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Upper-tropospheric (200 hPa) velocity potential is useful in identifying areas of rising or sinking atmospheric motions on varying temporal scales (e.g., weekly, seasonal, interannual) especially in the global tropics. These areas are associated with enhancement (rising motion) or suppression (sinking motion) of tropical convection and subsequent weather phenomena dependent on these processes (e.g., tropical cyclones). This study employed three commonly used global weather reanalysis datasets (NCEP/NCAR Reanalysis 1, JMA JRA-55, ECMWF ERA5) to calculate and compare upper-tropospheric velocity potential fields on varying temporal scales and quantify any differences that existed between them from 1958 to 2020 over four key regions of variability (Equatorial Africa, Amazon Basin, Equatorial Central Pacific, and Equatorial Indonesia). To supplement this analysis, the highly correlated variables to velocity potential of outgoing longwave radiation (OLR) and daily precipitation rate were used and directly compared with independent OLR and precipitation datasets to determine the reanalysis' level of agreement with the independent data. The ECMWF ERA5 held the highest agreement to these data over all regions examined and was reasoned to have the highest confidence in capturing the variability of upper-tropospheric velocity potential fields for the study period. Confidence was decreased in the usefulness of the NCEP/NCAR Reanalysis 1 as it consistently performed poorly over much of the study domain. The results of this study also emphasized the usefulness in ensemble-based approaches to assessing climate variability and understanding potential biases and uncertainties that are inherent in the data sources. / Master of Science / Historical weather data across the globe is analyzed using global weather reanalysis datasets which provide the most complete picture of how the atmosphere has evolved over the course of the last several decades. This data is a vital component in today's research investigating climate change and variability over time. This study examined how the history of upper-tropospheric velocity potential was captured in three commonly used global weather reanalysis datasets (NCEP/NCAR Reanalysis 1, JMA JRA-55, ECMWF ERA5) from 1958 to 2020 over four key regions of variability (Equatorial Africa, Amazon Basin, Equatorial Central Pacific, and Equatorial Indonesia). The variable of velocity potential is useful in identifying areas of rising or sinking atmospheric motions on varying time scales (e.g., weekly, seasonal, interannual) especially in the global tropics. These areas are associated with enhancement (rising motion) or suppression (sinking motion) of tropical convection (i.e., thunderstorms) and subsequent weather phenomena dependent on these processes (e.g., tropical cyclones). The analysis conducted found that the newest of the reanalysis datasets, the ECMWF ERA5, held the highest agreement to independent weather observations over all regions examined was reasoned to have the highest confidence in capturing the variability of upper-tropospheric velocity potential fields for the study period. Confidence was decreased in the usefulness of the NCEP/NCAR Reanalysis 1, the oldest of the reanalysis datasets, as it consistently performed poorly over much of the study domain. The results of this study also emphasized the usefulness in ensemble-based approaches to assessing climate variability and understanding potential biases and uncertainties that can be found in the data sources.

Identifying the drivers of Arctic climate variability is essential for understanding the recent rapid changes in local climate and determining the mechanisms that cause them. Remote tropical sea surface temperatures (SST) have been identified in previous studies as having contributed to the recent positive trends in surface temperature and geopotential height at 200 hPa over north Canada and Greenland (1979-2012) through poleward propagating Rossby waves. However, the source and direction of wave propagation on to north Canada and Greenland (NCG) differs across climate datasets indicating that there are still uncertainties surrounding the mechanisms for how the tropics influence the NCG climate. This thesis aims to further investigate the robustness of the trends over NCG and understand how circulation in this region responds to imposed tropical SST perturbations. The eddy 200 hPa geopotential height (Z200) trends over NCG are assessed in a number of different datasets and compared to the response of eddy Z200 over NCG to imposed tropical SST perturbations in a number of sensitivity studies using the HadGEM3 atmosphere-only model. These model experiments are forced with observed differences in SSTs from the beginning and end of the satellite record (1979-1988 and 2003-2012), with spatial perturbations for [i] the entire tropics, [ii] global SSTs, [iii] the tropical Pacific only, [iv] the tropical Atlantic SST only, [v] the tropical Indian Ocean only. The positive spatial trends of eddy Z200 over NCG from ERA-Interim reanalysis is largely captured in ensemble means of two available climate datasets, UPSCALE and AMIP, indicating that this is a robust climate pattern, however, these trends appear to be stronger in the latter part of the record specifically over the UPSCALE period (1985 to 2011). The model sensitivity studies show that a negative eddy Z200 anomaly over NCG was found in response to all imposed tropical SST perturbations (2003-2012) relative to a background state (1979-1988). This was due a stationary trough over the region that was able to intensify in response to a lack of a strong anomalous wave forcing from changes in mid-tropospheric temperature and zonal winds. The forcing from the tropical Atlantic, relative to the other tropical ocean basins, resulted in the largest eddy Z200 response over NCG, indicating its dominance in forcing the large scale tropical signal. The influence of extratropical SST perturbations relative to tropical SST perturbations were also investigated and it was demonstrated that this negative anomaly is largely driven by the change in tropical sea surface temperatures.

