This project takes advantage of recent advances in forward modeling of stable isotopic climate proxies. The researchers will combine modeling with new records of tropical Holocene climate variability from speleothems, lake records, and ice cores to better constrain the interpretation of terrestrial stable isotopic records of Oxygen and Deuterium at inter-annual to millennial timescales. Specifically, the researchers will evaluate the ability of the Goddard Institute for Space Studies (GISS) ModelE-R, an isotope-enabled ocean-atmosphere general circulation model, to reproduce Holocene climate variability in the tropics.
The overall scientific aim of the research is to understand the influence of orbital forcing on the location of the Inter Tropical Convergence Zone (ITCZ), changes in the Hadley and Walker circulation cells, and the intensity of monsoons in each hemisphere and their respective influence on isotopes in model output and climate proxy data.
The research has two specific research thrusts, as follows: 1) synthesize existing isotopic records into a spatio-temporal network that can be compared with GISS ModelE-R output at ~1,000 year time steps to obtain a dynamically consistent and physically plausible picture of changes in the tropical hydrological cycle throughout the entire Holocene; and 2) diagnose the climatic controls on isotopic variations in the model throughout the Holocene at various tropical proxy sites.
The primary broader impacts involve support for a graduate student and assessing a climate model's ability to reproduce past variations in the tropical hydrologic cycle and its ramifications for future anthropogenic climate change projections.
Detailed records of weather and climate observations only go back about 150 years. In the tropics the records are commonly even shorter and very sparse, yet the tropics host most of the world’s population and are home to large and often abrupt variations in climate, driven by phenomena such as ENSO or monsoon-related droughts or floods. Hence a better understanding of the natural amplitude of climate variability is desperately needed. Even though we do not have direct climate observations to fully document this envelope of climate variability and change in the region, natural archives, which incorporate the change in the stable isotopic composition of water, can be employed to reconstruct such changes back in time for thousands of years. Here we focus on archives extracted from tropical ice in Andean glaciers, lake sediments and cave deposits (stalagmites, so-called speleothems) to reconstruct climate variations on a variety of regional and spatial scales in the circum-tropics. Using this approach, for example, we were able to document how the monsoon has varied over tropical South America during the past 2,000 years and show that the South American monsoon intensity is at present rather weak, rivaled only by one similar period over a thousand years go, termed the Medieval Climate Anomaly. On the other hand the monsoon was exceptionally strong at the height of a cooling phase in the northern hemisphere, termed the Little Ice Age (see Figure below). These results are not only important to better understand how climate has varied in the tropics as a result of changes in solar, volcanic or orbital forcing, but they also provide an important test bed for climate models and different forcing reconstructions that are used to drive such models. In our project we used models that explicitly include the isotopic composition of precipitation, in order to avoid error-prone conversions of the isotopic signal into absolute temperature or precipitation amount The project also provided the basis for a variety of outreach and capacity building activities, including the training of one PhD student, production of educational material used in both the undergraduate and graduate classroom and international collaborations that helped train several South American graduate students in climate dynamics and paleoclimate research.
