The history of climatic changes in the Asian Summer Monsoon system over the Tibetan Plateau during the Holocene has been the subject of significant research due to the importance of the plateau as the headwaters for many major rivers providing water resources to the surrounding large, populous countries. In general, previous research has concluded that monsoon rainfall and summer temperatures peaked during the early Holocene (9-11 ka BP) in Tibet, coincident with peak Northern Hemisphere summer insolation. Atmospheric teleconnections with upstream Northern Hemisphere westerly circulation patterns influenced by North Atlantic sea surface temperature changes have also been noted at millennial and centennial timescales. However, recent studies have noted that the timing of peak monsoon warmth and wetness during the Holocene are not synchronous across the entirety of the Tibetan Plateau, and studies of modern precipitation indicate several distinct regions of monsoon precipitation variability at interannual scales, suggesting the monsoon response to past and future climate change may be regionally heterogeneous for the plateau. Clear assessment of this regionality within the monsoon climate region is a topic of continuing research, but it has been hindered by lack of climate records in remote areas, dating difficulties, and concerns over the comparability of interpreted climate-proxy relationships between the many different biological, hydrological, and geochemical proxies applied. The first part of this dissertation uses ¹⁴C and U-Th series geochronology, sedimentology, and GIS analysis of exposed lake shoreline sediments surrounding the numerous closed-basin lake systems of the central and western Tibetan Plateau to investigate regional heterogeneity in monsoon rainfall, and to develop a new well-dated lake level record from the Ngangla Ring Tso lake system in the poorly studied southwestern region. The major conclusions are: 1) peak early Holocene monsoon rainfall, recorded by the highest paleoshorelines surrounding 130 lake systems, intensified more relative to today in the western part (west of 86°E longitude) of the Tibetan Plateau when compared to eastern regions, closely following regions of modern rainfall variability; 2) monsoon rainfall in the Ngangla Ring Tso region peaked during the early Holocene insolation maximum, consistent with other records, remained significantly higher than modern until ~6.0 ka BP, but with abrupt reductions in monsoon rainfall associated with North Atlantic ice-rafted debris peaks. The warm and wet period of the early and middle Holocene was also likely coincident with the first major colonization of the Tibetan Plateau by prehistoric humans. Current research suggests early foragers employing stone tools first forayed into the middle elevation areas above 3,000 m elevation on the northeastern fringe of the plateau as early as 14.8 ka BP, and therefore the dominant hypothesis suggests plateau colonization proceeded from this direction, heading westward through the Holocene. However, well studied and dated archaeological sites from the high plateau are exceedingly rare, requiring further investigation. The second part of this dissertation presents new age controls for the Holocene Zhongba microlithic site in the southwestern Tibetan Plateau, using ¹⁴C dating of organic and carbonate-rich paleo-wetlands sediments hosting in situ stone artifacts. The major conclusions of this study are: 1) artifacts at the Zhongba site, which are typologically similar to microlithics across the plateau, can be no older than 6.5 ka BP, consistent with the prevailing east-to-west colonization hypothesis, and 2) microlithic tools continued to be important as late as 1.3 ka BP at the site, even though metal is found in sites of similar age elsewhere in Tibet.

Western science and Indigenous knowledge understand Southwest climate and the North American monsoon from different cultural perspectives. However, scant literature exists relating to climate and Indigenous communities in the Southwest. On the contrary, substantial climate research has occurred with Arctic Indigenous communities; however, a general aspiration among communities is Indigenous-led climate research and education. This requires more Native scientists and culturally responsive climate science curricula. Southwest Indigenous communities are primed to do this. This dissertation examines 1) the current scientific understanding of the North American monsoon, 2) the state of climate research in Indigenous communities, and 3) the development of culturally responsive climate science curricula. The first paper synthesizes the current scientific understanding of the monsoon and its interannual variability. Pacific Ocean-based teleconnections, such as ENSO-PDO combined indices do add skill in early-season monsoon forecasting. However, general circulation models continue to deal with computational-spatial resolution limitations challenging their application in future climate change projections of the monsoon. The second paper focuses on climate-related research in Indigenous communities in the Arctic and the Southwest to highlight lessons-learned. Climate researchers working with Native communities must exercise cultural considerations for Indigenous relationships with the climate and Indigenous protocols for acquiring and disseminating knowledge. Furthermore, increasing the number of Native students in science and Native scientists are ways to improve climate-related research in Indigenous communities. The third paper is a participatory action research approach to develop a culturally responsive climate science curriculum for Tohono O'odham high school and college students. This project worked with a community advisory board as well as Tohono O'odham Community College instructors and student interns. Pre-assessment surveys were given to community members learn of the most relevant weather and climate topics. The curriculum was developed incorporating local, culturally relevant topics. Climate workshops were offered in the communities using activities developed for the curriculum. Workshop evaluations were positive; however, they also addressed the need for more culturally relevant examples. The overlapping theme for these dissertation papers is cultural understanding for climate research and education in Indigenous communities toward a means for Indigenous-led climate research/education within their own communities.