Dans la zone intertropicale, les précipitations sont le principal marqueur climatique saisonnier et déterminent très largement l’hydrologie de surface et de nombreuses activités anthropiques. Le bassin amazonien est caractérisé par divers régimes régionaux de précipitations, dont la variabilité spatiale et temporelle est forte. De nombreux travaux ont montré que cette variabilité est liée à des forçages externes de large échelle, comme les températures de surface de l’océan. L’étude des précipitations dans cette région porte le plus souvent sur les tendances ou les extrêmes pluviométriques. En revanche, la détection d’années similaires constituant des sous-régimes régionaux et leur lien avec une configuration océano-atmosphérique particulière a été, jusqu’à présent, peu abordée. L’objectif principal de cette thèse est ainsi de créer une typologie des sous-régimes de précipitations régionaux dans le bassin amazonien et de les mettre en relation avec le contexte océano-atmosphérique pouvant en partie les expliquer. Dans ce but, des données issues de 205 pluviomètres répartis sur 5 pays du Bassin Amazonien ont été sélectionnées et soumises à une série de tests statistiques et de reconstruction. Cette thèse utilise également des données de nébulosité (Outgoing Longwave Radiation), de flux d’humidité et de température de surface de l’océan ainsi que des données satellitaires (TRMM3B42 version 7) qui permettent de compléter les informations sur la variabilité spatiale des pluies.Au sein de chacune des sept régions amazoniennes déterminées dans ce travail, deux à quatre sous-régimes de précipitations ont été détectés. Parmi les vingt-six sous-régimes, vingt sont associés à des anomalies de circulation des flux d’humidité et de température de surface des océans. Les sous-régimes de pluies de la moitié nord et les Andes de l’ouest du bassin sont le plus liés à des anomalies océaniques. De plus, comme cela est régulièrement décrit, des déficits ou excédents correspondent souvent à des phases El Niño ou La Niña, mais cette thèse met également en évidence le rôle important de l’Atlantique, en particulier sud, sur le déplacement de la ZCIT et sur les flux d’humidité ; et elle souligne également le lien entre la temporalité des événements océaniques et celle des anomalies de pluies.Le produit TRMM 3B42 V7 permet d’aller plus loin dans l’analyse de la variabilité spatiale intra-régionale des pluies de la région Nord-est du bassin amazonien et de relativiser la cohérence spatiale des sous-régimes de précipitations de cette région. / Precipitations are the main seasonal climate marker between the tropics and largely determine surface hydrolosy as well as many anthropogenic activities. The Amazon Basin is characterized by various regional rainfall patterns, whose spatial and temporal variability is high. Numerous studies have shown that this variability is related to large scale external forcing, such as sea surface temperatures. The analysis of precipitation in this region is generally related to trends or extreme of rainfall. However, the detection of similar years associated with regional sub-regimes and the analysis of their links with a specific ocean-atmosphere configuration has only been fewly addressed until now. The main objective of this thesis is to create a typology of regional precipitation sub-régimes in the Amazon Basin and to link them to ocean-atmosphere areas able to partly explain them. For that purpose data from 205 raingauges in 5 countries of the Amazon Basin were selected and submitted to a series of statistical tests and reconstruction. Outgoing longwave radiation, specific humidity, sea surface temperature, as well as satellite data (TRMM 3B42 version 7) were also used with the aim of improving the understanding of the spatial rainfall variability.Within each of the seven Amazon regions identified in this work, two to four precipitation sub-regimes were detected. Among the twenty six sub-regimes, twenty are associated with specific humidity and sea surface temperature anomalies. The precipitation sub-regimes of the northern half and the westernmost Andes of the Amazon Basin are most closely related to oceanic anomalies. Moreover, as previously described in the literature, reduction or surplus of rain often correspond to El Niño or La Niña phases, but this thesis also highlights the important role of the Atlantic, more specifically the southern part, on the move of the ITZC and on specific humidity. This work also stresses the link between the temporality of ocean events anomalies and rainfall anomalies.The TRMM 3B42 v7 product allows to enhance the analysis of the spatial variability of rainfall at the intra-regional scale of the North region of the Amazon Basin and to relativize the spatial coherence of its precipitation sub-regimes.

