Current Research Projects
20th Century ENSO As Seen By Tropical CoralsMy 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 am going to use an isotope-enabled global climate model, combined with a regional ocean model, 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. I'm working with a team of senior scientists on this project: Brian Powell and Mark Merrifield at UH, Kim Cobb at Georgia Tech, and David Noone at CU.
Mechanisms of Southwestern US Drought
I am currently working on understanding the drivers of prolonged (multi-year) droughts in the Southwestern United States. Periods of intense drought have been well documented in the Southwest, and a lot of work has been done showing that the ocean can have a profound influence on atmospheric circulation and associated droughts; but it is not yet clear whether or not ocean dynamics are required to create the types of droughts we see in the tree ring record. Rather, it is possible that variability internal to the atmosphere could create changes to precipitation comparable to those associated with changes to ocean conditions. I am running dedicated control experiments with the NCAR Community Earth System Model (CESM) with and without ocean coupling to examine the changes to atmospheric circulation patterns associated with drought, and the statistics of the events which occur in each simulation. The paper describing these experiments will be submitted to the Journal of Climate during summer 2013.
Completed Research Projects
Response of the El Nino/Southern Oscillation (ENSO) to Climate Change in IPCC ModelsPart of my PhD work focused on the impact of anthropogenic climate warming on ENSO dynamics. Since the oscillation varies in strength from decade to decade, it has been shown that you need to observe ENSO for several centuries to be sure that you have seen the majority of what it is expected to do. And in the climate change case, the background climate is decidedly not stable: this turns out to mean that you cannot be sure that changes to ENSO will become significant on timescales of a century. Our investigation of climate change in the NCAR Community Climate Sytem Model version 4 (CCSM4) has now been accepted by the Journal of Climate: Stevenson et al. (2012) and a follow-up study using all the CMIP5 models has been accepted by Geophysical Research Letters: Stevenson, 2012ó
Dynamical Controls on ENSO
Although the climate change problem is hard to understand, one can simplify the situation using model
simulations to try to identify the major physical processes driving ENSO changes. I have diagnosed
output from several equilibrated simulations with the CCSM3.5, to look at changes due to
CO2 increases in the absence of other complicating effects. I am particularly interested in the role of
changes to the wind-driven circulation and their impact on ENSO variability and the seasonal cycle.
My paper on the CCSM3.5 simulations has now been accepted by the Journal of Climate.
The online version of the paper may be found here: Stevenson et al. (2012b).
Using Paleoclimate Records for Model Validation
An often-cited goal of paleoclimate research is to extend the temporal baseline of observations further back into the past, to improve model validation efforts and therefore make future projections more reliable. But no perfect recorder of past climates exists, meaning that we have to work with the tools available from nature. As part of my IRP-funded project, I am working with Helen McGregor and Steven Phipps of the Universities of Wollongong and New South Wales on quantifying the limitations of coral oxygen isotope records in reconstructing the past amplitude of ENSO. We have found that including the seasonal cycle in oxygen isotope variability improves the accuracy of predicting oxygen isotopic composition of corals based on instrumental data. The manuscript on this topic has been submitted to Paleoceanography: Stevenson et al. (2013a)
ENSO Event Statistics and PredictabilityUsing coupled climate models, we can only run a certain number of ensemble members since the model simulations are each so expensive. I have developed a new statistical model for ENSO which can reproduce the gross statistical properties of coupled model output, to look at changes to the event statistics under global warming scenarios. I have found that although the overall strength of ENSO does not change significantly (see above), La Nina events may tend to become stronger in future climates.
This work has now been accepted by the Journal of Geophysical Research-Oceans.