Existing data and models present a confusing picture of how the Inter Tropical Convergence Zone (ITCZ), an integral part of the tropical climate system, responds under different boundary conditions. This research tests the hypothesis that the Pacific Marine ITCZ shifts to the south, and perhaps even below the equator, during glacial times. Using an innovative approach (measuring oxygen isotope ratios measured on planktonic foraminifera that calcify at the base of the photic zone) the researchers reconstruct changes in the position of the thermocline ridge that separates the North Equatorial Current from the North Equatorial Countercurrent. This ridge is currently situated just north of the modern ITCZ. The focus is on the last 150,000 years of Earth history. Funding supports education and training of a graduate student as well as involvement of undergraduates and high school students in research.
This proposal funded an investigation of changes in the patterns of atmospheric convection in the Tropical Pacific over the last ice age cycle. Today, the pattern of surface winds associated with the Intertropical Convergence Zone (ITCZ) result in a ridge (bowing upward) of the thermocline just north of the ITCZ. While the ITCZ today spends most of its time in the Northern Hemisphere, it has been suggested that during glacial time atmospheric convection shifted to the south. We hypothesized that a shift of the ITCZ to the Southern Hemisphere would create an analogous thermocline ridge in the Southern Hemisphere. We investigated the link between atmospheric circulation, the ITCZ, and the thermocline ridge in a general circulation model and used chemical and isotopic analysis on the shells of planktonic foraminifera preserved on the seafloor to reconstruct the thermocline structure both north an south of the equator in the Western Tropical Pacific. Intellectual Merit For the Last Glacial Maximum, we find evidence that the ridge in the thermocline just north of the present-day ITCZ persists during glacial times, indicating the continuing presence of a Northern Hemisphere ITCZ. However, we also evidence for a shallower thermocline in the Southern Hemisphere consistent with a Southern Hemisphere ITCZ, or, equivalently, a more zonal South Pacific Convergence Zone. The reconstructed thermocline structure is consistent with that produced in a General Circulation Model with both a Northern and Southern Hemisphere ITCZ. Our findings add dynamic evidence for a change in the pattern of convective activity in the tropical Pacific during the last ice age. Evidence so far has been based on past records rainfall amount, and we provide evidence for associated changes atmospheric circulation patterns. Broader Impacts This work formed the basis of a Ph.D. thesis for a student at Georgia Tech, and also supported the analytical efforts of an externally funded post-doctoral fellow. Our findings underscore previous suggestions that the patterns of convective precipitation in the tropics are sensitive to changes in climate, and provide a new target for climate model simulations under Last Glacial Maximum boundary conditions.