Little is known about the paleo-precipitation of the Florida Peninsula. In order to better understand Floridas late Holocene climate variability (last 4,200 years), the isotopic composition was analyzed of four speleothems from two caves, in West-Central Florida. Two speleothems were collected from BRC Cave in Hernando County, and two others from Briar Cave in Marion County. This study represents the first speleothem-based paleoclimate records for Florida. Uranium-series disequilibrium analyses were determined by using thermal ionization mass spectrometry (TIMS) to provide accurate determination of chronology of the deposition of the speleothems. Stable isotopic analyses of oxygen and carbon were performed using stable isotope mass spectrometry, which provided information regarding changing amounts of precipitation (increase in precipitation, decrease in the δ18Oc) and types of vegetation above the cave (increased forest density, decrease in the δ13Cc). Variations in the speleothems δ18O composition reveal abrupt changes in precipitation amount, fluctuations that appear both regional and hemispheric in nature. Strong similarities between the speleothem δ18O, Lake TulaneδD record (Cross et al. 2003; 2004) and the SE US tree-ring record (surrogate for spring precipitation - Stahle and Cleaveland 1992) suggests a regional atmospheric influence on Floridas precipitation. The major causes of changes in precipitation are proposed to be Atlantic Multi-decadal Oscillation (AMO), El Nino and changes in the relative positions of the Intertropical Convergence Zone (ITCZ)-North Atlantic High (NAH). Comparison between the δ18Oc and surrogates of these influences, show all three have some effect. AMO and El Nino have short-term (decadal) influence and ITCZ-NAH has a long term (centennial) influence. The contributions of these climatic effects have implications for teleconnections involving Floridas climate; the AMO correlation shows higher latitude influence, while El Nino and the ITCZ show tropical influence on subtropical Florida.

South Asia, a land of contrasting landscapes, seasons and climates, is highly vulnerable to climate variability over intra-seasonal to decadal time scales. In winter, precipitation over the western parts of south Asia and fog over the Indo-Gangetic (IG) plains are the two major climatic features. During summer most of the region comes under the grip of monsoon. Winter precipitation over the north-western parts of South Asia is associated with eastwards propagating ‘western disturbances’ originating mostly from Mediterranean. Both observations and regional climate-model simulations show that the winter precipitation increases/decreases during the positive/negative phases of the North Atlantic Oscillation (NAO) and the warm/cold phase of the El Niño-Southern Oscillation (ENSO). During these phases, the intensification of western disturbances results from the effect of an enhanced trough visible at sea-level as well as at higher altitudes over central Asia. The inter-annual variability of fog is coupled over IG plains with a significant trend in the fog frequencies, both in observations and ERA-Interim reanalysis data. This increase shows two distinct regime shifts in 1990 and 1998 with respect to mean and variance, this in contrast to a gradual increase of the humidity over the region. The thermodynamic analysis of the intra-seasonal summer monsoon active phases (APs) over Pakistan revealed that a few days before AP, an upper-level warm anomaly appears over the northern Hindu Kush-Himalaya region and is reinforced by surface heating. The baroclinic height anomalies, with a low-level anticyclone located east of the warming, causes a moisture convergence, strong enough to overcome the preexisting stable atmospheric conditions. The extratropical dynamics also play an important role for the inter-annual variation of the South-Asian monsoon. It is found that the two leading modes between the upper-level circulation in the Atlantic/European region and monsoon rainfall are the Circumglobal Teleconnection (CGT) and the summer NAO. The positive phase of the CGT is related to a widespread increase of monsoon rainfall, and a positive summer NAO is related to a precipitation dipole with its positive anomaly over Pakistan. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

