The timing of orbital-scale Indian monsoon changes

Ruddiman [2006. What is the timing of orbital-scale monsoon changes? Quaternary Science Reviews 25, 657–658.] interprets early climate model sensitivity tests as indicating that the timing of Northern Hemisphere (NH) summer monsoon strength has, or should have, near-zero phase relative to maxima of NH precession-driven radiation. Ruddiman used this “zero-phase hypothesis” to tune the Vostok methane record to mid-July 30 °N insolation with the assumption that tropical (monsoonal) sources control the orbital-scale variation of methane [Ruddiman and Raymo, 2003. A methane-based time scale for Vostok ice. Quaternary Science Reviews 22, 141–155.]. In contrast, we and others have measured the monsoon-related response in the Indian Ocean and find that the strongest monsoons occur about 8000 years after precession-driven NH radiation maxima when analyzed over the last 350,000 years. We interpret this resultant −120° phase as a combined response to multiple forcing mechanisms including NH and Southern-Hemisphere (SH) radiation as well as global ice volume. Here, we assess four issues raised by the Ruddiman Viewpoint: (1) the validity of Arabian Sea monsoon proxies, (2) the analogy between phase at the annual cycle (month) and phase at the precessional cycle (ka), (3) the phase implications of the early Kutzbach climate model sensitivity tests, and (4) the phase of ice core and speleothem records cited by Ruddiman in support of his zero-phase hypothesis.