Central Africa is, climatologically speaking, a poorly studied region (Clivar, 2000; Dezfuli and Nicholson, 2012; Nicholson and Dezfuli, 2012; Todd and Washington, 2004). It is considered as a knowledge gap in the understanding of the tropical climate system (Todd and Washington, 2004). Drivers of Central Africa rainfall are not well documented and deserve more attention. The aims of thesis are to enhance our fundamental understanding of Central Africa rainfall and the mechanisms involved in its seasonal and interannual variability as well as to assess how an atmospheric general circulation model forced by observed sea surface temperature (SST), the ECHAM5.3 model, does represent the main features of Central Africa hydroclimate variability. The seasonal cycle of Central Africa rainfall is primarily driven by change in the atmospheric low-pressure system of Central Africa landmass, water vapor and latent heat release rather than change of local temperature. From October to April, over Central Africa and its neighbouring regions, we highlight the existence in the mid-lower troposphere, between 1000 and 500 hPa of a dominant cyclonic and quasipermanent circulation pattern that drives the atmospheric large-scale circulation and its associated water vapor transports, namely the Central Africa Low. The Central Africa Low, with its variation strongly modulated by El Niño Southern Oscillations (ENSO), is characterized by strong convective activity due to an unstable atmosphere over central Africa, leading to high rainfall with less variance. Nevertheless, when the Central Africa Low prevails, Central Africa is a sink of water vapor, with the Indian Ocean as the main supplier. The weakening of the Central Africa Low, in May to September, is associated with the reversal of the water vapor transport at the northern boundary channel, leading Central Africa to become a source of moisture. During this season, both surrounding oceans are suppliers of moisture, with some additional contribution from the Congo basin rainforest. Central Africa rainfall variability is controlled by large-scale circulation variation, rather than variation in tropospheric water vapor. Year-round, the large-scale circulation is characterized by dominant easterly jets at middle (African easterly jets, AEJs) and upper (tropical easterly jets, TEJ) levels, owed by the Central Africa Low. At low-levels, there is a shallow zonal overturning circulation thermally direct, namely the Congo Basin Cell, driven by near-surface land-ocean thermal contrast between the warm central Africa landmass and the relatively cold Atlantic Ocean. The Congo Basin Cell, characterizes by eastward flow, persists year-round, with a maximum strength (-196.92±32.89 Sv) and width (30o degree) in August/September and minimum strength (-24.80± 17.83 Sv) and width (~6o degree) in May. The Congo Basin Cell does not play any crucial role in modulating Central Africa rainfall but it does regulate the rainfall distribution, through the seasonal position of the ITCZ. At midlevel, the atmospheric convective instability over Central Africa is controlled by the southward import of high moist static energy from the warmer Sahel associated with the AEJ over Central Africa. The saturation of the rising moist air at midlevel determines the location of high rainfall over central Africa year-round. Nevertheless, the absence of significant trend (- 0.013 mm per decade) of the Central Africa rainfall is associated with the weakening of the Central Africa Low in recent decades (1979 to 2015), consistent with Lau and Wu (2006). Further investigations on physical mechanisms affecting the Central Africa hydroclimate reveals that the Central Africa Low and land-ocean thermal contrasts are the main drivers of Central Africa rainfall variability at seasonal and interannual time scale, through the control of AEJs and the Congo Basin Cell strength and width. The analysis of ECHAM5.3 experiments provide a support to these mechanisms. Finally, to unravel what are the physical mechanisms shaping the rainfall anomalies patterns associated with the interannual variability of Central Africa rainfall, we found out that the Central Africa does reflect the regional-scale response of the atmosphere to the variation of the interbasin SST anomalies gradient (ΔSST) between tropical Atlantic and Indian Oceans. Likely, the zonal contrast of central Africa rainfall is owed by the Central Africa Low, which separates central Africa in two distinct regions of opposite polarity by regulating the strength of the low-level westerly and mid-upper easterly jets and their associated water vapor transports. This east-west dipole-like pattern of Central Africa rainfall is similar to the second leading mode obtained by empirical orthogonal functions (EOF) analysis of rainfall anomalies during the long rainy season. Thus, during the positive phase of ΔSST, the Central Africa Low area change induces an anomalous clockwise zonal overturning cell over Central Africa, with ascending branch over Atlantic, indicative of deep convection leading to rainfall surplus, and sinking branch over Indian Ocean, indicative of subsistence, which suppress convection and lead to rainfall deficit, consistent with the mechanism proposed by Dezfuli et al. (2015). However, the impact of ΔSST on Central Africa rainfall variability is asymmetrical during positive and negative phases of ΔSST.

