Collaborative Research: Investigating Inter-Hemispheric Phasing of Tropical Andean Hydroclimate in Response to Holocene Orbital Forcing

The primary objective of this research is to investigate the mechanisms of tropical South American hydroclimate variably during the last 10,000 years (the Holocene). For more than two decades, latitudinal (north-south) changes in the position of the tropical rain belt (also referred to as the Intertropical Convergence Zone; ITCZ) in response to variations in the distribution of solar energy across the Earth's surface have been viewed as the primary driver of tropical South American hydroclimate. A growing number of lake-level reconstructions from the tropical Andes, however, show that changes in Andean hydroclimate did not closely track long-term changes in solar energy. Instead, there were abrupt and marked changes between drought and pluvial (wet phases) conditions that lasted for centuries to millennia. How gradual changes in the global distribution of solar energy produced abrupt and long-lasting changes in tropical Andean hydroclimate remains an open question. Determining the mechanisms responsible for abrupt Andean climate changes in the past has profound implications for predicting the ways in which future climate change will impact this region which provides freshwater resources to over 350 million people across 11 countries. Because tropical climate variability directly impacts mid-latitude regions, including North America, this research will additionally provide critical insight into large-scale climate system interactions that will improve our understanding of how mid-latitude climate will change in the future.

The researchers will investigate Holocene tropical Andean hydroclimate by conducting biological, geochemical, and sedimentological analyses of sediment from Lake Tota, a high-altitude lake in the eastern cordillera of the Colombian Andes. The researchers will use these data to develop proxy records of mean annual temperature (MAT), mean annual precipitation (MAP), the hydrogen isotopic composition of precipitation and lake water, and lake levels. Lake Tota was specifically selected for this work because previous geophysical and sediment core research shows that it contains a thick sequence of Holocene sediments that archive the aforementioned hydroclimate proxies and can be accurately dated with radiometric techniques. These data will provide insight on hydroclimate responses to external forcing in the understudied core ITCZ region of South America. Integrating the proxy data with lake and climate model simulations and existing paleoclimate records, the researchers will test the hypothesis that Holocene Andean effective moisture (the balance between precipitation and evaporation) was driven by MAP (as controlled by ITCZ dynamics) and mean annual evaporation (as controlled by MAT) responses to the hemispheric distribution of summer insolation and that Northern Hemisphere (NH) Andean effective moisture was generally anti-phased with the Southern Hemisphere (SH) Andes. For the NH, it is hypothesized that Andean MAP was slightly lower during the early Holocene when the ITCZ was at its northerly most position, increased during the early middle Holocene as the ITCZ migrated southward, and generally lessened as the ITCZ migrated further southward as summer insolation decreased in the NH and increased in the SH. Changes in MAE, as moderated by MAT, mitigated or enhanced changes in ITCZ-driven MAP, resulting in non-insolation-like effective moisture trends.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.