Abstract
Abstract. Orbital forcing is a key climate driver over multi-millennial timescales. In particular, monsoon systems are thought to be driven by orbital cyclicity, especially by precession. Here, we analyse the impact of orbital forcing on global climate with a particular focus on the North African monsoon, by carrying out an ensemble of 22 equally spaced (one every 1000 years) atmosphere–ocean–vegetation simulations using the HadCM3L model, covering one full late Miocene precession-driven insolation cycle with varying obliquity (between 6.568 and 6.589 Ma). The simulations only differ in their prescribed orbital parameters, which vary realistically for the selected time period. We have also carried out two modern-orbit control experiments, one with late Miocene and one with present-day palaeogeography, and two additional sensitivity experiments for the orbital extremes with varying CO2 forcing. Our results highlight the high sensitivity of the North African summer monsoon to orbital forcing, with strongly intensified precipitation during the precession minimum, leading to a northward penetration of vegetation up to ~ 21° N. The modelled summer monsoon is also moderately sensitive to palaeogeography changes, but it has a low sensitivity to atmospheric CO2 concentration between 280 and 400 ppm. Our simulations allow us to explore the climatic response to orbital forcing not only for the precession extremes but also on sub-precessional timescales. We demonstrate the importance of including orbital variability in model–data comparison studies, because doing so partially reduces the mismatch between the late Miocene terrestrial proxy record and model results. Failure to include orbital variability could also lead to significant miscorrelations in temperature-based proxy reconstructions for this time period, because of the asynchronicity between maximum (minimum) surface air temperatures and minimum (maximum) precession in several areas around the globe. This is of particular relevance for the North African regions, which have previously been identified as optimal areas to target for late Miocene palaeodata acquisition.
Highlights
It is suggested that the Antarctic Ice Sheet was already present throughout this time period (e.g. Lewis et al, 2008; Shackleton and Kennett, 1975) while the presence of a much reduced Greenland Ice Sheet means that the Northern Hemisphere was nearly ice-free (Kamikuri et al, 2007; Moran et al, 2006)
Marzocchi et al.: Orbital control on late Miocene climate and the North African monsoon trations for the late Miocene, with values ranging from about 140 to 1400 ppm, but with most of the data converging between the preindustrial (280 ppm) and present-day (400 ppm) concentrations
The present-day configuration is the opposite of this, with the Northern Hemisphere on average 1.5 ◦C warmer than the Southern Hemisphere (Feulner et al, 2013, and references therein). This difference is caused by the open Panama Seaway in our late Miocene simulations (Lunt et al, 2008a), leading to a weaker Atlantic Meridional Overturning Circulation in the late Miocene, compared with that of the present day
Summary
Late Miocene (11.61–5.33 Ma; Hilgen et al, 2005; Gradstein et al, 2004) climate is thought to have been globally warmer and wetter than the present day, as indicated by the available proxy reconstructions and modelling studies (e.g. Bradshaw et al, 2012a, 2015; Bruch et al, 2007, 2011; Eronen et al, 2010, 2011; Pound et al, 2011, 2012; Utescher et al, 2011). Lewis et al, 2008; Shackleton and Kennett, 1975) while the presence of a much reduced Greenland Ice Sheet means that the Northern Hemisphere was nearly ice-free (Kamikuri et al, 2007; Moran et al, 2006) This period was characterised by significant tectonic reorganisation, such as the gradual closure of the Panama Gateway Zhang et al, 2013; Bolton and Stoll, 2013; LaRiviere et al, 2012) This is, still a matter of debate; model–data comparisons based on vegetation reconstructions argue that the presence of European seasonal temperate forests in the late Miocene (as indicated by the fossil record) is consistent with preindustrial CO2 concentrations (Forrest et al, 2015)
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