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Matías Méndez and Víctor Magaña

Abstract

Major prolonged droughts in Mexico during the twentieth century are mainly related to anomalous dry summers, such as those observed in the 1930s, the 1950s, or the 1990s. Droughts in northern Mexico frequently coincide with anomalously wet conditions over Mesoamerica (i.e., southern Mexico and Central America), and vice versa, displaying a dominant “seesaw” structure in persistent precipitation anomalies, mostly in relation to tropical sea surface temperature (SST) anomalies. A warmer North Atlantic Ocean, expressed as a positive phase of Atlantic multidecadal oscillation (AMO), is related to the occurrence of major droughts in North America associated with weaker-than-normal moisture flux into northern Mexico. Drought over northern Mexico may also be related to changes in transient activity in the Caribbean Sea. During the negative phase of the Pacific decadal oscillation (PDO), the Caribbean low-level jet (CLLJ) weakens and easterly wave (EW) activity increases, leading to more tropical convection over Mesoamerica and less moisture flux into northern Mexico. On the other hand, when EW activity is weak over the intra-Americas seas (IAS) (i.e., the Gulf of Mexico and the Caribbean Sea) because of a stronger-than-normal CLLJ, precipitation increases over northern Mexico. Therefore, the interaction between easterly waves and the trade winds over the IAS appears to be crucial to explain the spatial patterns of droughts that have affected Mexico. In addition, low-frequency modulators, such as AMO or PDO, may serve to explain the spatial patterns of severe prolonged droughts in Mexico during the nineteenth century.

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Víctor Magaña and Michio Yanai

Abstract

The relationship between the low-frequency oscillation of convective activity in the tropics and the intensity of planetary-scale upper-tropospheric (200 mb) systems in the subtropics during the Northern Hemisphere (NH) summer is studied using data obtained in the First GARP Global Experiment (FGGE). The mid-Pacific trough and the South Asian (Tibetan) and Mexican anticyclones undergo cycles of amplification and decay with the 30–60 day fluctuation of convective activities in the Indonesia-western Pacific (IWP) region and in the intertropical covergence zone (ITCZ) over Central America. The thermal contrast between the Asian continent and the North Pacific and the resulting east-west circulation show similar time variations. This circulation regulates the intensity of the South Asian anticyclone and the mid-Pacific trough. Divergent circulation associated with convection over Central America maintains the Mexican anticyclone.

The low-frequency transients in the upper troposphere around the mid-Pacific trough play a dominant role in the northward transport of westerly momentum. Their effect is recognizable in the variation of zonally averaged relative angular momentum, which originates near 10°N and then propagates northward. At the same time, the westerlies that develop southeast of the trough form an equatorial westerly “duct,” through which wave energy can propagate into the tropics from midlatitudes. Thus, the mid-Pacific trough acts as a two-way link between the tropics and midlatitudes.

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Nuria Vargas and Víctor Magaña

Abstract

During the second half of the twentieth century, rapid demographic growth and urban expansion led to the development of the Mexico City metropolitan area (MCMA) urban heat island (UHI). The thermal gradient between rural and urban regions is used to define the UHI in the transition zone along the 26°C isotherm of mean maximum temperature. As the MCMA expands, more natural vegetation is replaced with urbanization, and the spatial extent of the 26°C isotherm grows. The loss of natural vegetation, in a densely populated region of Mexico, leads to the formation of a canopy-layer UHI. The intensification of the MCMA UHI results in an increase in the frequency of daily maximum temperatures above 30°C (above 26°C on a weekly average), a threshold value that constitutes a natural hazard. Warm-spell occurrences are related to an increase in the number of acute diarrhea diseases (ADD), mainly in zones of the MCMA where the socioeconomic and environmental conditions are low (e.g., insufficient access to potable water). Vulnerable people are mostly located in new settlements along the periphery of the MCMA, where large numbers of hospital discharges due to ADD are reported. The combined effect of more frequent warm spells and increasing vulnerability results in higher levels of risk of suffering this type of health problem, mainly during the warmest part of the year. This analysis may serve to develop UHI mitigation strategies and early warning systems to manage high levels of ADD risk during warm spells.

