Thesis (M. S.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2010. / Committee Chair: Peter J. Webster; Committee Member: Judith A. Curry; Committee Member: Robert X. Black. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Asia, one of the most densely populated regions in the world, receives 50%-80% of annual rainfall during the summer monsoon season. While agricultural yield and water resource over this region benefit greatly from the summer rainfall, human lives and infrastructures are, at the same time, threatened by the frequently occurring heavy downpour. Although large efforts have been devoted to delineate the characteristics and variations of Asian monsoon extreme rainfall, its dynamical triggers and the physical mechanisms underlying the past and future changes remain poorly understood. To address the knowledge gap, this thesis aims to provide a process-oriented perspective on monsoon rainfall extremes with special attention given to the heavy-rain producing weather systems, namely the monsoon low-pressure systems (LPSs). In Chapter 1, an objective feature-tracking algorithm is adopted to compile the observed trajectories of monsoon LPSs over the East Asia monsoon region during the post-1979 satellite era. Two types of LPS are identified. One forms near the downwind side of the Tibetan Plateau (i.e., southwestern China) and travels northeastward toward north-central China. The other forms over the western North Pacific Ocean and migrates along the southern and western peripheries of the subtropical high. The two types of LPS together account for approximately half of the rainfall extremes. The terrestrial LPSs are responsible for a great majority of extreme rainfall over inland areas, whereas the influences of marine LPSs are primarily confined to the coastal regions where they frequently make landfall. The observed long-term change in extreme rainfall, featured by a “south flood-north drought” pattern, aligns well with the change in LPS activity. The decreasing number of northeastward-moving terrestrial LPSs leads to an extreme rainfall dipole with negative trends in north-central China and positive trends in southern China, while the increasing number of northward-recurving marine LPSs enhances the extreme rainfall along the southeastern China coast. These trends are driven by the weakening of the monsoonal southwesterlies and the eastward retreat of the subtropical high. Despite the great importance of terrestrial LPSs in modulating extreme rainfall over East Asia, these storms have so far received limited attention in research community because of the lack of a track archive. Chapter 2 further investigates the dynamical processes fueling the different evolution regimes of individual terrestrial LPSs and explores the environmental factors controlling their evolution. Chapters 3 and 4 concentrate on the South Asian monsoon region, where the long-term trend of LPS activity remains debatable owing to the potential errors arising from the manual and subjective identification of LPSs from weather charts. Using two different tracking algorithms, in Chapter 3 we find that the trends of extreme rainfall and LPS activity indeed exhibit a strong coherence. Over time, the LPSs propagate preferentially through south-central India rather than north-central India, imparting a corresponding dipole footprint in rainfall extremes. In agreement with previous studies that the LPS propagation is a combined effect of the northwestward-propagating component due to horizontal nonlinear adiabatic advection and the southwestward-propagating component due to diabatic heating, the LPSs traveling through south-central India have stronger updrafts on their west-southwestern flank than those passing through north-central India. The increased frequency of LPSs propagating through south-central India is likely due to a strengthened cross-equatorial moisture transport over the Arabian Sea, which favors more vigorous storm convection through the conditional instability of second kind mechanism. Chapter 4 then focuses specifically on the role of LPS in triggering the record-breaking Pakistan flood during summer 2022, when most of the South Asian LPSs were able to propagate into Pakistan with intensity and longevity far exceeding historical records. The abnormal LPS activity was fueled by a historically-high cross-equatorial moisture transport, which is in agreement with the fingerprint of anthropogenic warming in the Coupled Model Intercomparison Project - Phase 6 (CMIP6) models. The last chapter of this thesis proceeds to evaluate the performance of CMIP6 models in simulating the monsoon rainfall extremes and to explore whether the performance is affected by the degree to which the models could realistically capture the LPS activity. The modelled precipitation often occurs more frequently and the extreme events are commonly less intense than in observations. A robust improvement of model performance in simulating monsoon rainfall extremes as resolution increases is seen across most models, both in terms of spatial distribution and intensity. The dry biases get improved in the regions with high exposure to monsoon LPSs, such as central India, southern China, and western North Pacific. The improvement is associated with a better representation of LPSs, which become more frequent and stronger at finer resolution.

No description available.

