Last Glacial Maximum Tropical Pacific Thermocline Structure and Wind Field: A Data Synthesis

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The spatial pattern of wind and rainfall in the tropics is not fixed, but changes with time. The changes can be driven by the amount of greenhouse gasses in the atmosphere, changes in seasonality, or the presence of large expanses of ice outside of the tropics. By looking at these patterns during the height of the last ice age, ideas about the links between these drivers and the tropical climate can be tested. It would be difficult to directly deduce the patterns of the ice age winds. But when the winds blow on the upper ocean, they pile up warm water in some areas and bring cold water closer to the sea surface in others. In this project, the investigators will use tiny shells (foraminifera) that contain information about the temperature of seawater at different depths. These shells accumulate on the sea-floor and preserve a history of ocean temperature. The upper ocean temperature reconstructed from shells that fell to the seafloor during the height of the last ice age can then be used to deduce the wind patterns. The project would support two graduate students (one of whom is a woman from an underrepresented minority), as well as providing support for undergraduate summer researchers.The PIs proposes a plan of database development and analysis that will use the information about the thermocline structure recorded in the oxygen isotope composition of surface and subsurface planktonic foraminifera in the tropics to reconstruct the tropical thermocline and winds during the Last Glacial Maximum. The isotopic composition of the tests of different species will reflect the surface and subsurface density and can be used to reconstruct thermocline structure. The tight coupling between winds and thermocline structure will be exploited to infer changes in the winds through the thermocline reconstructions provided from the foraminifera proxy data. The PIs will first create a global database of tropical planktonic foraminifera oxygen isotope data from recent sediments. Using this data, models for foraminifera calcification habitat will be constructed and the uncertainty in the calcification habitat quantified. A variety of statistical approaches will then be explored in order to determine how to project the sparse information about the vertical structure of the water column from the foraminiferal proxy data into estimates of the tropical thermocline structure, and to determine how to optimally estimate the long-term mean winds associated with a given mean thermocline state. These analytical approaches will include Empirical Orthogonal Functions, Optimal Linear Estimators, and Inverse Methods. Finally, existing Last Glacial Maximum oxygen isotope data will be compiled for these same species. The database will be augmented in the Tropical Pacific with new measurements, as guided by the data analysis effort. Once the Last Glacial Maximum foraminiferal database is complete the data will be directly compared to results from isotope-enabled general circulation model simulations of ice age climate, and also used to produce an estimate of Tropical Pacific thermocline structure and winds using the data analysis tools developed as described above.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.