This project explores any spatial and temporal changes in precipitation over the Holocene (last 10,000 years) across the Western Pacific Warm Pool (WPWP) from hydrologic records generated from speleothems from Guam, Solomon Islands, Vanuatu, and the Philippines. The overall goal of the research is to determine if the tropical Pacific played an active or passive role in driving the changes in the global hydrological cycle over the Holocene.
The research questions being pursued include: Did insolation forcing directly cause meridional changes associated with the monsoon and InterTropical Convergence Zone (ITCZ) in the WPWP over the Holocene? Did external insolation forcing induce internal changes in the tropical Pacific via zonal changes, or ENSO, to cause precipitation changes in the WPWP over the Holocene?
Specific objectives of the project include: 1) measuring Uranium-series ages and oxygen isotopic composition of speleothem samples from Guam, Vanuatu, the Solomon Islands, and the Philippines for the last 10,000 years at decadal-scale resolution; 2) determining the spatial and temporal hydrologic variability across the WPWP for the last 10,000 years by comparing paleoclimate records from sites that experience different climate dynamics to separate the influence of ENSO dynamics and monsoon dynamics on WPWP; and 3) returning to the sites in the WPWP to collect additional speleothems, cave dripwaters, bedrock and soils to ensure that we accurately interpret the speleothem records
The broader impacts involve linking paleoclimate reconstructions with hydrologic studies. This provides potential benefits for regional and local water resources management. The project would also support a post-doctoral fellow at the University of Texas at Austin and a M.S. student at the University of Guam.
The Western Pacific Warm Pool (WPWP) is an important region as changes in the warm waters there cause climate to change across much of the world. The goal of this project was to determine how changes in the external forcing of insolation, as well as internal climate variability, affect rainfall over the WPWP. Modern climate specialists, who concentrate on the instrumental record, will have a perspective on longer-term rainfall variability in the WPWP based on these new records that is not possible due to the short duration of the instrumental record. This project has discovered new ways in which rainfall over the WPWP varied in the past by generating new paleoclimate reconstructions of rainfall. These new records are compared to previously published transient climate model simulations to help elucidate how these changes may have occurred. A combination of both paleoclimate datasets and global climate model comparisons facilitate in the identification of the mechanisms that control rainfall in this important area. Stalagmite samples were collected from several locations around the Western Pacific Warm Pool (WPWP), including western Philippines (Palawan), central Philippines (Negros), eastern Philippines (Samar), Guam, Vanuatu, and the Solomon Islands during field trips in 2011 and 2012. We provide 3 independent datasets to calibrate the conversion from the proxy of stalagmite stable isotopic composition (δ18O) to rainfall amount. All of the data suggest a relationship of a change of ~1 permil in stalagmite δ18O to ~1 m of rainfall change per year. The stalagmite δ18O records showed several new pieces of information about rainfall changes in the WPWP including: large multi-decadal variability, sensitivity to insolation forcing, and a connection to ocean circulation changes in the North Atlantic. Prominent multi-decadal variability recorded in stalagmite records over the past 500 years from Vanuatu and the Solomon Islands provide quantitative limits on the range, frequency, and pattern of internal climate variability in the South Pacific Convergence Zone (SPCZ), which is intimately linked to the WPWP. The multi-decadal changes in SPCZ rainfall do not correlate with records of external forcing, such that it appears internal variability is responsible for the multi-decadal changes. These changes are large, abrupt, and repeatable, suggesting a level of predictability to the system. Additionally, the reconstructions provide estimates of natural rainfall changes that have happened in the past for water resource planners of small islands nations, which are especially susceptible to extended periods of drought (see Figure). On longer timescales, a stalagmite record from western Philippines provides direct evidence that an abrupt climate change event called ‘The Younger Dryas’ caused a reduction in rainfall in the WPWP that lasted over 1,000 years. This rainfall reduction is present in climate models that are forced by a reduction in the North Atlantic overturning circulation due to the presence of freshwater in that region. Two stalagmite records also from western Philippines display a remarkable anti-correlation to local summertime insolation over the last 10,000 years, as well as to previously published stalagmite records of land monsoon changes in Southeast Asia. The anti-correlation is also present in several published climate model simulations during periods over the last 10,000 years. The anti-phased Palawan record results from complex interactions between the land and the ocean monsoons. The results of this research provided the grounds for the promotion of a post-doc (who was lead-PI) to Research Associate at the University of Texas at Austin (UT). From 2011-2013, an undergraduate was mentored on the project, and the results were presented work at the annual meeting of the American Geophysical Union (AGU) in December 2011 and 2013. During the summer of 2012, an undergraduate student was sponsored by this grant to participate in the Research Experience for Undergraduates (REU) program at UT. She presented a poster of her results locally at the end of program, as well as at AGU in December 2012. The lead-PI has attended AGU for the past three years (2011, 2012, 2013) presenting data that resulted directly from this grant. Additionally, the lead-PI attended the International Conference on Paleoceanography (ICP11) in Sep 2012 to present data resulting from this grant.
