AGS-PRF: Understanding Historical Trends in Tropical Pacific Sea Surface Temperature Patterns

Warming sea surface temperatures (SSTs) are an expected consequence of rising greenhouse gas levels, and the past several decades have seen a dramatic increase in global SST. But the eastern tropical Pacific has not warmed as expected despite substantial West Pacific warming. The reasons for the absence of East Pacific warming are not clear and the lack of warming stands in contrast to the SST trend patterns in model simulations, which typically show an El Nino-like pattern with greater warming to the east. The opposition between observed and simulated Pacific trend patterns has attracted considerable interest among climate scientists, as the warming pattern matters for both regional climate impacts and the overall amount of warming caused by greenhouse gas increases. Research conducted here seeks to understand the physical mechanisms that account for the difference in Pacific warming patterns between simulations and observations. The project involves comparisons between full-complexity climate models and the Zebiak-Cane (ZC) model, a highly idealized atmosphere-ocean model built to capture the basic dynamics of El Nino. The model produces an SST trend pattern similar to the observations and thus provides an important point of reference for understanding the physical mechanisms that produce the pattern. The work involves adding mechanisms to the ZC model and disabling the same mechanisms in a full-complexity climate model to understand the effects of particular mechanisms. A key consideration is the role of mean state bias in the simulated pattern, as earlier work suggests that simulations produce an incorrect trend pattern due to a well-known "cold tongue" bias in which the strip of cold SST along the eastern equatorial Pacific is too long and too cold. The work is of practical as well as scientific interest due to the implications of the SST pattern for regional climate impacts. These include the distribution and intensity of Pacific tropical cyclones (hurricanes and typhoons) and the effect of Pacific warming on drought in the US Southwest. In addition, the award provides for the career development of a Postdoctoral Research Fellow (PRF), who will work with mentors Tiffany Shaw and Malte Jansen at the University of Chicago. 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.