Thesis (Ph.D.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2009. / Committee Chair: Peter J. Webster; Committee Member: Robert X. Black; Committee Member: John A. Knox; Committee Member: Judith A. Curry; Committee Member: Yi Deng.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 92-96).

Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (leaves [186]-198).

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. James J. O'Brien, Florida State University, College of Arts and Sciences, Dept. of Meteorology. Title and description from dissertation home page (viewed Sept. 24, 2004). Includes bibliographical references.

A primary goal of paleoclimatology is to extend the instrumental record to capture a wider range of natural variability, documenting the climate system's response to past changes that have no analog in the historical record. Sediment archives of the recent geologic past, both marine and lacustrine, offer the opportunity to study how climate responds to a range of forcings and changing boundary conditions on timescales ranging from years to millennia. In this dissertation I use lacustrine and marine sediment to investigate changes late Quaternary climate, with particular focus on the Last Interglacial period (LIG). First, I use multiple approaches to reconstruct long-term changes in the West African Monsoon by investigating centennial-scale hydrologic variability recorded in Lake Bosumtwi sediments over the past 530,000 years. Over this interval, hydrology in the region is driven by a complex interplay of orbital forcing and glacial-interglacial boundary conditions. Lake level was generally much lower between 50 and 300 ka, likely due to the redistribution of rainfall from the tropics to the subtropics, driven by eccentricity's amplification of precession. Consequently, the Holocene highstand at the lake was both larger and longer lived than the maximum highstand during the LIG.Annual layers were continuously deposited through the LIG in Lake Bosumtwi, and I also present a new, 12,100 year-long, varve record spanning the interval from 128.6 to 116.5 ka. Over the course of the LIG, lake level generally tracks sea surface temperatures (SST) in Gulf of Guinea, including an abrupt drop in lake level that lasted about 500 years ca. 118 ka, coincident with cool SSTs in the North Atlantic and severe aridity in Europe. I find that the despite the generally drier conditions, hydrology varied on similar timescales as the late Holocene, with pronounced multidecadal to centennial-scale variability with non-stationary periodicities. I also investigate the contribution of ocean thermal expansion to sea level rise during the LIG, using a synthesis of paleoceanographic data and a climate model simulation. Globally, LIG SSTs were similar to, or slightly cooler than late Holocene SSTs, with the exception of the North Atlantic, which was several degrees warmer. Consequently, thermal expansion was likely a minor component of sea level rise during the interval, explaining between -0.3 and 0.4 m. of the 6 to 8 m highstand. Lastly, I tested the potential of Raman spectroscopy as a new, non-destructive technique to rapidly measure oxygen isotopic ratios in carbonates at extremely high resolution. Analyses on a suite a synthetic calcites indicate that ¹⁸O/¹⁶O ratios can be measured directly from the Raman spectra and have a 1:1 correspondence with traditional mass-spectrometry measurements. At present, the technique does not have the precision necessary to record natural variability, although there is considerable potential for improving the precision of the technique.

The location of the Inter-Tropical Convergence Zone plays an important role in the climatology of tropical regions. Yet, despite its importance, the basic physics that determine the location of the ITCZ are not fully understood. Observational analyses show that, where the cross-equatorial pressure gradient is strong, the maximum convection is not necessarily associated with the highest sea surface temperature,or correspondingly, the lowest sea level pressure. Tomas and Webster (1997) argue that if a strong enough cross-equatorial pressure gradient exists and the system is inertialy unstable, secondary ameliorating circulations will drive strong off-equatorial convection in regions where ITCZ location is determined by low tropospheric dynamics. The observational record is re-examined to test the inertial instability hypothesis using the new ECMWF reanalysis data set. Composite analyses are performed to study the structure of the summer meridional circulation for the tropical Eastern Pacific Ocean and Atlantic Ocean. In agreement with Tomas and Webster theory, we find that the magnitude of the cross-equatorial pressure difference appears to determine the intensity of convection with low values of outgoing longwave radiation always to the north of the zero absolute vorticity line, and the absolute vorticity advection equatorward of the this line. Also the observed oscillation period of the disturbance for the studied regions coincides with theoretical oscillation period of the inertial flow.

