This study addresses the susceptibility of atmospheric rivers (ARs) to the behavior of the North Atlantic Subtropical High (NASH). ARs are a major mechanism for meridional moisture transport often connected to heavy precipitation and mid-latitude troughs. The NASH, a semi-permanent anticyclone over the subtropical North Atlantic Ocean, has been shown to be significantly influential on precipitation variability over the southeastern United States. A self-organizing map (SOM) was trained on a 4 x 3 regular grid over 250 iterations using ERA5 derived 6-hourly 850 hPa Geopotential Heights ≥ 1535 gpm from 1979-2020. The 12 resulting "nodes" were analyzed with respect to ARs defined by objects of ERA5 derived integrated water vapor transport (IVT) > 500 m-1 s-1 with lengths > 2000 km. Composites of thresholded 850 hPa heights, AR-concurrent PRISM precipitation, AR spatial frequency distribution maps, and seasonal AR frequency histograms per node illustrate seasonal interactions between the NASH and ARs that demonstrate a tendency of more frequent ARs and higher mean AR-driven precipitation over the Mississippi embayment and Ohio River Valley in the summer months, believed to be representative of extreme moisture transport events, when the NASH exhibits increased intensity, spatial expansion, and southwestward migration. Conversely, AR frequency and AR-concurrent precipitation composites suggest wintertime events are mainly supported by dynamically-driven nor'easter and bomb type cyclones when the NASH is constricted, at higher latitudes, and further east. Findings suggest that extreme summertime water vapor transport events associated with an AR are enhanced by the warm season NASH due to its increased intensity and proximity to the eastern US that acts as a supplementary lifting mechanism amidst low dynamic influence. / Master of Science / This study aims to investigate the response of atmospheric rivers (ARs) to the behavior of the North Atlantic Subtropical High (NASH). ARs are a major vehicle for the poleward transport of moisture from the tropics and subtropics. ARs are often affiliated with heavy precipitation and mid-latitude cyclones and frontal boundaries. The NASH, a semi-permanent anticyclone over the subtropical North Atlantic Ocean, has been shown to be significantly influential on precipitation variability over the southeastern United States. A self-organizing map (SOM), a method of vector quantification, was trained on a 4 x 3 regular grid over 250 iterations using ERA5 derived 6-hourly 850 hPa Geopotential Heights ≥ 1535 meters from 1979-2020. The 12 resulting "nodes" were analyzed with respect to ARs defined by objects that result from masking the rate of transport of water vapor within a vertical column from 1000 hPa to 300 hPa of which that are greater than 2000 km long. Composites of thresholded 850 hPa heights, AR-concurrent precipitation, AR spatial frequency distribution maps, and seasonal AR frequency histograms per node illustrate seasonal interactions between the NASH and ARs that demonstrate a tendency of more frequent ARs and higher mean AR-driven precipitation over the Mississippi embayment and Ohio River Valley in the summer months, believed to be representative of severe precipitation events, when the NASH is stronger, larger, and further southwestward. Conversely, AR frequency and AR-concurrent precipitation composites suggest wintertime events are mainly supported by nor'easter and bomb type cyclones that occur when the Polar jet stream is strongest and when the NASH is constricted, at higher latitudes, and further east. Findings suggest that extreme summertime water vapor transport events associated with an AR are enhanced by the warm season NASH due to its increased intensity and proximity to the eastern US that acts as a supplementary lifting mechanism amidst low dynamic influence.

As global temperatures rise due to anthropogenic climate change, water resources, thus economies, are threatened. A geologically recent period of increased temperatures is the mid-Holocene and an investigation of its climate may allow for a better understanding of future precipitation and changes to regional water resources. The regions of interest are tropical Northern Central America and subtropical North America with Belize and Florida representing each climate zone. By reconstructing mid-Holocene climate in Florida and Belize, I hope to provide a better understanding of how increased temperatures and a reduced latitudinal temperature gradient impacts both precipitation patterns and variability. Today, drivers of changes in precipitation include climate systems such as the Atlantic Multidecadal Variability (AMV), North Atlantic Oscillation (NAO), and the Intertropical Convergence Zone (ITCZ). Therefore, it is imperative to determine their latitudinal influences during the mid-Holocene and consequently their potential impact on water resources in the near future. Speleothems from Chen Ha Cave, Vaca Plateau, Belize and Brown's Cave, West Central Florida, provided high-resolution (sub-annual to decadal) oxygen and carbon stable isotope data that allowed for a detailed investigation of mid-Holocene climate. The speleothems were sampled along the growth axis of a cross-section for oxygen and carbon isotopic analysis. 234U-230Th dating was used to create a chronology for each record and determine the time step between each isotope sample. Time series analysis with variations of Fourier transforms, including Lomb-Scargle, wavelet analysis, and multi-taper method, was used to extract periodicities for each oxygen isotope record. To determine which atmospheric-oceanic modes influenced mid-Holocene precipitation, the speleothem periodicities were compared to those of known periodicities of atmospheric-oceanic modes, such as the AMV and NAO. Finally, the Florida and Belize records were assessed for coherency using cross wavelet analysis. The Floridian speleothem recorded less precipitation compared to present levels due to a westward expansion and intensification of the North Atlantic Subtropical High (NASH) with a quasi-persistent but less influential AMV. Relative to today, the mid-Holocene in Belize was slightly wetter which I suggest is a result of a more northerly ITCZ and an intensification of the NASH that increased the strength of the Caribbean Lower Level Jet (CLLJ). The Seuss solar cycle was also significant in Belize, contributing 7.2% of the precipitation variability. Wavelet coherency assessment reveal very little connectivity between the Florida and Belize speleothem reconstructions, potentially due to the blocking influence of the ITCZ. Comparison to other records from the mid-Holocene supports the hypothesis of an intensified NASH and more northerly ITCZ. A future increase in precipitation in Belize may lead to increased soil erosion, the need for crop adaptation, and risk to the population of low lying areas, such as Belize City. In Florida, reduced precipitation may result in a decrease in agricultural output and threats to the state's freshwater supply.