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.

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Thesis advisor: Corinne I. Wong / Investigating controls on past variability of South American hydroclimate is critical to assessing its response to future warming scenarios. δ¹⁸O records from South America offer insight into past variability of the South American Monsoon System (SAMS). The controls on precipitation δ¹⁸O values, however, can be decoupled from precipitation amount and thereby limit investigations of variability in local moisture conditions. Here we use a principle components analysis to assess the coherence of speleothem and lake core Holocene δ¹⁸O records in tropical South America to evaluate the extent to which δ¹⁸O variability reflects changes in SAMS intensity at different sites across the region. The main mode of variability across Holocene δ¹⁸O records (PC1) closely tracks austral summertime insolation, consistent with existing work. PC1 is strongly expressed at sites towards the periphery of the continent, while variability at interior sites bear little similarity that implicates controls, in addition to monsoon intensity, on these δ¹⁸O records. Further, we develop speleothem ⁸⁷Sr/⁸⁶Sr records spanning the Holocene from Tamboril Cave (Brazilian Highlands), Paraíso Cave (eastern Amazon Basin), Jaraguá Cave (Mato Grosso Plateau), and Botuverá Cave (Atlantic coastal plain) to investigate coupling between reconstructed monsoon variability and local moisture conditions. Speleothem ⁸⁷Sr/⁸⁶Sr variability is interpreted as a proxy of local moisture conditions, reflecting the degree of water-rock interaction with the cave host rock as driven by variations in water residence time. Speleothem ⁸⁷Sr/⁸⁶Sr records from all the sites, except Botuverá cave, do not co-vary with PC1, suggesting that local moisture conditions do not necessary follow variations in monsoon intensity at these interior sites. These speleothem ⁸⁷Sr/⁸⁶Sr records, however, generally suggest dry mid-Holocene conditions, consistent with interpretations of other paleo-moisture records in the region. These results highlight that dynamics, in addition to SAMS variability, might influence δ¹⁸O variability as well as local moisture conditions at interior sites, and highlight the need for δ¹⁸O-independent reconstructions of moisture conditions. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Mono Lake (CA) is a hydrologically closed lacustrine basin well-known for its paleo-shorelines, which record fluctuations in water level for the last deglacial and late Holocene. Mono Lake is a sentinel of California’s water supply, situated in the rain shadow of the Sierra Nevada, a mountain range whose snowpack is a vital source of freshwater for urban and agricultural districts to the west and south. Recent droughts, floods, and wildfires show that California is threatened by climate change, but how these changes impact and get recorded by Mono Lake sediments remains poorly known. Here, we use a new radiocarbon-dated deepwater sediment core from Mono Lake to test the hypothesis that organic facies development is controlled by climate and limnological change. An integrated stratigraphic analysis of the core reveals seven lithostratigraphic units that track environmental changes from ~16-4 ka. When compared to available paleo-shoreline and shallow water core data, our results show that high amplitude lakelevel fluctuations of the late Pleistocene produce different patterns of sedimentation and organic enrichment than lower-amplitude water level changes of the early and middle Holocene. The results have implications for understanding patterns of paleo-production and hydroclimate change at Mono Lake.