The phasing of various teleconnection patterns has been linked to variability of tornado activity in various geographic regions. These links have been used to improve long-term tornado forecast models. Oklahoma has been long-considered the center of Tornado Alley, has remained vulnerable to tornado hazards despite mitigation efforts, and as such would benefit greatly from improvements to tornado forecasting. This study compares phases of four teleconnection patterns considered to be primary climate influencers in North America (El Niño-Southern Oscillation, North Atlantic Oscillation, Arctic Oscillation, and Pacific/North American Pattern) to tornado activity in Oklahoma. The phases of these teleconnection patterns were individually compared to Oklahoma tornado day frequency via χ2 statistical testing. It is shown that there are potentially linkages between the negative phases of the North Atlantic Oscillation, the Arctic Oscillation, and the Pacific-North American Pattern and Oklahoma tornado activity.

This research presents a new multivariate statistical approach to downscale hydroclimatic variables associated with global climate signals, from low-resolution Global Climate Models (GCMs) to high-resolution grids that are appropriate for regional and local hydrologic analysis. The approach uses Principal Component Analysis (PCA) and Multichannel Singular Spectrum Analysis (MSSA) to: 1) evaluate significant variation modes among global climate signals and spatially distributed hydroclimatic variables within certain spatial domain; 2) downscale the GCMs' projections of the hydroclimatic variables using these significant modes of variation and 3) extend the results to other correlated variables in the space domain. The approach is applied to the Colorado River Basin to determine common oscillations among observed precipitation and temperature patterns in the basin and the global climate signals El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). These common oscillations serve as a basis to perform the downscaling of ENSO-related precipitation and temperature projections from GCMs, using a new gap-filling algorithm based on MSSA. The analysis of spatial and temporal correlations between observed precipitation, temperature and vegetation activity (represented by the Normalized Difference Vegetation Index, NDVI) is used to extend the downscaling of precipitation to vegetation responses in ten ecoregions within the basin. Results show significant common oscillations of five and 15-year between ENSO, PDO and annual precipitation in the basin, with wetter years during common ENSO and PDO positive phases and dryer years during common negative phases. Precipitation also shows an increase in variability in the last 20 years of record. Highly correlated responses between seasonally detrended NDVI and precipitation were also identified in each ecoregion, with distinctive delays in vegetation response ranging from one month in the southern deserts (in the fringe of the monsoon precipitation regime), to two months in the mid latitudes and three months to the north, affected by seasonal precipitation. These results were used to downscale precipitation and temperature from two GCMs that perform well in the basin and have a distinctive ENSO-like signal (MPI-ECHAM5 and UKMO-HADCM3) and to extend the downscaling to estimate vegetation responses based on their significant correlations with precipitation.

The goal of this study is to improve seasonal tornado outbreak forecasting by creating a statistical model that forecasts tornado outbreak frequency in the US using teleconnection indices as predictors. For this study, a tornado outbreak is defined as more than 6 tornado reports associated with a single synoptic system and an event N15 rating index of 0.5 or higher. The tornado outbreak season is confined to all months after February for a given calendar year. Monthly teleconnection indices are derived from a rotated principal component analysis (RPCA) of the geopotential height fields. Various regression techniques were trained with a sample of monthly teleconnection indices, tested on new data, and optimized to achieve the highest predictive skill. The outcome of this study could potentially allow forecasters the ability to predict tornado outbreak potential on a climatological scale with months of lead-time, allowing for better preparation strategies for tornado outbreak seasons.