This study has been performed as a pilot study for a project regarding the meridional migration of the Intertropical Convergence Zone (ITCZ) and its relationship with lightning activity in the Eastern Tropical Pacific (ETP). Objectives of this study were to analyze and improve lightning data to be used for such a study and to decide on a method and proper time scale of data analysis and ITCZ index development for this study. Exploratory data analysis has been practiced with World Wide Lightning Location Network (WWLLN) data and ITCZ index data. Results suggest that the most beneficial time-scale to be used for the above study is 15 days and that ITCZ estimations can be obtained via the use of precipitation index and cloud top temperature. Lightning data originated from atmospheric systems not associated with the ITCZ has been analyzed. This report proposes that Uppsala University should become part of the World Wide Lightning Location Network, enabling further work regarding this and similar projects.

Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 320-325. / by Mark A. Cane. / Ph.D.

The physical mechanisms and predictability associated with extreme daily rainfall in South East South America (SESA) are investigated for the December-February season. Through a k-mean analysis, a robust set of daily circulation regimes is identified and then it is used to link the frequency of rainfall extreme events with large-scale potential predictors at subseasonal-to-seasonal scales. This basic set of daily circulation regimes is related to the continental and oceanic phases of the South Atlantic Convergence Zone (SACZ) and wave train patterns superimposed on the Southern Hemisphere Polar Jet. Some of these recurrent synoptic circulation types are conducive to extreme rainfall events in the region through synoptic control of different meso-scale physical features and, at the same time, are influenced by climate phenomena that could be used as sources of potential predictability. Extremely high rainfall (as measured by the 95th- and 99th-percentiles) is preferentially associated with two of these weather types, which are characterized by moisture advection intrusions from lower latitudes and the Pacific; another three weather types, characterized by above-normal moisture advection toward lower latitudes or the Andes, are preferentially associated with dry days (days with no rain). The analysis permits the identification of several subseasonal-to-seasonal scale potential predictors that modulate the occurrence of circulation regimes conducive to extreme rainfall events in SESA. It is conjectured that a cross-timescale interference between the different climate drivers improves the predictive skill of extreme precipitation in the region. The potential and real predictive skill of the frequency of extreme rainfall is then evaluated, finding evidence indicating that mechanisms of climate variability at one timescale contribute to the predictability at another scale, i.e., taking into account the interference of different potential sources of predictability at different timescales increases the predictive skill. This fact is in agreement with the Cross-timescale Interference Conjecture proposed in the first part of the thesis. At seasonal scale, a combination of those weather types tends to outperform all the other potential predictors explored, i.e., sea surface temperature patterns, phases of the Madden-Julian Oscillation, and combinations of both. Spatially averaged Kendall’s τ improvements of 43% for the potential predictability and 23% for realtime predictions are attained with respect to standard models considering sea-surface temperature fields alone. A new subseasonal-to-seasonal predictive methodology for extreme rainfall events is proposed, based on probability forecasts of seasonal sequences of these weather types. The cross-validated realtime skill of the new probabilistic approach, as measured by the Hit Score and the Heidke Skill Score, is on the order of twice that associated with climatological values. The approach is designed to offer useful subseasonal-to-seasonal climate information to decision-makers interested not only in how many extreme events will happen in the season, but also in how, when and where those events will probably occur. In order to gain further understanding about how the cross-timescale interference occurs, an externally-forced Lorenz model is used to explore the impact of different kind of forcings, at inter-annual and decadal scales, in the establishment of constructive interactions associated with the simulated “extreme events”. Using a wavelet analysis, it is shown that this simple model is capable of reproducing the same kind of cross-timescale structures observed in the wavelet power spectrum of the Niño3.4 index only when it is externally forced by both inter-annual and decadal signals: the annual cycle and a decadal forcing associated with the natural solar variability. The nature of this interaction is non-linear, and it impacts both mean and extreme values in the time series. No predictive power was found when using metrics like standard deviation and auto-correlation. Nonetheless, it was proposed that an early warning signal for occurrence of extreme rainfall in SESA may be possible via a continuous monitoring of relative phases between the cross-timescale leading components.

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.