The Arctic is an important component of the Earth’s climate system, and it is a region dynamically coupled to climate phenomena at lower latitudes, through both atmospheric and oceanic paths. The coupling has significant effects on the hydroclimate variability in the Arctic, including effects on sea ice and Arctic precipitation. In this dissertation, we explore the coupling of the lower latitudes and the Arctic hydroclimate through atmospheric mechanisms with dynamical and thermodynamical components, with a focus on the following examples of variability: i) the decadal variability of boreal winter Arctic precipitation, ii) the variability of the strength of the stratospheric polar vortex in boreal winter, and iii) the initial melt of Arctic sea ice in late boreal spring. The goal of the research is to understand what drives the Arctic hydroclimate variability in each of these examples through improved knowledge of the mechanisms linking them to the tropics and Northern Hemisphere midlatitudes. In the first part of the analysis, we explore the mechanisms responsible for the decadal variability of boreal winter Arctic precipitation. We find that the decadal variability of cool-season Arctic precipitation is at least partly connected to decadal modulation of tropical central Pacific sea surface temperatures related to the El Niño-Southern Oscillation (ENSO). The modulation can be described as the oscillation between periods favoring central and eastern Pacific warming events [CPW and EPW, respectively], which are two common types of ENSO variability. By analyzing a collection of CPW and EPW events in reanalysis data, we establish the following connecting mechanism. First, the increase of central Pacific SSTs drive a Rossby wave train that destructively interferes with the zonal wavenumber 1 component of the background extratropical planetary wave in the subpolar region. Next, as a result of this interference, the magnitude of the vertical Rossby wave propagation from the troposphere to the stratosphere decreases and the stratospheric polar vortex strengthens. Finally, the strengthening of the vortex translates into a tendency towards a positive Arctic Oscillation (AO) in the troposphere and a poleward shift of the Northern Hemisphere midlatitude storm tracks, increasing moisture transport from lower latitudes and increasing total Arctic precipitation. In a further investigation of a crucial component of the above mechanism, the initial response of the stratospheric polar vortex to the influence of CPW and EPW is investigated. A 20-member ensemble run of an idealized model experiment in the NCAR Whole Atmosphere Community Climate Model (WACCM) is conducted with prescribed CPW and EPW pattern SST anomalies. Both CPW and EPW events weaken the polar vortex in the ensemble mean. The weakening is mainly tied to changes in the eddy-driven mean meridional circulation, with some contribution from eddy momentum flux convergence. There is a significant spread between ensemble members with identical CPW and EPW forcing, where a few of the ensemble members exhibit a weak strengthening response. The initial conditions of the extratropical atmosphere and subsequent internal variability after the introduction of the CPW and EPW forcing help drive the spread in response between individual members. In the last part of the analysis, using MERRA reanalysis data, the means by which atmospheric eddies affect the trend and variability of the initial melt of Arctic sea ice are explored. We focus specifically on the effects of lower troposphere (i.e. 1000-500 mb average) meridional heat transport by atmospheric eddies, a dynamical component of the atmospheric eddy mechanism, and eddy-generated surface downwelling shortwave and longwave radiation anomalies, a thermodynamical component. Although in a climatological sense, atmospheric eddies in all major frequency bands transport heat poleward into the Arctic, we find that the lower-troposphere eddy meridional heat transport does not contribute to the trend of an earlier initial melt date. However, eddy heat transport still plays an important role in the initialization of individual episodes of initial melt with large areal coverage. In the investigation of two specific episodes, the meridional heat transport term that represents the interaction between the eddy wind and mean temperature fields (i.e. the product of the meridional eddy wind and the mean temperature fields) is most associated with the initial melt in both episodes. Additionally, melt in one of the episodes is also associated with surface downwelling longwave and shortwave radiation anomalies, a result of eddy-generated cloud cover anomalies. Therefore, in individual melt events, the combination of direct eddy meridional heat transport and surface longwave and eddy-driven shortwave radiation anomalies may significantly contribute to the initial melt of Arctic sea ice. This combination may be especially important in episodes where significant initial melt occurs over a large area and over a period of a few days.

abstract: Rapid urbanization and population growth occurring in the cities of South Western United States have led to significant modifications in its environment at local and regional scales. Both local and regional climate changes are expected to have massive impacts on the hydrology of Colorado River Basin (CRB), thereby accentuating the need of study of hydro-climatic impacts on water resource management in this region. This thesis is devoted to understanding the impact of land use and land cover (LULC) changes on the local and regional hydroclimate, with the goal to address urban planning issues and provide guidance for sustainable development. In this study, three densely populated urban areas, viz. Phoenix, Las Vegas and Denver in the CRB are selected to capture the various dimensions of the impacts of land use changes on the regional hydroclimate in the entire CRB. Weather Research and Forecast (WRF) model, incorporating the latest urban modeling system, is adopted for regional climate modeling. Two major types of urban LULC changes are studied in this Thesis: (1) incorporation of urban trees with their radiative cooling effect, tested in Phoenix metropolitan, and (2) projected urban expansion in 2100 obtained from Integrated Climate and Land Use Scenarios (ICLUS) developed by the US Environmental Protection Agency for all three cities. The results demonstrated prominent nocturnal cooling effect of due to radiative shading effect of the urban trees for Phoenix reducing urban surface and air temperature by about 2~9 °C and 1~5 °C respectively and increasing relative humidity by 10~20% during an mean diurnal cycle. The simulations of urban growth in CRB demonstratedii nocturnal warming of about 0.36 °C, 1.07 °C, and 0.94 °C 2m-air temperature and comparatively insignificant change in daytime temperature, with the thermal environment of Denver being the most sensitive the urban growth. The urban hydroclimatic study carried out in the thesis assists in identifying both context specific and generalizable relationships, patterns among the cities, and is expected to facilitate urban planning and management in local (cities) and regional scales. / Dissertation/Thesis / Masters Thesis Civil, Environmental and Sustainable Engineering 2017