Over the 1951-2009 time period, 47% of all tropical systems in the Atlantic Basin transitioned to post-tropical storms. These storms are capable of producing hurricaneforce winds, torrential, flooding rains and storm surge that floods coastal areas. This study adds to previous climatological work by completing a case-study of Hurricane Ike (2008) and examining how teleconnections such as the El Niño Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) contribute to the strength of a transitioning post-tropical storm. T-tests performed show strong statistical relationships between an increase (decrease) in post-tropical storm frequency and warm PDO – La Niña (cold PDO – La Niña), cold PDO – ENSO neutral (warm PDO – ENSO neutral), and warm (cold) AMO conditions. Moreover, nearly significant results were found for the same increase (decrease) and La Niña seasons since (pre) 1980 and for cold (warm) PDO conditions. Modeling the MJO suggests that increased (decreased) relative humidity associated with the wet (dry) phase could increase (decrease) precipitation output from the storm and decrease (increase) forward speed of the storm, decreasing (increasing) wind speeds observed at the surface.

The Quasi-Biennial Oscillation (QBO) is the dominant pattern of variability in the tropical stratosphere. Despite a well established theory regarding its generation in the atmosphere, the simulation in global climate models remains difficult. A set of metrics assessing the quality of model simulations is presented in this study. The QBO simulations in models submitted to the CMIP5 and CCMVal-2 intercomparison projects are characterised and compared to radiosonde observations and reanalysis datasets. Common model biases and their potential causes are addressed. As the QBO has a long intrinsic period, knowing its influences on other parts of the climate system can be used to improve long range forecasts. These teleconnections of the QBO in observations are investigated using composite analysis, multilinear regression and a novel approach called causal effect networks (CEN). Findings from these analyses confirm previous results of the QBO modulating the stratospheric polar vortex and subsequently the North Atlantic Oscillation (NAO). They also suggest that it is important to take the equatorial zonal mean zonal wind vertical profile into account when studying teleconnections, rather than the more traditional method of using just one single level. While QBO influences on the Northern Hemisphere winter polar vortex and the NAO are more clearly established, interactions within the tropics remain inconclusive. Regression analysis does not show a connection between the QBO and the MJO, whereas the CEN analysis does. Further studies are needed to understand the interaction mechanisms near the equator. Finally, following the unprecedented disruption of the QBO cycle in the winter 2015/16, the event is described and a model analogue from the MPI-ESM-MR historical simulation is presented. Future implications are unclear, although model projections indicate more frequent QBO irregularities in a warming climate.

abstract: ABSTRACT Famine is the result of a complex set of environmental and social factors. Climate conditions are established as environmental factors contributing to famine occurrence, often through teleconnective patterns. This dissertation is designed to investigate the combined influence on world famine patterns of teleconnections, specifically the North Atlantic Oscillation (NAO), Southern Oscillation (SO), Pacific Decadal Oscillation (PDO), Atlantic Multidecadal Oscillation (AMO), or regional climate variations such as the South Asian Summer Monsoon (SASM). The investigation is three regional case studies of famine patterns specifically, Egypt, the British Isles, and India. The first study (published in Holocene) employs the results of a Principal Component Analysis (PCA) yielding a SO-NAO eigenvector to predict major Egyptian famines between AD 1049-1921. The SO-NAO eigenvector (1) successfully discriminates between the 5-10 years preceding a famine and the other years, (2) predicts eight of ten major famines, and (3) correctly identifies fifty out of eighty events (63%) of food availability decline leading up to major famines. The second study investigates the impact of the NAO, PDO, SO, and AMO on 63 British Isle famines between AD 1049 and 1914 attributed to climate causes in historical texts. Stepwise Regression Analysis demonstrates that the 5-year lagged NAO is the primary teleconnective influence on famine patterns; it successfully discriminates 73.8% of weather-related famines in the British Isles from 1049 to 1914. The final study identifies the aggregated influence of the NAO, SO, PDO, and SASM on 70 Indian famines from AD 1049 to 1955. PCA results in a NAO-SOI vector and SASM vector that predicts famine conditions with a positive NAO and negative SO, distinct from the secondary SASM influence. The NAO-famine relationship is consistently the strongest; 181 of 220 (82%) of all famines occurred during positive NAO years. Ultimately, the causes of famine are complex and involve many factors including societal and climatic. This dissertation demonstrates that climate teleconnections impact famine patterns and often the aggregates of multiple climate variables hold the most significant climatic impact. These results will increase the understanding of famine patterns and will help to better allocate resources to alleviate future famines. / Dissertation/Thesis / Doctoral Dissertation Geography 2017