In this thesis I present the results of a comprehensive assessment of the Pacific-North American (PNA) teleconnection pattern in general circulation models (GCMs) and a regional climate model (RCM). The PNA teleconnection pattern is a quasi-stationary wave field over the North Pacific and North America that has long been recognized as a robust feature of Northern Hemisphere atmospheric circulation, and directly affects the interannual variability of North American temperature and precipitation. The teleconnection is evaluated under present (1950-2000) and future (2050-2100) climate in a coupled GCM (MPI/ECHAM5) and a high-resolution regional climate model (RegCM3). I further assess the PNA in 27 atmosphere-ocean GCMs and earth system models (ESMs) from the ongoing fifth phase of the Coupled Model Intercomparison Project (CMIP5). The National Centers for Environmental Prediction and Atmospheric Research (NCEP/NCAR) Reanalysis serves a quasi-observational baseline against which the models are evaluated. For each analysis, changes in the spatial and temporal patterns of the PNA spatial are assessed for both the present and future climates, and these changes are then related to changes in climate and surface hydrology in North America. Coupling the NCEP and ECHAM5 GCMs with RegCM3 is very successful in that the PNA is resolved in both models with little loss of information between the GCMs and RegCM3, thereby allowing an assessment of high-resolution climate with an inherent skill comparable to that of the global models. The value of the PNA index is generally independent of the method used to calculate it: three- and four-point modified linear pointwise calculations for both the RegCM3 and ECHAM5 model simulations produce very similar indices compared with each other, and compared with those extracted from a rotated principle component analysis (RPCA) which is also used to determine the PNA spatial pattern. The spatial pattern of the PNA teleconnection emerges as a leading mode of variability from the RPCA, although the strength of the teleconnections are consistently weaker than NCEP as defined by four main "centers of action". This discrepancy translates into the strength of the controls of the PNA on surface climate. Maps of the correlations between the GCM PNA indices and RCM surface climate variables are compared to the results from the NCEP/NCAR Reanalysis. I find that correlation patterns with temperature and precipitation are directly related to the positioning of the Aleutian low and Canadian high, the two main drivers of upper-atmospheric circulation in the PNA sector. The CMIP5 models vary significantly in their ability to simulate the quasi-observed features of the PNA teleconnections. The behavior of the models relative to NCEP is more definite than the trends within the models. Most models are unable to resolve the temporal variability of NCEP; however, on the other hand most of the models are able to capture the PNA as a low-frequency quasi-oscillation. Many of the models are unable to simulate the barotropic instability that initiates wave energy propagation through the 500-hPa geopotential height field, thereby leading to phase-locking and thus the positive and negative modes of PNA are indistinguishable. The behavior and the spatial patterns of the PNA throughout the 21st century are consistent with other projections of future climate change in that most models exhibit a lengthening of the eddy length scale and a poleward shift of the mid-latitude jet stream associated with polar amplification of greenhouse-gas driven global warming. Finally, my analyses underscore the robustness of multi-model means, suggesting that the cumulative results of multiple climate models outperform the results from individual models because ensemble means effectively cancel discrepancies and hereby expose only the most robust common features of the model runs. While ensembles provide better representation of the average climate, they potentially mask climate dynamics associated with inter-annual and longer time scales. Relying on ensemble means to limit model spread and uncertainties remains a necessity in using models to project future climate. / Graduation date: 2013

The occurrence of large scale and rapid climate shifts at millennial time-scales (suborbital) remains an enigma between records from high and low latitudes spanning the Late Quaternary. This thesis studies such variations in the eastern tropical North Pacific (ETNP) using marine sediment cores retrieved from Mexico and Nicaragua. The main goals are to understand the nature of millennial timescale climate-changes in the Pacific low latitudes, to identify the atmospheric and oceanic teleconnections involved, to document the impacts on the biogeochemical cycles of carbon, nitrogen and silicon, and their potential to regulate Greenhouse Gas (GHG) concentrations during the last two glacial cycles (the last 240,000 years before present). In this thesis, we use a suite of multi-proxy records from the Core MD02-2519, which are compared to others records from adjoining regions to study the climatic history of the ETNP at millennial timescales. The Core MD02-2519, was retrieved from 955 mbsl off NW Mexico. It is strategically located within the North Pacific Intermediate Water (NPIW), underlying the coastal upwelling and denitrification zones of the ETNP. The paleoceanography of the region is studied using proxy records of productivity, denitrification, intermediate water circulation and radiocarbon activity, which are discussed in 5 separated chapters. In Chapter 1, we use records of organic carbon (%OC) and diffuse spectral reflectivity (DSRa*) to document changes in productivity, which are shown in phase with Northern Hemisphere (NH) timing at millennial scale, suggesting a direct atmospheric teleconnection with higher northern latitudes. In Chapter 2, reconstruction of nitrogen isotope records (δ15N) show that abrupt changes in denitrification are in phase with NH timing over the last glacial period; however, the advection of heavy nitrate from southern sources is also documented, possibly from the denitrification zone off Peru-Chile. Records of opal (%opal – Chapter 3) and carbon isotopes from benthic foraminifera (δ13C-Uvigerina – Chapter 4) support the inference of oceanic teleconnections between the ETNP and the South Pacific via subthermocline circulation. In Chapter 4, the δ13C records also suggest that intermediate water circulation changed over glacial periods and terminations, being the result of intrusion of southern component waters. In Chapter 5, the reconstruction of radiocarbon activity (Δ14C) records from surface (planktonic foraminifera) and intermediate water (benthic foraminifera) suggest oceanic degassing of old-carbon from the deep ocean during the last termination. In this way, the ETNP upwelling system could be an important locus of CO2 release at millennial timescales.