The southwest monsoon is responsible for a summer precipitation maximum for much of the southwest U.S. Biostratigraphies of pollen, plant macrofossils, and aquatic fossils in lakes from near modern monsoon boundaries in conjunction with climate modelling suggests variations in strength of the monsoon system during the late Quaternary. At Montezuma Well, Arizona, high percentages of Pinus and Juniperus pollen as well as maximum influxes of Quercus and Gramineae pollen suggest a shift from dominantly winter to summer precipitation between ca. 12,000 and 9000 yr BP. Maximum aridity occurred 7000-4000 yr BP, coincident with lowest lake levels. In the High Plateaus region of the Colorado Plateau, high Artemisia to Chenopodiaceae-Amaranthus pollen ratios suggests precipitation maxima until ca. 6000 yr BP at Fryingpan Lake and 5000 yr BP at Posy Lake. Pollen records suggest that Pinus edulis, P. ponderosa, and Quercus gambelii, were present on the western Colorado Plateau throughout the Holocene. Expansion of shadscale steppe vegetation at low elevations and upslope movement of ecotones for Pinus edulis, P. ponderosa, and Q. gambelii after ca. 6000 yr BP and low lake levels ca. 5000-3700 yr BP, suggest a period of maximum aridity from decreased summer precipitation. In the San Luis Valley, Colorado, pollen records from Head Lake on the basin floor suggest an expansion of oaks and junipers at the Pleistocene/Holocene boundary that may indicate increased summer precipitation. Lake levels of Head Lake fell sharply after ca. 9500 yr BP. Pollen records from Como Lake in the Sangre de Cristo Mountains suggest that Pinus ponderosa was established in the area by ca. 12,000 yr BP and Pinus edulis by ca. 9500 yr BP. Highland regions such as the High Plateaus (until ca. 6000-5000 yr BP) and central Colorado (until ca. 4000 yr BP) may have experienced Holocene summer precipitation maxima later into the Holocene than sites in lower elevation regions. Regional orographic uplift as a catalyst for convective summer precipitation may be responsible for the duration of summer precipitation maxima in these regions. On a longitudinal gradient, sites to the west such as in the southern Great Basin and Mohave Desert may have recorded enhanced summer precipitation earlier, reflecting different histories of the low-level jets in the southwest. The paleoecologic record generally confirm predictions of general circulation models (GCMs) that southwest monsoon circulation was enhanced from 12,000-6000 yr BP in response to peaks in annual (11,500-11,000 yr BP) and summer insolation (10,000-9000 yr BP) during the late Quaternary.

Almost one-half of the annual precipitation in the southwestern United States occurs during the North American monsoon (NAM). Given favorable synoptic-scale conditions, organized monsoon thunderstorms may affect relatively large geographic areas. Through an objective analysis of atmospheric reanalysis and observational data, the dominant synoptic patterns associated with NAM extreme events are determined for the period from 1993 to 2010. Thermodynamically favorable extreme-weather-event days are selected on the basis of atmospheric instability and precipitable water vapor from Tucson, Arizona, rawinsonde data. The atmospheric circulation patterns at 500 hPa associated with the extreme events are objectively characterized using principal component analysis. The first two dominant modes of 500-hPa geopotential-height anomalies of the severe-weather-event days correspond to type-I and type-II severe-weather-event patterns previously subjectively identified by Maddox et al. These patterns reflect a positioning of the monsoon ridge to the north and east or north and west, respectively, from its position in the "Four Corners" region during the period of the climatological maximum of monsoon precipitation from mid-July to mid-August. An hourly radar gauge precipitation product shows evidence of organized, westward-propagating convection in Arizona during the type-I and type-II severe weather events. This new methodological approach for objectively identifying severe weather events may be easily adapted to inform operational forecasting or analysis of gridded climate data.

A presente dissertação tem por objetivo as edições fac-similar e semidiplomática justalinear de manuscritos datados do século XIX da cidade de Porto Feliz, e, com base nesses manuscritos, realizar, sucintamente, uma análise codicológica e paleográfica, e, por fim preparar um índice remissivo dos topônimos e antropônimos encontrados nos referidos documentos. Ao todo serão 105 manuscritos compostos por 101 ofícios e 4 atestados. Serão analisados também fatores históricos fundamentais para a formação da cidade de Porto Feliz e, conseqüentemente, de sua variedade lingüística. A partir dessa pesquisa, obtemos dados relevantes para o estudo da formação e expansão do dialeto caipira formado na região ribeirinha do Rio Tietê, e que se expandiu até o centro-oeste do Brasil, mais precisamente até o Estado de Mato-Grosso, por meio das monções. / The present dissertation has for objective the editions fac-similar and semidiplomatic line by line of dated manuscripts of century XIX of the Porto Feliz city and with base in these manuscripts, to make it, briefly, a codicological and paleographic analysis and finally to prepare a remissive index of the place names and antroponyms found in these documents. To all they will be 105 manuscripts composites for 101 offices and 4 certified. It will also be analyzed historical factors essentials for the formation of the Porto Feliz and consequently, of its linguistic variety. From this research, we get relevant dices for the study of the formation and expansion of the rustic dialect formed in the riverside region of the River Tietê and that it expanded until the center-west of Brazil, more necessarily until the Mato Grosso State, by means of the monsoons.

