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Sergio Reyes and Daniel L. Cadet

Abstract

Some meteorological features have been examined as part of a more extensive study on general circulation in the eastern Pacific and Central American region. The analysis is based on twice-daily data from May to August 1979. Different FGGE data sets (Level III-b, TIROS-N) were used to obtain fields of precipitable water (PW), 1000 mb wind, and surface water vapor fluxes. The results concerning precipitable water and wind are presented as monthly mean fields. The time and spatial evolution of three layers of PW (surface to 700 mb, 700–500 mb and 500–300 mb) show the synoptic characteristics prevailing during this period. The fields of PW reveal fluctuations that are associated with the number and trajectories of tropical perturbations observed over the Atlantic and Pacific Oceans. The time series of the 1000 mb water vapor flux for latitudinal and longitudinal segments around Mexico and the eastern equatorial Pacific 0cean were determined. The strongest horizontal mean flux takes place over the Gulf of Mexico and the eastern Pacific Ocean. Minimum intensity occurs during August, coincident with the month of observed fewer-than-normal tropical perturbations.

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Sergio Reyes and Daniel L. Cadet

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The physical characteristics of the southwest branch of the North American monsoon system during the summer of 1979 are studied with the FGGE dataset. The combined features such as the low pressure trough over northwestern Mexico, the penetration of the easterly flow from the Atlantic Ocean, and the intensification. of the anticyclonic gyre of the South Pacific, are responsible, during the summer, for a well established cross-equatorial surface air flow along the eastern tropical Pacific Ocean and western Mexico. Mean monthly maps of vertically integrated water-vapor flux show the development of a low-level jet favoring the penetration, into western Mexico, of a cross-equatorial moist flow which originates over the southern Pacific. It is shown that the South Pacific anticyclone gyre is an important feature which brings moist air along western Mexico. The mean monthly evolution of the net water-vapor flux divergence suggests a strong association with the precipitation pattern observed over Mexico.

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Michael W. Douglas, Robert A. Maddox, Kenneth Howard, and Sergio Reyes

Abstract

The pronounced maximum in rainfall during the warm season over southwestern North America has been noted by various investigators. In the United States this is most pronounced over New Mexico and southern Arizona; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the “Mexican monsoon,” is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data. In particular, the authors note the geographical extent and magnitude of the summer rains, the rapidity of their onset, and the timing of the month of maximum rainfall. Finally, the difficulty in explaining the observed precipitation distribution and its timing from monthly mean upper-air wind and moisture patterns is discussed.

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V. Brahmananda Rao, J. Pablo Reyes Fernandez, and Sergio H. Franchito

Abstract

A primitive equation global zonally averaged general circulation model is used to study the effects of the topography on the atmospheric annual cycle. A smoothed zonally averaged topography that has a form similar to that observed was used. The control experiment showed that the model was capable of capturing the zonally averaged behavior of the annual cycle. The model is able to capture some characteristics of the monsoonlike circulation such as the seasonal wind reversal and the easterly jet in the boreal summer. Even in the absence of topography the model was able to reproduce the monsoonlike features. However, the circulation was weak and the position of its components was altered. This suggests that the topography has an important role in modifying the intensity and position of the monsoon circulation. Sensitivity tests were made in order to investigate the effects of high elevation and its steep southern slope. Two experiments were performed: 1) increasing the elevation of orography without changing the steepness of the slope, and 2) increasing both the elevation and the steepness of the slope. The results indicated that the steepness of the southern slope seems to control the monsoonlike flow in the model. The model was also capable of reproducing a monsoonlike response to changed external conditions. When the values of the earth’s orbital parameters (precession, obliquity, and eccentricity) were changed to those of 9000 yr BP, the precipitation and circulation intensified, which seems to agree with paleoclimatic evidence.

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V. Brahmananda Rao, Sergio H. Franchito, and J. Pablo Reyes Fernandez
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Sergio Reyes, Gerard Vogel, Edgar Pavía, and Alejandro Parés

Abstract

Synoptic winds, obtained from surface pressure maps, and local surface winds, registered around Todos Santos Bay, are analyzed and filtered to isolate the high-frequency band (diurnal scale) and the low-frequency band (greater than a few days) to study, independently, their physical phenomena. Two methods of analysis are used: rotary spectra and empirical orthogonal functions (EOF). The surface data show the highest energy at diurnal frequency, indicating a strong sea-land breeze regime. On the other hand, the synoptic data show the highest energy at longer periods. In general, the low-frequency synoptic wind is much stronger than the low-frequency local surface wind. The differences are explained by means of the frictional effect and the higher energy dissipation over land than over sea. The EOF fire mode of the low-frequency wind data, representing 77% of the total variance, shows the characteristics of the most energetic phenomena, i.e., the synoptic regime. The correlation between synoptic and surface eigenvectors is best for the island data; poor correlations are found between the synoptic eigenvector and those for the city and shoreline sites. Similarly, the synoptic-reduced surface wind, obtained by the power law equation, has the best correlation with the island data. The EOF first mode of the high-frequency wind data, representing 62% of the total variances, shows the characteristics of the sea land breeze eigenvectors.

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Marco Turco, Sonia Jerez, Markus G. Donat, Andrea Toreti, Sergio M. Vicente-Serrano, and Francisco J. Doblas-Reyes

Abstract

Accurate and timely drought information is essential to move from postcrisis to preimpact drought-risk management. A number of drought datasets are already available. They cover the last three decades and provide data in near–real time (using different sources), but they are all “deterministic” (i.e., single realization), and input and output data partly differ between them. Here we first evaluate the quality of long-term and continuous climate data for timely meteorological drought monitoring considering the standardized precipitation index. Then, by applying an ensemble approach, mimicking weather/climate prediction studies, we develop Drought Probabilistic (DROP), a new global land gridded dataset, in which an ensemble of observation-based datasets is used to obtain the best near-real-time estimate together with its associated uncertainty. This approach makes the most of the available information and brings it to the end users. The high-quality and probabilistic information provided by DROP is useful for monitoring applications, and may help to develop global policy decisions on adaptation priorities in alleviating drought impacts, especially in countries where meteorological monitoring is still challenging.

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Marco Turco, Sonia Jerez, Markus G. Donat, Andrea Toreti, Sergio M. Vicente-Serrano, and Francisco J. Doblas-Reyes
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Marco Turco, Sonia Jerez, Markus G. Donat, Andrea Toreti, Sergio M. Vicente-Serrano, and Francisco J. Doblas-Reyes
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