Research Themes and Current Projects
Improving Model/Proxy Comparisons
Paleoclimate reconstructions are our only source of information on climate before the start of the modern instrumental record. Much of my work is focused on making sure climate models are able to simulate the features we see in those reconstructions; but there are several complicating factors to consider.
My NSF Ocean Sciences proposal addresses an outstanding problem in paleoclimate: how do we relate the variations we can measure in "proxy" records to changes in large-scale atmosphere/ocean circulation? I am looking at reconstructions made from tropical coral skeletons, where the ratio of oxygen-18 to oxygen-16 taken up by the coral as it grows is controlled by changes to temperature and the composition of the seawater the reef is sitting in. Many people have shown that these variations are related to ENSO, but since we don't have good observations of conditions right near the reef it can be hard to interpret the details of what is going on. So I have created an isotope-enabled version of the Regional Ocean Modeling System, to "fill in the gaps" in the observational record and see exactly how El Nino and La Nina events over the past 50 years have led to the coral "delta 18O" variations we see.
We are also conducting a field campaign at Palmyra Atoll and Christmas Island, funded in part by an NSF Physical Oceanography RAPID award, to measure the ongoing El Nino/La Nina cycle which began in 2015.
Arid regions like the southwestern United States experience periods of extreme drought from time to time, with severe impacts on water resources. As severe as some droughts have been over the 20th century, paleoclimate evidence indicates that past droughts have been even more intense and prolonged at times. These ‘megadroughts’ are expected to occur naturally, and to become more severe due to future climate change: so understanding the conditions which lead to these events, and how well climate models are able to represent their occurrence, is an important step towards being able to prepare for future megadroughts.
I am part of an interdisciplinary team at NCAR and the University of Arizona, working on quantifying megadrought risk using both paleoclimate records and simulations with the Community Earth System Model. Our group website is here: www.megadrought.org.
Much of our work uses the CESM Last Millennium Ensemble, a new community resource including the largest set of simulations of the 850-2005 period ever created with a single climate model. This is a suite of 30 different simulations of the 850-2005 period, which uses climate "forcings" from solar variability, volcanic activity, orbital changes, land use/land cover, ozone/aerosols, and all of those things combined. As part of this project, I am looking at various aspects of how Last Millennium drought in arid regions relates to the tropical oceans.
The El Nino/Southern Oscillation and Climate Change
My dissertation work focused on the response of the El Nino/Southern Oscillation (ENSO) to anthropogenic climate change. Because of the long timescales of internal (randomly generated) ENSO variability, multiple centuries of data are required to ‘average out’ changes to ENSO statistics, which makes it hard to isolate forced signals.
In the past few years, I have been working on other ways to identify effects of climate change on ENSO: changes to the characteristics of El Nino/La Nina events, or so-called ‘ENSO diversity’, can be easier to detect than changes to the overall strength of variability. Different aspects of climate change can also affect ENSO differently: for example, the effects of greenhouse gas increases, anthropogenic aerosol emissions, and land use changes are quite distinct from one another. I am also quite interested in understanding why 21st century ENSO projections can vary dramatically across different climate models.