A presente dissertação tem por objetivo as edições fac-similar e semidiplomática justalinear de manuscritos datados do século XIX da cidade de Porto Feliz, e, com base nesses manuscritos, realizar, sucintamente, uma análise codicológica e paleográfica, e, por fim preparar um índice remissivo dos topônimos e antropônimos encontrados nos referidos documentos. Ao todo serão 105 manuscritos compostos por 101 ofícios e 4 atestados. Serão analisados também fatores históricos fundamentais para a formação da cidade de Porto Feliz e, conseqüentemente, de sua variedade lingüística. A partir dessa pesquisa, obtemos dados relevantes para o estudo da formação e expansão do dialeto caipira formado na região ribeirinha do Rio Tietê, e que se expandiu até o centro-oeste do Brasil, mais precisamente até o Estado de Mato-Grosso, por meio das monções. / The present dissertation has for objective the editions fac-similar and semidiplomatic line by line of dated manuscripts of century XIX of the Porto Feliz city and with base in these manuscripts, to make it, briefly, a codicological and paleographic analysis and finally to prepare a remissive index of the place names and antroponyms found in these documents. To all they will be 105 manuscripts composites for 101 offices and 4 certified. It will also be analyzed historical factors essentials for the formation of the Porto Feliz and consequently, of its linguistic variety. From this research, we get relevant dices for the study of the formation and expansion of the rustic dialect formed in the riverside region of the River Tietê and that it expanded until the center-west of Brazil, more necessarily until the Mato Grosso State, by means of the monsoons.

Thesis. 1976. M.S. cn--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Bibliography: leaves 113-116. / M.S.cn

Open grassy environments in Africa have been key landscapes for the development and evolution of humans and our hominid ancestors for millions of years. These environments have not been static, however, as global climate changes have strongly shaped their nature and location over time. In the modern, at least 80 million people living in Sub-Saharan Africa rely on agriculture and pastoralism within the grasslands and savannas of the Sahel region alone for food security. Devastating droughts and associated famine in the region over the past several decades have highlighted this region’s potential vulnerability to future climate change. Wildfires play a unique and critical role in maintaining Africa’s grasslands and savannas, especially in the Sahel region. Emissions from these fires have additional ramifications for the Earth’s radiative balance and global cycles of carbon and nutrients. As populations in Africa rise over the coming century from ~1.3 billion to 4 billion people by 2100, increasing demand for food, rising temperatures, and highly uncertain changes in rainfall and wildfire patterns are poised to put the people and ecosystems of this region in jeopardy. In the face of potentially novel environmental conditions resulting from anthropogenic climate change, this research aims to better understand the long-term interconnections of climate, ecology, and human presence in Northwest Africa and how these linkages may vary under broad shifts in climate. Accurate projections of future climate and ecosystem change not only require the mechanistic understanding of climate forcings and climate-ecosystem interactions that can be gleaned from modern relationships, but also information about how these interactions may vary as a function of changes in the background climate state itself (on centennial to million year timescales). Highly spatially-resolved satellite measurements relevant for asking such questions only extend back a few decades and thus only provide a limited perspective on whether or not modern climate-ecosystem interactions are stationary through time. This thesis is focused on developing and applying paleoclimate reconstruction techniques to generate new records of hydroclimate, ecosystem structure, and fire activity in Northwest Africa over a broad set of time scales. These new records are used to assess the governing controls of climate variability and evaluate the evolution of climate-ecosystem interactions across a diversity of background climate states. We seek specifically to (1) improve our understanding of the natural climate forcings that dictate changes in Northwest African monsoon rainfall, (2) evaluate how changes in rainfall and other climate parameters––namely atmospheric CO₂ concentrations––together affect ecosystem distributions and compositions in Northwest Africa, (3) ground-truth the use of increasingly popular molecular proxies of fires applied to marine sediment archives, (4) assess the relative environmental and human controls on fires in Northwest African savannas over time, and (5) develop interpretive frameworks for understanding multiproxy records of environmental changes in Northwest Africa to draw conclusions about how climate-ecosystem interactions may have evolved over time. To address these goals, this dissertation is broken down into four chapters. The first two chapters focus on the orbital-scale to multimillion year forcings in the climate system that control the strength and tempo of the Northwest African monsoon and how these changes impact the distributions and compositions of ecosystems in the region over time. In both Chapters 1 and 2, we develop new reconstructions of hydroclimate using the hydrogen isotopic composition of plant-waxes and extraterrestrial 3He normalized dust fluxes from marine sediment core MD03-2705 taken off the coast of Mauritania along the Northwest African margin. We further reconstruct ecosystem change using the carbon isotopic composition of plant-waxes. Chapter 1 is centered on the late-Pleistocene while Chapter 2 takes a wider perspective and explores long-term trends in Northwest African hydroclimate and vegetation structure from the Pliocene to the late-Pleistocene. In the second half of this work, the focus is narrowed to center the role of fire in Northwest African savannas and how the nature of burning in this region has changed since the last glacial maximum. In Chapter 3, we use atmospheric back trajectory modeling and a transect of marine core sediments taken aboard the research vessels Vema and Conrad that spans the southern European to southern west African margin to test if molecular biomarkers of vegetation (plant-wax n-alkanes and pentacyclic triterpene methyl ethers) and fires (pyrosugars and polycyclic aromatic hydrocarbons) preserved in marine sediment archives capture modern distributions of ecosystems and biomass burning on the landscape. In Chapter 4, we generate new records of the fire history of Northwest Africa from the last glacial maximum to the late-Holocene from marine sediment core OCE 437-7 GC68. We compare the relative influences of changes in rainfall, ecosystem structure and human activities on fire across the most recent deglaciation and ‘Green Sahara’ period. From these new records, we are able to draw several conclusions. In Chapter 1 we find that the location, timing, and intensity of northwest African monsoon rainfall is controlled by low-latitude insolation gradients and that while increases in precipitation are associated with the expansion of grasslands into desert landscapes, changes in pCO2 predominantly drive the C3/C4 composition of savanna ecosystems. In Chapter 2 we observe that low latitude insolation gradients best explain both the tempo and amplitude of orbital scale variations in Northwest African rainfall over the last five million years, however strengthening sea surface temperature gradients in the Atlantic Ocean during the mid-Pleistocene likely led to a precipitous and sustained decline in monsoon strength ~900 thousand years ago independent of any change to orbital insolation forcing. Furthermore, changes in the relationship between rainfall and vegetation in Northwest Africa can be used to track changes in the northward extent of ecosystems, augmenting previous pollen-based reconstructions, which together show shifts in ecosystem distributions over the Plio-Pleistocene likely related to changes in ecosystem disturbances and climate-vegetation interactions. In Chapter 3 we show that by accounting for the effects of long-range transport of biomarkers, good agreement is found between ecosystem composition and biomass burning patterns on the African continent and the distribution of terrestrial plant and fire biomarkers in marine core top sediments. This provides strong justification for applying molecular indicators of fires to the paleorecord. In Chapter 4, we show that rainfall is the dominant natural control on the amount of biomass burned in Northwest African savannas, but increased human presence and land-use change during the mid- to late-Holocene likely fundamentally changed the fire regime of Northwest Africa to this day.

