Orbital Control and Climate Feedbacks on Ice Age Cycles during Plio-Pliestocene.

Abstract

Orbital forcing of Pleistocene climate has been firmly established since the development of the benthic δ18O record, a proxy for glacial ice volume, three decades ago. However, our understanding of ice age cycles and the involving feedback remains limited. Prediction based on Milankovitch theory that both ~21kyr and ~41kyr cycles exist in the ice volume record failed to explain Pleistocene benthic δ18O record which is dominated by ~41kyr periodicity before 0.8 Ma. This failure of Milankovitch theory has inspired new hypotheses to account for the unexplained features of the ice volume record. The variety of these hypotheses indicates the complication of the climate-orbital linkage: instead of simple radiative forcing of high latitudes, internal climate feedbacks might play an important role in the cause of ice age cycles. However, these hypotheses are largely untested to date. Marine proxy records demonstrate tropical climate oscillations in accordance with high-latitude climate changes. Based on these records, tropical region has been recognized as a possible amplifier of the obliquity insolation change. Nevertheless, the pathways through which the high-latitude climate influenced tropical climate condition are not understood.

This study seeks to understand the climate response to obliquity forcing in coupled ocean-atmosphere simulations. Pleistocene experiments were developed and analyzed to determine the mechanisms that link high and low latitudes during the late Pliocene to early Pleistocene. Surface gyre circulation in the South Pacific was identified as an oceanic mechanism that connects high-latitude and tropical oceans. This mechanism, tight closely to South Ocean sea ice extent, explains much of the tropical climate variability in Plio-Pleistocene proxy records especially the obliquity imprints in tropical records. By comparing simulated climate response to obliquity and precessional forcing, this study test a couple existing hypotheses account for the prominence of obliquity period in Pleistocene ice volume record and identify factors that explain the large sensitivity of ice volume record to obliquity. Within an obliquity cycle, seasonal as well as mean-annual forcing are both important in changing high-latitude snowfall. Net global snowfall and Antarctic snow accumulation tend to magnify the influence of obliquity forcing on the glacial ice volume.