The El Niño Southern Oscillation (ENSO), which refers to a coupling between equatorial Pacific Ocean and atmosphere anomalies, is a major source of interannual climate variability. Although it is fundamentally a tropical Pacific phenomena, both warm (El Niño) and cold (La Niña) events alter atmospheric circulations -- and subsequently temperature and precipitation patterns -- well into the mid- latitudes. Furthermore, both El Niño and La Niña have characteristic multi-year life cycles of sea surface temperature and zonal wind anomalies. The research in this thesis focuses on understanding whether the global teleconnections and multi-year evolution of El Niño and La Niña imposes a risk of synchronous or sequential crop failures relevant to global food production. In the first chapter, which focuses on maize, wheat and soy in the Americas, we analyze the dynamics underlying ENSO life cycles to illustrate which aspects of the system are most important for agriculture. In North America, the same-season teleconnections affecting soybean and maize have been well studied, but we demonstrate the importance of lagged soil moisture teleconnections for wheat in the southern Great Plains. In South America, peak ENSO sea surface temperature (SST) teleconnections are concurrent with, and therefore critical for, wheat and maize growing seasons while soil moisture memory in Argentina plays an important role during the soybean growing season In the second chapter we show how the teleconnections from chapter one lead to correlated crop production anomalies in North and South America. We estimate the magnitude of ENSO-induced Pan-American production anomalies and discuss how increasing crop harvesting frequency may affect Pan-American production variability. We find that ENSO-induced production anomalies are greatest for maize, with median anomalies of about 5% of Pan-American production. After broadly characterizing ENSO-induced production anomalies, we demonstrate that they are not static in time. Increasing crop harvesting frequency in Brazil has affected the correlated risks posed by ENSO to soybeans and maize. In the third chapter we expand our analysis of agriculturally relevant teleconnections to the greater Pacific Basin region, and move beyond observations into model simulations. In this chapter we propose a coherent framework for understanding how trans-Pacific ENSO teleconnections pose a correlated risk to crop yields in major agricultural belts of the Americas, Australia and China over the course of an ENSO life cycle. The potential for consecutive ENSO-induced yield anomalies is of particular interest in these major food producing areas, where modest changes in yield have significant effects on global markets. We demonstrate that ENSO teleconnections relevant for crop flowering seasons are the result of a single trans-Pacific circulation anomaly that develops in boreal summer and persists through the following spring. These trans-Pacific ENSO teleconnections are often (but not always) offsetting between major producing regions in the Americas and those in northern China or Australia. Multi-year La Niñas, however, only tend to force multi-year growing season anomalies in Argentina and Australia. In our final chapter we estimate of the relative contribution of major modes of climate variability to crop yield variability at the global scale. We consider the influence of not only ENSO, but also the Indian Ocean Dipole (IOD), tropical Atlantic variability (TAV) and the North Atlantic Oscillation (NAO). We find that modes of climate variability account for 18.4%, 7.4% and 5.4% of globally aggregated maize, soy and wheat production variability, respectively. All modes of variability are important in at least one region studied, but only ENSO has a significant influence on global production. The low fractions of global-scale soy and wheat production variability attributable to climate is a result of significant but offsetting ENSO-induced yield anomalies in major production regions. Our findings represent an observationally-derived limit on the importance of climate variability to crop production stability that is not dependent on the fidelity of present generation of climate or crop models. In terms of synchronous crop failures within a single harvest year, we find that ENSO poses a significant correlated risk to maize yields but that it has a much smaller effect on global wheat and soy production. ENSO-forced maize production anomalies offset less than wheat and soy at the global scale because production is concentrated in regions with same-sign yield anomalies, notably the United States and Southeast Africa. To illustrate this point, we show that ENSO is largely responsible for the largest synchronous maize failure in the post-1960 historical record. These results demonstrate how the distribution of global cropland in relation to ENSO teleconnections contributes significantly to the presence for maize or absence for wheat and soy of synchronous global crop failures