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Carolina Neri and Víctor Magaña

Abstract

Prolonged droughts severely affect the economic, social, and environmental sectors in Mexico. The interest in reducing the costs of drought is now focused on prevention by means of vulnerability reduction. The present study proposes a methodology to estimate vulnerability and risk to drought, considering the physical, economical, and social factors that make regions of Mexico prone to experiencing hydrological and agricultural droughts. Recognizing that there is no universally accepted way to describe vulnerability, the proposed method defines the object under study, the natural hazard, and vulnerability factors by means of indicators. The vulnerability factors are related to water infrastructure, the condition of aquifers or water reservoirs, the levels of wastewater treatment, water productivity in agriculture, hydraulic infrastructure, and water tariffs. A drought vulnerability model for each Hydrological Administrative Region (RHA) in Mexico is obtained by combining the vulnerability indicators. The product of vulnerability and hazard results in risk estimates that are compared with impact data to validate the approach. Information on agricultural or hydrological drought is used as impact data. The validation process is an important step in the methodology, since it allows examination of the causes of disasters by the vulnerability factors and leads to risk management strategies. It is found that although vulnerability to meteorological drought in the agricultural and hydrological sectors in Mexico has decreased in recent years, the drought risk is still high and results in severe economic losses, such as those registered in central and northern Mexico during the 2011–12 prolonged drought.

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Victor Magaña and Michio Yanai

Abstract

The mechanisms associated with the excitation of mixed Rossby-gravity waves (MRGWs) in the upper troposphere are studied using wind and outgoing longwave radiation (OLR) data from 1979 to 1991. The largest anomalies in meridional wind associated with MRGWs at 200 mb generally appear in the Northern Hemisphere summer–fall periods and they are pronounced in the central/eastern Pacific where equatorial westerlies form. The OLR field in the intertropical convergence zone shows a spectral peak with time and space scales similar to those of MRGWs at 200 mb. However, tropical convective activity does not show a clear contrast between years of strong and weak 200-mb MRGW activity.

During the Northern Hemisphere summer, weak easterlies or westerlies often form over the equatorial central/eastern Pacific allowing disturbances to propagate from the Southern Hemisphere midlatitudes into the deep Tropics. Some of these disturbances that possess spatial and temporal scales similar to those of the observed MRGWs (zonal wavenumber 4–6 and period 5–7 days) appear to project onto MRGWs. MRGWs are then intensified when the flow associated with them, and that with extratropical disturbances, are favorably superposed. The extratropical disturbances propagating into the Tropics possess a baroclinic vertical structure in the midlatitude troposphere. As they approach the Tropics, the disturbances appear to confine themselves to the upper troposphere under the effect of the “easterly dome.”

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Victor Magaña, Jorge A. Amador, and Socorro Medina

Abstract

The annual cycle of precipitation over the southern part of Mexico and Central America exhibits a bimodal distribution with maxima during June and September–October and a relative minimum during July and August, known as the midsummer drought (MSD). The MSD is not associated with the meridional migration of the intertropical convergence zone (ITCZ) and its double crossing over Central America but rather with fluctuations in the intensity and location of the eastern Pacific ITCZ. During the transition from intense to weak (weak to intense) convective activity, the trade winds over the Caribbean strengthen (weaken). Such acceleration in the trade winds is part of the dynamic response of the low-level atmosphere to the magnitude of the convective forcing in the ITCZ. The intensification of the trade winds during July and August and the orographic forcing of the mountains over most of Central America result in maximum precipitation along the Caribbean coast and minimum precipitation along the Pacific coast of Central America.