In the southwestern United States, the North American monsoon (NAM) is the main driver of severe weather and accounts for nearly half the annual precipitation. How the monsoon has behaved in the past and how it will change in the future is a question of major importance for natural resource management and infrastructural planning. In this dissertation, I present the results of three studies that have investigated North American monsoon variability and change from the perspective of paleoclimate records, future climate change projections, and simulation of the low-frequency variability with the longest retrospective atmospheric reanalysis. In the first study, a monsoon-sensitive network of tree-ring chronologies is evaluated within its ability to reproduce NAM variability during the past four centuries. Matrix methods are used to detect the low-frequency spatiotemporal variability. The treering chronologies can reasonable characterizes the dominant modes of NAM climate variability. The monsoon tree-ring network is able to reproduce the interannual variability of cool and warm season precipitation, in a manner similar to the period of the instrumental record. Earlywood and latewood adjusted chronologies reveal low frequency climate variability at decadal and longer timescales that is beyond the ability of the instrumental record to temporally well resolve. This low-frequency climate variability seems to be part of a much larger cycle that coincides with the occurrence of multiyear persistent droughts. In the second study, we consider the modes of natural climate variability identified in the previous study to objectively assess the degree of physical uncertainty in climate change projections for NAM from Regional Climate Models (RCMs) used in the North American Regional Climate Change Assessment Program (NARCCAP). Climate change projection models are evaluated mainly on their ability to represent warm season driven by quasi-stationary Rossby wave trains and El Niño Southern Oscillation – Pacific Decadal Variability (ENSO-PDV). It is concluded that use of the NARCCAP model ensemble mean for NAM climate projections is probably not suitable. NARCCAP RCMs are largely a slave to their driving global models and their error in the specification of large-scale atmospheric circulation. Only one out of eight NARCCAP RCMs has a reasonable representation of the seasonal cycle of monsoon precipitation and ENSOdriven interannual variability in both the 20th and 21st centuries. No decadal variability was observed in any of the NARCCAP RCMs. In the third study, the low-frequency drought signal found with tree-ring chronologies is further explored within the framework of a regional climate modeling. Version 2 of the Twentieth-Century Reanalysis (DD-20CR) is dynamically downscaled over a contiguous U.S.-Mexico domain. Statistic analysis of the DD-20CR suggests that the low-frequency drought signal in the Southwest is driven by atmospheric circulation changes on global to continental scales that affect precipitation in Central American as well. DD-20CR reproduces the spatial patterns of precipitation associated with climate variability at decadal and longer timescales in a manner that compares well with observational records and tree-ring chronologies. Low-frequency climate variability is therefore likely responsible for the multiyear persistent droughts in the last four centuries, as independently evaluated from the tree-ring monsoon-sensitive network.

