatures were performed. Measurements show that Lake Chapala has a quite complex thermodynamic regime and a significant vertical stratification. The differences in temperatures detected between the surface and bottom in the center of the lake were between 0.5 and 1°C and in the eastern part, from 2 to 3°C. The warmest water was located in the lake’s eastern shallow part. The temperature of the water in the central and southern parts of the lake was 1–2°C lower than that of the water at the northern shore. In all the cross-sections, ordinary spatial variations of 3°C on a distance of just 100 to 500 m were registered with a towed device. A buoy station registered the movements of the internal thermal front with a temperature fluctuation of 2°C. The advanced part of the front was accompanied by intense internal waves. The measurements show that the currents in the lake depend strongly on the intensity of the daytime breeze, which enhances wind velocities up to 10 cm s−1 resulting in increases of the water levels of up to 10–30 mm near the shoreline.

Changes in Aridity across Mexico in the Second Half of the Twentieth Century

AbstractSix regions in Mexico, with typical interannual changes in the aridity index, have been defined by the 1951–2001 meteorological dataset. Peak months of rainfall differ within the regions. Most of the land in the Mexican terrain has had a slow aridization since the early 1980s. The decline in the aridity index in the early 1950s and late 1990s was caused by droughts in the area. The distinctive features of the aridization of Mexican dry lands are characterized by steady and extensive droughts during 1948–57, 1960–65, and 1994–2003 in the second half of the twentieth century. During the drought of 1951–57 substantial aridization in most of the dry lands was observed, including the Sierra Madre Occidental, Sierra Madre Oriental, and Mexican Altiplano. Aridization of dry lands during the drought in 1960–65 affected mostly the southern part of the Mexican Altiplano, the Sierra Madre del Sur, and the Yucatan Peninsula. For the drought in the 1990s, one special feature of the aridization was its propagat...

Climatic Change in a Large Shallow Tropical Lake Chapala, Mexico

Measurements of temperature, currents and lake level taken in 2005–2014 are ana‐ lyzed and discussed. We obtained a conceptually new data set on the formation of the thermocline in Lake Chapala. It is showed that the thermocline in the lake occurs only during the daytime, in the top 0.5–1.0-m layer of the water column, whereby the vertical temperature gradient reaches 2.5°C/m within that layer. At night, the top layer is cooled, which causes strong vertical mixing down to the bottom. Moored measurements of temperature and level from Lake Chapala reveal the presence of seiches oscillations with periods of 5.7 and 2.8 hours with amplitudes of 15.4 and 8.1 mm. Temperature measurements on sections across the lake showed that in the northern part of the lake, the water column is warmer that in southern 2–3°C in all seasons. The lake currents were simulated for wet and dry seasons. The model results are in good agreement with the acoustic Doppler current profiler (ADCP) data. The presence of an anticyclone gyre in the central part of the lake in both seasons is detected.