O presente trabalho foi particularmente motivado pela necessidade de se compreender a variabilidade do sinal intrasazonal relacionado a eventos extremos da Oscilação de Madden-Julian (OMJ) fator consensual na mudança do clima em diversas regiões do globo terrestre, em virtude de seus padrões de teleconexão atmosférica. Tal necessidade exige habilidades diferenciadas, como as apresentadas para o modelo OLAM v3.3 no decorrer do presente estudo. Foram utilizados dados observacionais da Reanálises II do NCEP (campo de vento em 200 e 850 mb) assim como variáveis obtidas por satélites (Radiação de Onda Longa Emergente ROL) para avaliar a estrutura atmosférica na escala de tempo intrasazonal. O campo diário de TSM foi assimilado pelo modelo numérico como principal forçante atmosférica para a geração do sinal intrasazonal; além disso, aninhamentos de grade foram acionados para melhor resolver os processos de menor escala essenciais para formar os processos na grande escala, os quais são intrínsecos ao sinal intrasazonal. Métodos estatísticos com um nível de significância em 5% foram aplicados para validar os resultados obtidos com a modelagem numérica em detrimento as observações. As observações mostraram que o ano de 2002 apresentou uma maior variabilidade intrasazonal na região do INDO-PACÍFICO associada a eventos da OMJ em relação aos outros anos em análise, tanto para o verão quanto para o inverno no HS. De outra forma, para a modelagem numérica, os anos de 2001/2002 apresentaram maior variabilidade na escala de tempo intrasazonal na região de controle INDI com forte influência remota na região da América do Sul/ZCAS para o verão de 2002. O estudo de caso observacional de 22 de dezembro de 2002, mostrou que o principal mecanismo para a interação remota entre a região de controle INDI e a ZCAS2 foi gerado por uma combinação entre o PSA-curto e o guia preferencial de ondas 2. A modelagem numérica sugere que a variabilidade intrasazonal representada pelo modelo OLAM v3.3 independe da distribuição temporal dos campos de TSM. No entanto, evidências mostraram que o sinal modelado será tanto melhor quanto maior a variabilidade da energia intrasazonal no campo de TSM assimilado pelo modelo numérico. Em detrimento à convenção de Grell, a parametrização de cúmulos profundo do tipo Kuo apresentou maior variabilidade temporal na região de controle INDI para a DIV200mb em todo período analisado, favorecendo uma maior atividade convectiva para aquela região, inclusive na escala de tempo intrasazonal. Sucessivos aninhamentos de grade sugerem que a energia intrasazonal tende a aumentar significativamente à medida que se aumenta o número de grades aninhadas. Para o estudo de caso de 01 de julho de 2001, via modelagem numérica, foram necessários 30 dias para a OMJ inverter seu padrão na região de controle INDI, e somente após essa inversão foi encontrado atividade convectiva na escala de tempo intrasazonal sobre a região da ZCAS. Dessa forma, o OLAM v3.3 superestima o tempo de meio ciclo dessa oscilação e consequentemente o tempo de resposta sobre a AS, em particular na região da ZCAS2. Outro aspecto relevante se refere à diferença na quantidade de energia intrasazonal que o OLAM v3.3 simula na região INDI quando há aninhamento de grade na região da ZCAS. Este fato, juntamente com a inversão de sinal descrita acima, sugere uma interação do tipo gangorra convectiva entre a região INDI e a região ZCAS. O espectro de energia da TSO para a divergência ao nível de 200 mb, mostrou que o OLAM v3.3 subestima a energia do sinal intrasazonal na região do oceano Índico em quase a metade do valor real observado. Todavia, as observações mostraram que a energia espectral intrasazonal da divergência em 200 mb na região de controle ZCAS2, para a escala de 43 dias, foi da ordem de 0,42 x 10-10 s-2, resultando em uma diferença positiva de 0,08 x 10-10 s-2 em relação ao valor numérico obtido. Por