Humans’ ability to combat heat stress through sweat-based evaporative cooling is modulated by air temperature and humidity, rendering human health highly sensitive to humid heat extremes. While the field of climate science has studied extreme dry bulb temperatures for decades, exploration of the physical drivers producing extreme humid heat is nascent in comparison. Two major areas of development for the evolving field are: 1) improving understanding of the local drivers of extreme humid heat, and 2) collating a set of universal physical mechanisms which generate humid heat extremes across the planet. The four chapters of this dissertation together advance each of these goals. Chapter 1 relates the occurrence of extreme humid heat in the Persian Gulf and South Asia to two related modes of intraseasonal climate variability, namely the Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO). Wet bulb temperatures (Tw) sufficiently high to impact human health are found to be almost twice as likely during certain oscillation phases than in others. Humid heat anomalies in each region are driven by distinct local circulation patterns and variations in moisture. Chapter 2 evaluates the influence of monsoon onset and subseasonal precipitation variability on the occurrence of extreme Tw across South Asia. Extreme Tw events often occur on rainy days during the monsoon season. However, the influence of precipitation on Tw varies with the background specific humidity climatology. In climatologically drier areas, positive Tw anomalies tend to occur when precipitation increases due to either early onset or wet spells during the monsoon. In contrast, in climatologically humid areas, positive Tw anomalies occur during periods of suppressed precipitation, including delayed onset and dry spells during the monsoon. Chapter 3 analyzes the dynamics of the record-breaking extreme heat event in Rio de Janeiro in November 2023. The heatwave was associated with persistent atmospheric blocking, potentially linked to developing El Niño conditions. Temperatures were intensified by declining soil moisture and elevated local sea surface temperatures, and the event was finally terminated due to the onset of precipitation. This chapter also evaluates the influence of climate change on the frequency of heat extremes, finding a significant increase in the frequency of high heat days throughout Brazil over the past four decades. Further, the frequency of spring heat extremes is expected to increase in the future, though highly dependent upon our future emissions pathway. Chapter 4 explores the combinations of temperature and humidity contributing to humid heat experienced across the globe. In addition to using traditional metrics, this chapter derives a novel variable named “stickiness,” which quantifies the relative contributions of temperature and specific humidity to a given Tw. Consistent across metrics, high magnitudes of Tw tend to occur in the presence of anomalously high moisture, with temperature anomalies of secondary importance. Nonetheless, there is a broad range of stickiness observed for a given Tw across moderate-to-high Tw thresholds relevant to socioeconomic impacts.