Tropical Pacific climate response to obliquity forcing in the Pleistocene

Marine proxy records of Pleistocene seawater temperature and productivity in the tropical Pacific Ocean vary over a 41,000-year period that has been attributed to Earth's obliquity cycle. The proxy records are paradoxical both because obliquity has a small effect on low-latitude insolation and because tropical seawater temperature and productivity were anticorrelated with obliquity insolation forcing. In this study, we investigate the response of the tropical Pacific climate to obliquity forcing using a coupled ocean-atmosphere model to reconcile the proxy records with climate theory. Two glacial and two modern simulations were completed with extreme high and low axial tilts of 24.5 degrees and 22.2 degrees. In response to an increase in axial tilt, tropical sea surface temperatures decrease by as much as 0.8 degrees C because of the local reduction in insolation. Subsurface water temperatures in the eastern and central equatorial Pacific increase by nearly 1 degrees C. Anomalous heating through high-obliquity forcing also generates dynamical responses that weaken mean annual midlatitude westerlies and subtropical trade winds, contributing to a similar to 20% reduction in the subtropical gyre circulation. Analyses using a Lagrangian transport model indicate that low-latitude subsurface warming is due to a reduction in heat export from the tropics and the advection and ventilation of anomalously warm South Pacific extratropical waters through the thermocline circulation. The model's response to obliquity is consistent with Pleistocene proxy data that indicate the tropical Walker circulation and thermocline slope were not strongly influenced by changes in axial tilt. The model results also support the hypothesis that Earth's obliquity influences climate through its control on meridional insolation gradients.