Special Seminar: Understanding the Tropical Pacific Response to Climate Change: Roadblocks, Progress, and Future Challenges
Abstract: Interannual variability in the tropical Pacific strongly influences both ocean ecosystems and worldwide weather patterns: this takes place mainly through the El Nino/Southern Oscillation (ENSO), an irregular coupled atmosphere/ocean phenomenon which is expected to be extremely sensitive to anthropogenic climate change. Model projections of 21st century ENSO strength vary dramatically, but naturally occurring modulations in ENSO strength make it difficult to completely explain the physical causes of inter-model differences. In this seminar, I will address two methods for gaining better insight into the ENSO dynamical response to climate change: idealized model experiments and comparison with paleoclimate proxy data. Using 1000-year 'control' experiments with the NCAR Community Climate System Model at a variety of fixed atmospheric CO2 concentrations, it can be seen that the interaction between ENSO and the annual cycle takes place primarily via climatological changes in the trade winds, rather than wave reflection off the western boundary of the Pacific Basin. This suggests that improvements to model representation of seasonal winds are an important target for correctly capturing El Nino evolution. Proxy records of ENSO during past epochs can potentially provide a valuable check on model results like these, and the oxygen isotopic composition of tropical coral skeletons are an ideal recorder of past ENSO variability. However, an error analysis shows that in the absence of better conversions between climate variables (temperature/salinity/currents) and seawater oxygen isotopic composition, obtaining sufficiently accurate variance estimates is not possible. Corals are able to capture the observed late-20th century weakening of the Walker circulation with reasonable accuracy, and provide high fidelity longer-term Walker reconstructions as well. More quantitative ENSO studies will require a fully isotope-enabled ocean simulation at multiple spatial scales, which is currently being developed for use during future research.