Changes in the divergent (convergent) low-level winds over the “warm pool” off the west coast of southern Mexico and Central America determine the evolution of the MSD. Maximum deep convective activity over the northern equatorial eastern Pacific, during the onset of the summer rainy season, is reached when sea surface temperatures exceed 29°C (around May). After this, the SSTs over the eastern Pacific warm pool decrease around 1°C due to diminished downwelling solar radiation and stronger easterly winds (during July and August). Such SST changes near 28°C result in an substantial decrease in deep convective activity, associated with the nonlinear interaction between SST and deep tropical convection. Decreased deep tropical convection allows increased downwelling solar radiation and a slight increase in SSTs, which reach a second maximum (∼28.5°C) by the end of August and early September. This increase in SST results once again in stronger low-level convergence, enhanced deep convection, and, consequently, in a second maximum in precipitation.

The MSD signal can also be detected in other variables such as minimum and maximum surface temperature and even in tropical cyclone activity over the eastern Pacific.

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Yukari N. Takayabu, George N. Kiladis, and Victor Magaña

Abstract

Insights by Professor Michio Yanai on tropical waves, which have been vital ingredients for progress in tropical meteorology over the last half-century, are recollected. This study revisits various aspects of research on tropical waves over the last five decades to examine, in Yanai’s words, “the nature of ‘A-scale’ tropical wave disturbances and the interaction of the waves and the ‘B-scale’ phenomena (cloud clusters),” the fundamental problem posed by Yanai at the design phase of the GARP Atlantic Tropical Experiment (GATE) in 1971. The various contributions of Michio Yanai to the current understanding of the dynamics of the tropical atmosphere are briefly reviewed to show how his work has led to several current theories in this field.

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Gregg Garfin, Nancy Lee, Victor Magaña, Ronald Stewart, J. Terry Rolfe, and Jamie McEvoy

No abstract available.

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Christian M. Appendini, Michel Rosengaus, Rafael Meza-Padillaand, and Victor Camacho-Magaña

Abstract

Tropical cyclones and their associated impacts along the western and eastern Mexican coastlines have led to the recent announcement of the creation of a National Hurricane and Severe Storms Center in Mexico. While Mexico falls under the responsibility of the Regional Specialized Meteorological Center in Miami, the newly announced center aims to provide local warning advisories to local governments and emergency managers. This study developed their first operational tool, which provides rapid forecasts of hazard areas under the presence of waves and storm surges from tropical cyclones threatening Mexico. The tool is based on precomputed wave parameters and storm surges from 3,100 synthetic tropical cyclones. Maximum envelope maps for all of the events are stored in a system database that is accessed through a graphical interface. Using a search function of synthetic events, the user can select those events most analogous to the tropical cyclone in question in order to make an assessment of warning areas. The tool allows users to plot maximum envelope maps for individual events or maxima of maximum maps combining several events, either using precomputed values for the different parameters (wind, waves, and storm surge) or a normalized map. To demonstrate the capabilities of the operational tool, we present an example application based on Hurricane Patricia (2015). This tool could also be implemented by developing countries affected by tropical cyclones, which otherwise are often limited by numerical modeling capabilities, time, and budgets.

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Siegfried Schubert, David Gutzler, Hailan Wang, Aiguo Dai, Tom Delworth, Clara Deser, Kirsten Findell, Rong Fu, Wayne Higgins, Martin Hoerling, Ben Kirtman, Randal Koster, Arun Kumar, David Legler, Dennis Lettenmaier, Bradfield Lyon, Victor Magana, Kingtse Mo, Sumant Nigam, Philip Pegion, Adam Phillips, Roger Pulwarty, David Rind, Alfredo Ruiz-Barradas, Jae Schemm, Richard Seager, Ronald Stewart, Max Suarez, Jozef Syktus, Mingfang Ting, Chunzai Wang, Scott Weaver, and Ning Zeng

Abstract

The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Niño–Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern.

One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific forcing pattern, with a cold Pacific leading to reduced precipitation and a warm Pacific leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitation response over the United States tends to occur when the two oceans have anomalies of opposite signs. Further highlights of the response over the United States to the Pacific forcing include precipitation signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. The response to the positive SST trend forcing pattern is an overall surface warming over the world’s land areas, with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all of the models.

It is hoped that these early results, as well as those reported in the other contributions to this special issue on drought, will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.

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