Víctor Magaña

Monsoons: Processes, predictability, and the prospects for prediction

The Tropical Ocean-Global Atmosphere (TOGA) program sought to determine the predictability of the coupled ocean-atmosphere system. The World Climate Research Programmes (WCRP) Global Ocean-Atmosphere-Land System (GOALS) program seeks to explore predictability of the global climate system through investigation of the major planetary heat sources and sinks, and interactions between them. The Asian-Australian monsoon system, which undergoes aperiodic and high amplitude variations on intraseasonal, annual, biennial and interannual timescales is a major focus of GOALS. Empirical seasonal forecasts of the monsoon have been made with moderate success for over 100 years. More recent modeling efforts have not been successful. Even simulation of the mean structure of the Asian monsoon has proven elusive and the observed ENSO-monsoon relationships has been difficult to replicate. Divergence in simulation skill occurs between integrations by different models or between members of ensembles of the same model. This degree of spread is surprising given the relative success of empirical forecast techniques. Two possible explanations are presented: difficulty in modeling the monsoon regions and nonlinear error growth due to regional hydrodynamical instabilities. It is argued that the reconciliation of these explanations is imperative for prediction of the monsoon to be improved. To this end, a thorough description of observed monsoon variability and the physical processes that are thought to be important is presented. Prospects of improving prediction and some strategies that may help achieve improvement are discussed.

The Midsummer Drought over Mexico and Central America

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...

A U.S. Clivar project to assess and compare the responses of global climate models to drought-related SST forcing patterns: Overview and results

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 Nino–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 for...

The NAME 2004 Field Campaign and Modeling Strategy

The North American Monsoon Experiment (NAME) is an internationally coordinated process study aimed at determining the sources and limits of predictability of warm-season precipitation over North America. The scientific objectives of NAME are to promote a better understanding and more realistic simulation of warm-season convective processes in complex terrain, intraseasonal variability of the monsoon, and the response of the warm-season atmospheric circulation and precipitation patterns to slowly varying, potentially predictable surface boundary conditions. During the summer of 2004, the NAME community implemented an international (United States, Mexico, Central America), multiagency (NOAA, NASA, NSF, USDA) field experiment called NAME 2004. This article presents early results from the NAME 2004 campaign and describes how the NAME modeling community will leverage the NAME 2004 data to accelerate improvements in warm-season precipitation forecasts for North America.

Regional aspects of prolonged meteorological droughts over Mexico and Central America.

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 convect...

Mixed Rossby–Gravity Waves Triggered by Lateral Forcing

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 MRGW...

Tropical-Midlatitude Interaction on the Time Scale of 30 to 60 Days during the Northern Summer of 1979

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-Pa...

Climate and freshwater resources in Northern Mexico: Sonora, a case study.

An analysis of current trends in water availability in the Mexican border state of Sonora is presented to illustrate what may be faced under climate change conditions. Precipitation, streamflow and even dam levels data are examined to estimate what changes have been experienced in recent decades. There are indications that the more frequent occurrence of El Niño/Southern Oscillation (ENSO) events have resulted in more winter precipitation and consequently in a slight increase in water availability in northwestern Mexico. However, water demands grow much faster than such trends in water availability, mainly due to a rapid increase in population in urban areas and in socio-economic activities such as those related to agriculture, industry and power generation. Some strategies to adapt or mitigate climate change conditions are proposed.

The 40‐ and 50‐day oscillations in atmospheric angular momentum at various latitudes

Intraseasonal oscillations in atmospheric angular momentum at various latitudes from 1979 to 1989 are analyzed and compared with fluctuations in tropical convective activity associated with the Madden-Julian oscillation. Oscillations with periods of approximately 40 and 50 days are observed in both the globally integrated angular momentum M and tropical convective activity. M is maximum when the convective activity increases over the western/central Pacific and weakens over the Indian Ocean. Cross-spectral analyses show that the main contributions to both 40- and 50-day oscillations in M and in length-of-day (LOD) come from the tropics where fluctuations in regional atmospheric angular momentum with the same periodicities exist. In general, the signal of the oscillation propagates from the tropics to the subtropics. In the northern hemisphere (NH) mid-latitudes, however, a small-amplitude 40-day oscillation in atmospheric angular momentum usually exists regardless of the presence or absence of fluctuations with the same periodicity in the tropics. In the southern hemisphere (SH) a dominant 50-day oscillation exists independently from those in the tropics. However, the 40- and 50-day oscillations in the NH and SH do not seem to drive oscillations with the same periodicity in LOD or M.

Climate change impacts and adaptation strategies in temperate forests in Central Mexico: a participatory approach

Worldwide temperate forests are vulnerable to climate change because climate anomalies may impact tree mortality and forest productivity, as well as the economic and social dynamics of the people that depend on forest services. Evaluation of their vulnerability is a key element for the impact scenarios under climate change and for the design of adaptation strategies. The objective of this study is to analyze the case of forests in central Mexico and the importance of land degradation as a factor that increases vulnerability to warmer than normal temperatures that may result in forest fires. By means of data analyses and participative workshops, current vulnerability factors of temperate forests ecosystem to a warmer climate have been identified and found to be related to local traditional practices, like slash and burn, illegal extraction, deforestation and land use change that have led to loss of biodiversity and ecosystem services. However, various stakeholders now admit that some of their practices on the use of forest resources lead to vulnerability to climate anomalies. If current trends in vulnerability continue, and climate change makes drier conditions more frequent, forest extension in the Central Mexico could be severely reduced mainly by wildfires, pests and loss of ecological services. In a number of workshops, stakeholders’ from the forests of Tlaxcala suggested three adaptation measures: i) forest conservation, ii) forest restoration and iii) sustainable forest management. Particular adaptation strategies are considered more appropriate for some communities that expect long term benefits (augmenting water resources, soil erosion mitigation and flood control). Therefore, it is concluded that perception and ancestral knowledge of stakeholders need to be incorporated in the vulnerability analysis in order to have them involved in the adaptation process and to empower them in the implementation of the adaptation strategies.