The aim of the research undertaken in this dissertation was to use medium-range to seasonal precipitation forecasts for hydrologic applications for catchments in the core North American Monsoon (NAM) region. To this end, it was necessary to develop a better understanding of the physical and statistical relationships between runoff processes and the temporal statistics of rainfall. To achieve this goal, development of statistically downscaled estimates of warm season precipitation over the core region of the North American Monsoon Experiment (NAME) were developed. Currently, NAM precipitation is poorly predicted on local and regional scales by Global Circulation Models (GCMs). The downscaling technique used here, the K-Nearest Neighbor (KNN) model, combines information from retrospective GCM forecasts with simultaneous historical observations to infer statistical relationships between the low-resolution GCM fields and the locally-observed precipitation records. The stochastic nature of monsoon rainfall presents significant challenges for downscaling efforts and, therefore, necessitate a regionalization and an ensemble or probabilistic-based approach to quantitative precipitation forecasting. It was found that regionalization of the precipitation climatology prior to downscaling using KNN offered significant advantages in terms of improved skill scores.Selected output variables from retrospective ensemble runs of the National Centers for Environmental Predictions medium-range forecast (MRF) model were fed into the KNN downscaling model. The quality of the downscaled precipitation forecasts was evaluated in terms of a standard suite of ensemble verification metrics. This study represents the first time the KNN model has been successfully applied within a warm season convective climate regime and shown to produce skillful and reliable ensemble forecasts of daily precipitation out to a lead time of four to six days, depending on the forecast month.Knowledge of the behavior of the regional hydrologic systems in NAM was transferred into a modeling framework aimed at improving intra-seasonal hydrologic predictions. To this end, a robust lumped-parameter computational model of intermediate conceptual complexity was calibrated and applied to generate streamflow in three unregulated test basins in the core region of the NAM. The modeled response to different time-accumulated KNN-generated precipitation forcing was investigated. Although the model had some difficulty in accurately simulating hydrologic fluxes on the basis of Hortonian runoff principles only, the preliminary results achieved from this study are encouraging. The primary and most novel finding from this study is an improved predictability of the NAM system using state-of-the-art ensemble forecasting systems. Additionally, this research significantly enhanced the utility of the MRF ensemble forecasts and made them reliable for regional hydrologic applications. Finally, monthly streamflow simulations (from an ensemble-based approach) have been demonstrated. Estimated ensemble forecasts provide quantitative estimates of uncertainty associated with our model forecasts.

A high-resolution multi-proxy reconstruction of the Arabian Sea Summer Monsoon (ASSM) intensity over the past 90,000 years has been determined using two marine sediment cores: one from the Somali margin and one from the Indian margin. This reconstruction indicates that changes in monsoon- induced upwelling, primary productivity and denitrification have varied in synchrony with Dansgaard-Oeschger (D-O) cycles. Increased monsoon intensity correlates with warm climate events (interstadials) and decreased monsoon intensity, which coincides with stadials and Heinrich Events, is confirmed by elevated dust concentrations in the marine cores. A comparison of the Somali and Indian margin cores with previously reported studies from the Northern and Western Basin allows the identification of discrete sediment signals from the Indus River, the Arabian Peninsula and from local riverine runoff. Sedimentary deposition on the Indian margin during interglacials is dominated by local terrestrial runoff, whereas during glacial periods increased dust input from the Arabian Peninsula is evident. Both signals are related to changes in the intensity of the ASSM. Monsoon intensity has decreased during the Holocene as the Intertropical Convergence Zone (ITCZ) has moved to a more southerly position. The ASSM-ITCZ relationship (increased ASSM intensity and a northern ITCZ, decreased ASSM intensity and a southern ITCZ) has remained consistent over the last glacial cycle suggesting that global millennial scale climatic variability is in part driven by modulations in tropical hydrological cycle. This ASSM reconstruction provides evidence that rearrangements in the tropical convection system affected atmospheric dust concentrations as well as the concentration and location of atmospheric water vapour. In addition to modulating terrestrial and marine emissions of greenhouse gases, variation in the tropical hydrological cycle provides a mechanism of amplifying and perpetuating millennial-scale climatic changes.

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.

abstract: Ecohydrological responses to rainfall in the North American monsoon (NAM) region lead to complex surface-atmosphere interactions. In early summer, it is expected that soil properties and topography act as primary controls in hydrologic processes. Under the presence of strongly dynamic ecosystems, catchment hydrology is expected to vary substantially in comparison to other semiarid areas, affecting our understanding of ecohydrological processes and the parameterization of predictive models. A large impediment toward making progress in this field is the lack of spatially extensive observational data. As a result, it is critical to integrate numerical models, remote sensing observations and ground data to understand and predict ecohydrological dynamics in space and time, including soil moisture, evapotranspiration and runoff generation dynamics. In this thesis, a set of novel ecohydrological simulations that integrate remote sensing and ground observations were conducted at three spatial scales in a semiarid river basin in northern Sonora, Mexico. First, single site simulations spanning several summers were carried out in two contrasting mountain ecosystems to predict evapotranspiration partitioning. Second, a catchment-scale simulation was conducted to evaluate the effects of spatially-variable soil thickness and textural properties on water fluxes and states during one monsoon season. Finally, a river basin modeling effort spanning seven years was applied to understand interannual variability in ecohydrological dynamics. Results indicated that ecohydrological simulations with a dynamic representation of vegetation greening tracked well the seasonal evolution of observed evapotranspiration and soil moisture at two measurement locations. A switch in the dominant component of evapotranspiration from soil evaporation to plant transpiration was observed for each ecosystem, depending on the timing and magnitude of vegetation greening. Furthermore, spatially variable soil thickness affects subsurface flow while soil texture controls patterns of surface soil moisture and evapotranspiration during the transition from dry to wet conditions. Finally, the ratio of transformation of precipitation into evapotranspiration (ET/P) and run off (Q/P) changed in space and time as summer monsoon progresses. The results of this research improve the understanding of the ecohydrology of NAM region, which can be useful for developing sustainable watershed management plans in the face of anticipated land cover and climate changes. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2014

