This thesis evaluates the nature and magnitude of tropical climate variability from the Last Glacial Maximum to the present.
The temporal variability of two specific tropical
climate phenomena is examined. The first is the position of the Intertropical Convergence Zone (ITCZ) in the Atlantic basin, which
affects sea surface temperature (SST) and precipitation
patterns throughout the tropical Atlantic. The second is the strength of the Indian Monsoon, an important component of both tropical and
global climate.

Long-term variations in the position of the ITCZ in the Atlantic region are determined using both organic geochemical techniques
and climate modeling. Upwelling in Cariaco Basin
is reconstructed using chlorin steryl esters as proxies for phytoplankton community structure.
We find that the diatom population was larger during the Younger Dryas cold event, indicating that upwelling was enhanced and the mean position of the ITCZ was farther south
during the Younger Dryas than it is today. A climate simulation using an
ocean-atmosphere general circulation model confirms these results by demonstrating that the ITCZ shifts southward in response to high-latitude cooling.

The climate of the Arabian Sea region
is dominated by the Indian Monsoon. Results from
modern sediments from a suite of cores located throughout the Arabian Sea suggest that wind strength is well represented
by the accumulation rate and carbon isotopic composition of terrestrially-derived plant waxes in sediments.
Arabian Sea SST patterns, reconstructed from a suite of sediment cores representing four
time slices utilizing the Mg/Ca SST proxy, suggest that
both the summer and winter monsoons were enhanced 8,000 yr BP relative to today while the summer monsoon was weaker and the winter monsoon stronger at 15,000 and
20,000 yr. These results are confirmed by a
time-series reconstruction of SST on the Oman Margin that reveals that SST
at this site is sensitive to both regional and global climate processes.
The results of this thesis demonstrate that tropical climate, as evaluated by a number of different proxies as well as climate models,
has varied substantially over the past 20,000 years and is closely coupled to climate at high-latitudes.