El Niño Southern Oscillation Dynamics and History

The see-sawing of atmospheric mass and surface ocean waters across the tropical Pacific Ocean known as the El Niño Southern Oscillation (ENSO) has the potential to affect the lives of billions of people via its impacts on Pacific fisheries and global rainfall and temperatures, yet we continue to lack the ability to predict future ENSO variability in response to changes in radiative forcing from greenhouse gases.  Brown faculty study Pacific climate using marine and lake sediments, providing key new insights into the dynamics of ENSO on annual to Plio-Pleistocene timescales. 

Indonesian lake sediments can provide important insights into high-resolution records of ENSO variability, but an a very underutilized archive. Recent research by Jim Russell analyzing the minerology and stable isotopic composition of carbonate minerals in laminated sediments from a maar crater lake in Eastern Java (Crausbay et al., 2006) indicates that the frequency and severity of El Niño events has varied dramatically during the past 800 years, with intervals of strong El Niño events between about 1450 and 1700 AD, and weak El Niño prior to that time. These changes may result from the combined radiative effects of changes in solar output, volcanic eruptions, and trace gases, indicating a strong sensitivity of the ENSO system to natural (and perhaps anthropogenic!) radiative forcing. Current research seeks to extend these results using additional crater lakes in Eastern Java, and to investigate long-term ENSO dynamics using sediments from large, tectonic lakes on the island of Sulawesi.

Current research by Tim Herbert on the Peru margin is breaking new ground in an area that has been very difficult to work in. Deposition of marine sediments along the continental margin is frequently interrupted by hiatuses (marked by phosphorite layers) and erosion. Furthermore, the sediments lack carbonate microfossils for 14C dating and conventional paleoceanographic measurements. We have been using organic proxies to monitor past sea surface temperature and productivity, and isotopes of Nitrogen from the organic fraction to monitor the sub-surface geochemistry. The Peru margin today is home of the world’s strongest open-ocean oxygen minimum zone. An important consequence of anoxia is denitrification, which leaves a strong isotopic imprint on the Nitrogen upwelled to surface plankton. Our reconstruction (see below) indicates that oceanography and ecology along the Peru margin have changed dramatically over the past 20,000 years. The modern association of relatively cool waters, high productivity, and intense dentrification, appear to be features of only the past 6,000 years.

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