abstract: The North American Monsoon (NAM) is characterized by high inter- and intra-seasonal variability, and potential climate change effects have been forecasted to increase this variability. The potential effects of climate change to the hydrology of the southwestern U.S. is of interest as they could have consequences to water resources, floods, and land management. I applied a distributed watershed model, the Triangulated Irregular Network (TIN)-based Real-time Integrated Basin Simulator (tRIBS), to the Beaver Creek basin in Arizona. This sub-basin of the Verde River is representative of the regional topography, land cover, and soils distribution. As such, it can serve to illustrate the utility of distributed models for change assessment studies. Model calibration was performed utilizing radar-based NEXRAD data, and comparisons were done to two additional sources of precipitation data: ground-based stations and the North American Land Data Assimilation System (NLDAS). Comparisons focus on the spatiotemporal distributions of precipitation and stream discharge. Utilizing the calibrated model, I applied scenarios from the HadCM3 General Circulation Model (GCM) which was dynamically downscaled by the Weather Research and Forecast (WRF) model, to refine the representation of Arizona's regional climate. Two time periods were examined, a historical 1990-2000 and a future 2031-2040, to evaluate the hydrologic consequence in the form of differences and similarities between the decadal averages for temperature, precipitation, stream discharge and evapotranspiration. Results indicate an increase in mean air temperature over the basin by 1.2 ºC. The average decadal precipitation amounts increased between the two time periods by 2.4 times that of the historical period and had an increase in variability that was 3 times the historical period. For the future period, modeled streamflow discharge in the summer increased by a factor of 3. There was no significant change in the average evapotranspiration (ET). Overall trends of increase precipitation and variability for future climate scenarios have a more significant effect on the hydrologic response than temperature increases in the system during NAM in this study basin. The results from this study suggest that water management in the Beaver Creek will need to adapt to higher summer streamflow amounts. / Dissertation/Thesis / M.S. Civil and Environmental Engineering 2012

This study deploys compound-specific multi-proxy isotopic study of lipid biomarkers to understand Neogene climatic and ecological variabilities in the Himalayan foreland. The investigation of compound-specific carbon and hydrogen isotopes along with glycerol dialkyl glycerol tetraether (GDGT) is the first of its kind for the Nepalese Siwalik. A total of 49 mudstone (and some paleosol) samples were collected from the paleomagnetically age-constrained Siwalik strata in the Surai Khola and Karnali River sections. δ13C results suggest a domination of C3 trees between 12 and 8.5 Ma, and a stepwise expansion of C4 grasses starting gradually at 8.5 Ma and culminating rapidly around 5.5 Ma. δD results show an overall gradual increase in rainfall since 12 Ma, with a rapid intensification around 5.5 Ma. The negative correlation between rainfall and GDGT-derived paleotemperature prior to 5.5 Ma indicates that the region experienced higher rainfalls during colder periods and vice versa. We propose that this negative correlation could be related to the strong presence of mid-latitude westerlies in the region because of the subdued Himalayas, when summer monsoon winds were weaker, that brought enhanced winter-precipitation particularly during colder periods. After 5.5 Ma, our data show a conspicuous positive correlation between rainfall and annual temperature, indicating the onset of modern-style seasonality in rainfall in the Indian subcontinent, which generates more rainfall during summer than during winter. Notably, this initiation of the Indian monsoon around 5.5 Ma favored the dominance of C4 grasses over C3 trees that is reflected in our δ13C data.

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