fim, a metodologia do traçado de raios mostrou que os números de onda 2, 3 e 4 são bem representados pelo OLAM v3.3 na região tropical, corroborando com a habilidade do modelo em reproduzir os padrões de teleconexão atmosférica gerados no evento da OMJ de 01 de julho de 2001. / This work was particularly motivated by the need to understand the variability of the intraseasonal signal, in relation to extreme events of the Madden-Julian Oscillation consensual factor in the weather changes at different regions of the globe, due its atmospheric teleconnection patterns. For this need, it\'s totally necessary special skills, such as those presented in this job for the OLAM model v3.3. In this job, observational datasets were used from the Reanalysis II/NCEP (wind fields at 200 and 850 mb), as also variables obtained by satellites (OLR) to assess the atmospheric profile in the intraseasonal time scale. The SST daily field was assimilated by the numerical OLAM model v3.3 to forcing the sign in the intraseasonal time scale. However, mesh refinement level also was activated for better resolve the smaller scale processes essentials to form key processes in large scale and relevant to intraseasonal signs generation. Statistical methods with 5% significance level, were applied to validate the results obtained with the numerical modeling in detriment to the observational results. The observations has shown that the year 2002 presented a higher intraseasonal variability in the INDO-PACIFIC region associated with MJO events, in detriment of the other years under review, both for summer as for Austral winter. Otherwise, for the numerical modeling, the years 2001/2002 presented higher variability in the intraseasonal time scale over the Indian ocean region showing strongest remote influences over the South America/SACZ to the Austral summer of 2002. The observational case study of December 22, 2002, showed that the main mechanism for the remote interaction between control region over Indian ocean and the SACZ2 control region, was generated by combination among a short-PSA and a preferential wave guide 2. The numerical modeling suggests that the intraseasonal variability represented by the OLAM model v3.3 is independent of the temporal distribution of the SST fields. However, evidences has shown that the sign will be better represented how much greater the intraseasonal energy variability in the SST fields assimilated by the numerical model. In detriment to Grell\'s convention, the Kuo\'s deep cumulus parameterization has showed greater temporal variability in the Indian ocean region for the divergence at 200 mb throughout analyzed period, favoring convective activity in the intraseasonal time scale for that region. Successive nesting grids suggests that the intraseasonal energy tends to increase significantly, when increases the number of nested grids. For the case study of July 1, 2001, via numerical modeling, were necessary 30 days to reverse the MJO\'s signal pattern in the Indian ocean region, and only after this reversal, was found convective activity in the intraseasonal time scale over the SACZ region. Thus, the results obtained with the OLAM model v3.3 suggests overestimation of the half cycle of oscillation and, consequently, the time response over the South America region, in particular over the SACZ2 region. Another important aspect refers to the difference at the intraseasonal energy amount simulated by the OLAM model v3.3 for the Indian ocean region, when is applied nesting grids over the SACZ region. This fact, together with the sign inversion described above, suggests an interaction of the type \"convective seesaw\" between the Indian ocean region and the SACZ region. The wavelets power spectrum for the divergence at 200 mb has shown that OLAM model v3.3 underestimates the intraseasonal signal energy over the Indian ocean region in about half the actual value observed. However, observations has shown that the spectral intraseasonal energy of the divergence at 200 mb in the ZCAS2 region, for 43 day\'s scale, was approximately 0.42 x 10-10 s-2, resulting in a positive difference of 0.08 x 10-10 s-2 in relation to the numerical value obtained. Finally, the methodology of the ray tracing showed that wave numbers 2, 3 and 4 were well represented by the OLAM model v3.3 for the tropical region, confirming the model ability to reproduce the atmospheric teleconnection patterns, as shown for MJO\'s event July 1, 2001.

