Global climate and monsoons response to orbital forcing in the Late Miocene

Abstract:

Global climate during the Late Miocene (11.61–5.33 Ma) is thought to have been generally warmer and wetter than at present day. The Northern Hemisphere was characterised by nearly ice-free conditions (with respect to the extent of the Greenland ice sheet) and some of the main marine gateways were undergoing opening or closure (e.g. Central American gateway, Bering Strait, and Indonesian Throughflow). Vegetation distribution was also generally more extensive than it is today, both at high and low latitudes.

There is geological evidence of orbitally-forced cyclicity in sedimentary sections throughout the globe, especially in marginal basins such as the Mediterranean Sea. In the Late Miocene the entire North African catchment drained in the Eastern Mediterranean, constituting the main fresh water input into the basin, regulated by the North African monsoon. In addition, the Tibetan Plateau underwent substantial uplift throughout this time period, which strengthened the Asian monsoon system.

The Late Miocene therefore represents an ideal scenario to investigate the impact of orbital forcing on the North African and Asian monsoon systems, the establishment of their teleconnections, and the associated vegetation changes. There still is considerable uncertainty in the reconstructed atmospheric CO₂ levels for this time period, due to the patchy distribution (both spatially and temporally) of the available proxy record. Hence, we also explore the sensitivity of global climate to changing CO₂ levels with different orbital configurations.

We carried out a new series of 22 fully coupled atmosphere-ocean-vegetation numerical simulations, run at evenly spaced intervals (1kyr) through a full late Miocene precession cycle (~6.5 Ma), using a full-complexity general circulation model (HadCM3L).

These model results show substantial changes to sea surface temperatures and regional atmospheric circulation on sub-precessional time scales. This triggers responses in the North African and Asian monsoon systems, which modify their timing, onset, intensity and seasonality. This has important consequences for vegetation distribution, with highly intensified precipitation patterns at times of minimum precession (maximum insolation) resulting in a greening of both the Sahel and Indian regions.