Abstract. The Tropical Ocean-Global Atmosphere (TOGA) program sought to determine the predictability of the coupled ocean-atmosphere system. The World Climate Research Programme's (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. IntroductionThe annual cycle of the monsoon systems has led the inhabitants of monsoon regions to divide their lives, customs, and economies into two distinct phases: the "wet" and the "dry." The wet phase refers to the rainy season during which warm, moist, and very disturbed winds blow inland from the warm tropical oceans. The dry phase refers to the other half of the year when winds bring cool and dry air from the winter continents. This distinct variation of the annual cycle occurs over Asia, Australia, west Africa, and in the Americas. In some locations (e.g., in the Asia-Australia sector) the dry winter air flows across the equa- Agricultural practices have traditionally been tied strictly to the annual cycle. Whereas the regularity of the warm and moist and cool and dry phases of the monsoon would seem to be ideal for agricultural societies, their very regularity makes agriculture susceptible to small changes in the annual cycle. Small variations in the timing and quantity of rainfall have the potential for significant societal consequences. A weak monsoon year (i.e., significantly less total rainfall than normal) generally corresponds to low crop yields. A strong monsoon usually produces abundant crops, although too much rainfall may produce devastating floods. In addition to the importance of the strength of the overall monsoon in a particular year, forecasting the onset of the subseasonal vari...

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 29ЊC (around May). After this, the SSTs over the eastern Pacific warm pool decrease around 1ЊC due to diminished downwelling solar radiation and stronger easterly winds (during July and August). Such SST changes near 28ЊC 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 (ϳ28.5ЊC) 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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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 Niñ o-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 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.

Major prolonged droughts in Mexico during the 20th century are mainly related to anomalous dry summers, such as those observed in the 1930´s, the 1950´s or the 1990's. Droughts in northernMexico frequently coincide with anomalously wet conditions over Mesoamerica (i.e, southern Mexico and Central America), and vice versa, displaying a dominant "see-saw" structure in persistent precipitation anomalies, mostly in relation to tropical sea surface temperature (SST) anomalies. Changes in the mean flow during the positive phase of the Atlantic Multidecadal Oscillation (AMO) and associated with weaker than normal moisture flux have resulted in some of the most severe droughts in North America. However, 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 IntraAmericas Seas (IAS) (i.e. the Gulf of Mexico and the Caribbean Sea) due to 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 19th century.

Abstract. MCMA-2003 was a major field campaign investigating the atmospheric chemistry of the Mexico City Metropolitan Area (MCMA) in April of 2003. This paper describes the wind circulation patterns during the campaign both within the Mexico City basin and on the regional scale. ''Time roses'' are introduced to concisely analyze the diurnal wind patterns. Three episode types were identified that explain the conditions encountered: ''O3-South'', ''Cold Surge'' and ''O3-North''. These can be diagnosed from a combination of synoptic and basin observations based on whether the day was predominantly cloudy, or whether the O3 peak was in the north or south of the basin. O3-South days have weak synoptic forcing due to an anti-cyclone over the eastern Pacific. Strong solar heating leads to northerly flows in the basin and an evening shift due to a gap flow from the south-east. Peak ozone concentrations are in the convergence zone in the south of the city. Cold Surge days are associated with ''El Norte'' events, with strong surface northerlies bringing cold moist air and rain. Stable conditions lead to high concentrations of primary pollutants and peak ozone in the city center. O3-North days occur when the sub-tropical jet is closer to Mexico City. With strong westerlies aloft, the circulation pattern is the same as O3-South days except for a wind shift in the mid-afternoon leading to ozone peaks in the north of the city. This classification is proposed as a means of understanding pollutant transport in the Mexico City basin and as a basis for future meteorological and chemical analysis. Furthermore, model evaluation and design of policy recommendations will need to take into account the three episode types.

The present analysis describes the temporal evolution of various meteorological parameters over the Americas warm pools that are related to the occurrence of the Mid Summer Drought (MSD). Contrary to what has been suggested by some authors, the bimodal structure of precipitation over the Pacific side of Mesoamerica is not a form of intraseasonal variability in convective activity related to the Madden Julian Oscillation, but a characteristic of the annual cycle in precipitation. The MSD – sea surface temperature (SST) ‐ radiation relationship proposed by Magaña et al. [1999] partially hold during the summer of 2001, when the Climate Experiment over the Americas Warm Pools (ECAC) field campaigns were conducted, with the SST exhibiting a bimodal structure in the MSD region. The maximum in tropical convection over the Central America‐Caribbean coast appears to play an important role in modulating convective activity in the surrounding regions, through induced subsidence related to direct circulations. The Caribbean Low Level Jet (CLLJ) is in phase with maximum western Caribbean Sea convective activity, reaching maxima intensities in July.

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