O presente trabalho foi particularmente motivado pela necessidade de se compreender a variabilidade do sinal intrasazonal relacionado a eventos extremos da Oscilação de Madden-Julian (OMJ) fator consensual na mudança do clima em diversas regiões do globo terrestre, em virtude de seus padrões de teleconexão atmosférica. Tal necessidade exige habilidades diferenciadas, como as apresentadas para o modelo OLAM v3.3 no decorrer do presente estudo. Foram utilizados dados observacionais da Reanálises II do NCEP (campo de vento em 200 e 850 mb) assim como variáveis obtidas por satélites (Radiação de Onda Longa Emergente ROL) para avaliar a estrutura atmosférica na escala de tempo intrasazonal. O campo diário de TSM foi assimilado pelo modelo numérico como principal forçante atmosférica para a geração do sinal intrasazonal; além disso, aninhamentos de grade foram acionados para melhor resolver os processos de menor escala essenciais para formar os processos na grande escala, os quais são intrínsecos ao sinal intrasazonal. Métodos estatísticos com um nível de significância em 5% foram aplicados para validar os resultados obtidos com a modelagem numérica em detrimento as observações. As observações mostraram que o ano de 2002 apresentou uma maior variabilidade intrasazonal na região do INDO-PACÍFICO associada a eventos da OMJ em relação aos outros anos em análise, tanto para o verão quanto para o inverno no HS. De outra forma, para a modelagem numérica, os anos de 2001/2002 apresentaram maior variabilidade na escala de tempo intrasazonal na região de controle INDI com forte influência remota na região da América do Sul/ZCAS para o verão de 2002. O estudo de caso observacional de 22 de dezembro de 2002, mostrou que o principal mecanismo para a interação remota entre a região de controle INDI e a ZCAS2 foi gerado por uma combinação entre o PSA-curto e o guia preferencial de ondas 2. A modelagem numérica sugere que a variabilidade intrasazonal representada pelo modelo OLAM v3.3 independe da distribuição temporal dos campos de TSM. No entanto, evidências mostraram que o sinal modelado será tanto melhor quanto maior a variabilidade da energia intrasazonal no campo de TSM assimilado pelo modelo numérico. Em detrimento à convenção de Grell, a parametrização de cúmulos profundo do tipo Kuo apresentou maior variabilidade temporal na região de controle INDI para a DIV200mb em todo período analisado, favorecendo uma maior atividade convectiva para aquela região, inclusive na escala de tempo intrasazonal. Sucessivos aninhamentos de grade sugerem que a energia intrasazonal tende a aumentar significativamente à medida que se aumenta o número de grades aninhadas. Para o estudo de caso de 01 de julho de 2001, via modelagem numérica, foram necessários 30 dias para a OMJ inverter seu padrão na região de controle INDI, e somente após essa inversão foi encontrado atividade convectiva na escala de tempo intrasazonal sobre a região da ZCAS. Dessa forma, o OLAM v3.3 superestima o tempo de meio ciclo dessa oscilação e consequentemente o tempo de resposta sobre a AS, em particular na região da ZCAS2. Outro aspecto relevante se refere à diferença na quantidade de energia intrasazonal que o OLAM v3.3 simula na região INDI quando há aninhamento de grade na região da ZCAS. Este fato, juntamente com a inversão de sinal descrita acima, sugere uma interação do tipo gangorra convectiva entre a região INDI e a região ZCAS. O espectro de energia da TSO para a divergência ao nível de 200 mb, mostrou que o OLAM v3.3 subestima a energia do sinal intrasazonal na região do oceano Índico em quase a metade do valor real observado. Todavia, as observações mostraram que a energia espectral intrasazonal da divergência em 200 mb na região de controle ZCAS2, para a escala de 43 dias, foi da ordem de 0,42 x 10-10 s-2, resultando em uma diferença positiva de 0,08 x 10-10 s-2 em relação ao valor numérico obtido. Por fim, a metodologia do traçado de raios mostrou que os números de onda 2, 3 e 4 são bem representados pelo OLAM v3.3 na região tropical, corroborando com a habilidade do modelo em reproduzir os padrões de teleconexão atmosférica gerados no evento da OMJ de 01 de julho de 2001. / This work was particularly motivated by the need to understand the variability of the intraseasonal signal, in relation to extreme events of the Madden-Julian Oscillation consensual factor in the weather changes at different regions of the globe, due its atmospheric teleconnection patterns. For this need, it\'s totally necessary special skills, such as those presented in this job for the OLAM model v3.3. In this