Collaborative Research: Testing the Indian Ocean El Nino Hypothesis

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This research will explore whether the Indian Ocean could sustain climate variability resembling El Nino under continued greenhouse warming. Testing this hypothesis is important because the activation of such a mode could drive large changes in year-to-year variability and hydrological extremes over the densely populated Indian Ocean rim. This mode of climate variability has not been observed in historical data, and therefore paleoclimate reconstructions are required to study conditions that favor its emergence, its spatial characteristics, and the ability of climate models to simulate it realistically. According to climate simulations, the activation of this mode depends on the state of the Indian Ocean, which currently does not exhibit upwelling nor has a shallow thermocline (the zone of the ocean water-column where temperatures rapidly decrease with depth) -- features needed to support strong, large-scale interactions between the ocean and atmosphere. Paleoclimate records indicate that these features could have existed in the geological past, particularly during the Last Glacial Maximum, roughly 19,000-21,000 years before present, albeit with cooler average conditions. Accordingly, simulations indicate greater cooling in the eastern Indian Ocean concurrent with strong upwelling and a shallow thermocline, which is reflected in the paleo-data as an altered basin-wide (east-west) temperature gradient. Preliminary data also reveal greatly increased seasonal and year-to-year temperature variations in this region, consistent with a state favoring stronger air-sea interactions. Climate simulations of this interval show that this increase in variability could have been caused by the activa- tion of the hypothesized "Indian Ocean El Nino". Throughout this project, the researchers will further test this hypothesis by generating new datasets of seasonal and year-to-year climate variability as well as investigating new model simulations that will help determine whether, and when, this mode will emerge in the future.This project builds on preliminary model-data investigations that indicate dramatically intensified seasonal and interannual climate variability in the eastern Indian Ocean during the Last Glacial Maximum. These data suggest that changes in oceanic conditions similar to those predicted for the future could cause the emergence of a presently unobserved mode of climate variability in the Indian Ocean that resembles modern El Nino in the Pacific. In order to test this hypothesis, the researchers will develop new paleoclimate datasets and model analyses to explore climate variability in the Indian Ocean under altered conditions. The proposed new paleodata will isolate seasonality and thermocline depth signatures and thus more rigorously test the "Indian Ocean El Nino" hypothesis. Advanced model-proxy comparison techniques using existing and new model simulations from the Paleoclimate Modelling Intercomparison Project (PMIP) will be developed to perform additional tests. Historical observations and model simulations will be analyzed to determine whether this mode is distinct from present-day climate variability, and address questions related to its precursors and background conditions under which it can become active. These questions will also be explored using existing and new climate model simulations from the Coupled Model Intercomparison Project (CMIP). These simulations will be used to assess uncertainties regarding the potential activation of the "Indian Ocean El Nino" in the near future. Constraints from the model-proxy comparison will be used to determine whether these uncertainties could be reduced. Further broader impacts include student and postdoc training, and support for a new faculty member at the University of Arizona.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.