The impacts of climate variability and change on streamflow are of increasing concern, particularly as human demands on water supplies compete with the needs of natural ecosystems. The consequences on the hydrological cycle are predicted to be most severe for mid- to high-latitude regions. Of particular concern is reduced mountain snow accumulation and related reductions in the snow- and glacier-derived water supply. In northern British Columbia (BC), recent snowpack declines have caused a unique water management challenge. Diminishing water security in a region considered water-abundant has intensified over the last decade. Characterizing the climate controls on hydrologic variability is a priority for developing baseline information required for water supply forecasting. This research focuses on developing multi-century, annually-resolved records of snow water equivalent (SWE) and streamflow to provide a better understanding of long-term hydroclimate variability for the design and implementation of management strategies that balance riverine ecosystem services, such as recreation and fish habitat, with increasing economic and social demands. Climate sensitive tree-ring chronologies provide the opportunity to extend instrumental records of hydroclimate by capitalizing on the influence of climate on both annual radial growth and seasonal runoff. Traditional dendrohydrology relies on moisture-limited tree species from dry, continental settings. This dissertation presents a new method by focusing on mid- to high-elevation conifers sensitive to snowpack variability. Ring-width and maximum latewood density records from mountain hemlock (Tsuga mertensiana (Bong.) Carriere), white spruce (P. glauca (Moench) Voss), and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) stands were collected at sites in northern BC. Dendrochronological techniques were used to develop a: 1) 223-year record of April 1 SWE for the Stikine River basin; 2) 417-, 716-, and 343-year record of summer streamflow for the Skeena, Nass and Stikine rivers, respectively; and, 3) a 193-year reconstruction of summer-run Skeena River steelhead abundance based on the influence of ocean-atmospheric forcings on both radial tree growth and steelhead escapement. The April 1 SWE record suggests that there has been considerable variability in snowpack levels in the Stikine basin and a distinct in-phase relationship with seasonalized Pacific Decadal Oscillation (PDO) indices, not seen in basins to the south. The summer streamflow records also support a north-south “see-saw” effect, suggesting an association between moisture transport and atmospheric-ocean circulation in the region. In addition to the snow-sensitive tree-ring data, the streamflow models incorporated paleo-hemispheric records to improve predictive skill. Finally, the steelhead model described alternating intervals of persistently above-average and below-average abundance that corresponded to oceanic PDO-like influences and describe links to “warm-warm” ENSO-PDO years associated with in-river low flow periods. The reconstructions suggest that: 1) recent snowpack and streamflow declines are a rare event over a multi-century context; and, 2) existing instrumental records do not adequately represent the historic range of basin-specific hydroclimate variability necessary for new planning horizons. Mid- to high-elevation, snow-sensitive conifers have strong potential as paleohydrological proxies and for expanding the application of dendrohydrology to non-arid settings. Current conditions in northern BC, compounded by land use changes and climate change, are predicted to become more severe in the future. It is important that planning regimes incorporate long-term hydroclimate data to better understand and quantify how water supply and ecosystems will respond to future changes. / Graduate

Indiana University-Purdue University Indianapolis (IUPUI) / The Indian summer monsoon (ISM) is the dominant hydrometeorological phenomenon that provides the majority of precipitation to southern Asia and southeastern Tibet specifically. Reliable projections of ISM rainfall are critical for water management and hinge on our understanding of the drivers of the monsoon system and how these drivers will be impacted by climate change. Because instrumental climate records are limited in space and time, natural climate archives are required to understand how the ISM varied in the past in response to changes in climatic boundary climate conditions. Lake sediments are high-resolution natural paleoclimate archive that are widely distributed across the Tibetan Plateau, making them useful for investigating long-term precipitation trends and their response to climatic boundary conditions. To investigate changes in monsoon intensity during the Holocene, three lakes were sampled along an east-west transect in southeastern Tibet: Galang Co, Nir’Pa Co, and Cuobu. Paleoclimate records from each lake were developed using isotopic (leaf wax hydrogen isotopes; δ2H), sedimentological, and geochemical proxies of precipitation and lake levels. Sediments were sampled at high temporal frequencies, with most proxies resolved at decadal scales, to capture multi-decadal to millennial-scale variability in monsoon intensity and local hydroclimate conditions. The ISM was strongest in the early Holocene as evidenced by leaf-wax n-alkane δ2H at both Cuobu and Galang Co corresponding with Cuobu’s higher lake levels and effective moisture. Monsoon intensity declined at Cuobu and Galang Co around 6 ka which corresponds to reduced riverine sediment influxes at Cuobu and deeper lake levels at Galang Co. The antiphase relationship between lake levels and monsoon intensity at Galang Co is attributed to air temperatures and effective moisture, with a warmer and drier local hydroclimate driving early Holocene low lake levels. The late Holocene ISM was more variable with wet and dry periods, as seen in the Nir’Pa Co lake level and leaf wax n-alkane δ2H record. These records demonstrate coherent drivers of synoptic and local hydroclimate that account for Holocene ISM expression across the southeastern Tibetan Plateau, indicating possible drivers of future monsoon expression under climate change.

We present a continuous high-resolution precisely dated multiproxy record of hydroclimate variability at Anjohibe in northwestern Madagascar using speleothem AB13. The record spanned ~4,484 to ~2,863 years BP and showed general agreement with previously published speleothem records from the same approximate location. However, a speleothem record from Rodrigues Island, located ~1,600 km to the east of Madagascar, did not align, suggesting that paleoclimate records from Rodrigues Island may not serve as accurate proxies for northwestern Madagascar, as has been previously suggested. Stalagmite AB13 also provides a detailed record of rainfall variability during the 4.2 ka event, the abrupt climate disturbance associated with the collapse of several early human civilizations. Between ~3,900 – 4,300 years BP, Anjohibe experienced two periods of moderate drying. The most significant climate perturbation in the record was a drought that lasted ~300 years with peak dryness at ~3,000 years BP. This extended drought may have contributed to the reduction of the local perennial wetland environments and thus may have implications for the extirpation of Malagasy pygmy hippopotamuses in this part of the dry deciduous forest.

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