job, observational datasets were used from the Reanalysis II/NCEP (wind fields at 200 and 850 mb), as also variables obtained by satellites (OLR) to assess the atmospheric profile in the intraseasonal time scale. The SST daily field was assimilated by the numerical OLAM model v3.3 to forcing the sign in the intraseasonal time scale. However, mesh refinement level also was activated for better resolve the smaller scale processes essentials to form key processes in large scale and relevant to intraseasonal signs generation. Statistical methods with 5% significance level, were applied to validate the results obtained with the numerical modeling in detriment to the observational results. The observations has shown that the year 2002 presented a higher intraseasonal variability in the INDO-PACIFIC region associated with MJO events, in detriment of the other years under review, both for summer as for Austral winter. Otherwise, for the numerical modeling, the years 2001/2002 presented higher variability in the intraseasonal time scale over the Indian ocean region showing strongest remote influences over the South America/SACZ to the Austral summer of 2002. The observational case study of December 22, 2002, showed that the main mechanism for the remote interaction between control region over Indian ocean and the SACZ2 control region, was generated by combination among a short-PSA and a preferential wave guide 2. The numerical modeling suggests that the intraseasonal variability represented by the OLAM model v3.3 is independent of the temporal distribution of the SST fields. However, evidences has shown that the sign will be better represented how much greater the intraseasonal energy variability in the SST fields assimilated by the numerical model. In detriment to Grell\'s convention, the Kuo\'s deep cumulus parameterization has showed greater temporal variability in the Indian ocean region for the divergence at 200 mb throughout analyzed period, favoring convective activity in the intraseasonal time scale for that region. Successive nesting grids suggests that the intraseasonal energy tends to increase significantly, when increases the number of nested grids. For the case study of July 1, 2001, via numerical modeling, were necessary 30 days to reverse the MJO\'s signal pattern in the Indian ocean region, and only after this reversal, was found convective activity in the intraseasonal time scale over the SACZ region. Thus, the results obtained with the OLAM model v3.3 suggests overestimation of the half cycle of oscillation and, consequently, the time response over the South America region, in particular over the SACZ2 region. Another important aspect refers to the difference at the intraseasonal energy amount simulated by the OLAM model v3.3 for the Indian ocean region, when is applied nesting grids over the SACZ region. This fact, together with the sign inversion described above, suggests an interaction of the type \"convective seesaw\" between the Indian ocean region and the SACZ region. The wavelets power spectrum for the divergence at 200 mb has shown that OLAM model v3.3 underestimates the intraseasonal signal energy over the Indian ocean region in about half the actual value observed. However, observations has shown that the spectral intraseasonal energy of the divergence at 200 mb in the ZCAS2 region, for 43 day\'s scale, was approximately 0.42 x 10-10 s-2, resulting in a positive difference of 0.08 x 10-10 s-2 in relation to the numerical value obtained. Finally, the methodology of the ray tracing showed that wave numbers 2, 3 and 4 were well represented by the OLAM model v3.3 for the tropical region, confirming the model ability to reproduce the atmospheric teleconnection patterns, as shown for MJO\'s event July 1, 2001.

Snow/ice events are indeed a rare occurrence in the southeast United States. As a result, residents of the Southeast often exemplify a passive attitude towards winter weather and are often unprepared when it strikes. This study analyzed every recorded winter weather event that struck the Southeast (Mississippi, Alabama, Georgia) from the winter season of 1961-'62 through 2000-'01 from both a spatial and temporal standpoint. Through the results of this study, it was evident that Georgia saw the most overall winter weather events and January seemed to be the most productive month overall. This study also analyzed teleconnection (ENSO, PNA, NAO, AO) indices per study period season in order to deduce correlations with active/inactive Southeast winters. Through statistical analyses, correlations were deemed insignificant.