Obliquity-driven changes in East Asian seasonality
Introduction
Data and modeling studies have explored the influence of obliquity on changes in the East Asian monsoon. In winter, obliquity, as a factor controlling differences in meridional insolation (Bosmans et al., 2015; Mantsis et al., 2014), affects the Siberian high and the nearby Asian monsoon flow (Liu et al., 2015; Shi et al., 2011). In summer, obliquity can influence the intrinsic dynamical mode in the East Asia–Western North Pacific (WNP) region, including the formation dynamics of the WNP monsoon trough and the Bonin high (Chen et al., 2011; Wu et al., 2016b). Nevertheless, controversy remains regarding East Asian proxy records that have shown a precessional signal overriding the contribution of obliquity (Liu et al., 2014; Wang et al., 2008). Whether this contribution is crippled remains a matter for further investigation.
Wyrwoll et al. (2007) found that the strength of the Australian summer monsoon can be related to obliquity forcing coupled with oceanic feedback. Tuenter et al. (2003) suggested that the amplitude of the precipitation response to obliquity may depend on precession during the African summer monsoon. Following the hypothesis of Liu et al. (2003) that the tectonic uplift of the Tibetan Plateau amplified orbital-scale East Asian monsoon variability, modeling results in Wu et al. (2018b) suggested that the contribution obliquity makes to changes in the Asian summer monsoon might be crippled due to the existence of the Tibetan Plateau and Maritime Continent. Furthermore, the influence of obliquity can be considerable in mid-to-late summer when subtropical monsoons develop widely over the Afro–Asia–Pacific region. This may be comparable to the dominant control of precessional forcing on the continental monsoons (Wu et al., 2018b). The competing relationship between monsoons with an inland/highland origin (i.e., South and East Asia) and monsoons with tropical/oceanic origin (i.e., West Africa and the subtropical WNP) (Wu et al., 2018a) might relate to a signal offset between the orbital parameters.
Several paleoclimate studies have focused on the role of greenhouse gases (GHG) in modulating orbital-scale climate changes (Lu et al., 2018; Lu et al., 2013; Otto-Bliesner et al., 2014; Weber and Tuenter, 2011), although challenges remain in isolating the effects of GHG on monsoon dynamics from paleoclimate records (Mohtadi et al., 2016). With regard to ice sheets, GHG may be considered a source of forcing at the period of precession band and a source of feedback at the obliquity band (Ruddiman, 2006). The modeling results in Otto-Bliesner et al. (2014) confirmed the effect of GHG on the development of interglacial conditions in African monsoon regions. In terms of the obliquity band, Weber and Tuenter (2011) indicated that GHG with varying ice sheet can explain most of the variance in simulated African and Indian monsoons, with orbital forcing playing a minor role, whereas orbital forcing on the East Asian monsoon remains dominant. Changes in GHG concentrations have also been considered a mechanism for a substantial contrast in orbitally-driven precipitation changes before (trace to mid-Holocene) versus after (until the present) preindustrial time (Berman et al., 2017). Sun et al. (2015) emphasized the different spatial effects of GHG (with a dominant influence in north China) and insolation (stronger effect in south China).
The role of GHG in modulating high-latitude atmospheric temperature might influence the contribution obliquity makes to the East Asian climate. This study aimed to understand (1) the influence of obliquity on East Asian seasonality and (2) whether the identified contribution of obliquity is sensitive to changes in GHG concentration. The results can provide insights into the debate regarding whether present-day GHG-driven warming is influenced by the accelerated decrease in obliquity. Insights into climate modeling might therefore be advanced further, especially regarding serious differences between orbital forcing on shortwave radiation (primarily at the surface) and GHG forcing on longwave radiation (mostly in the atmosphere). The remainder of the paper is organized as follows. Section 2 introduces the models and the experimental design. Section 3 presents the major differences in summer and winter monsoons between low versus high obliquity. Section 4 explores the influence of GHG concentrations on obliquity-driven changes; we also investigated the sensitivity of the obliquity contribution to a degree shift in perihelion precession. Conclusions are presented in Section 5.
Section snippets
Model description and experimental design
To explore atmospheric changes between high versus low obliquity, we used the Community Atmospheric Model (CAM) version 5.1 of the National Center for Atmospheric Research (NCAR) and the slab ocean model (SOM) (http://www.cesm.ucar.edu). Details of CAM are provided in Neale et al. (2013). Compared with the previous CAM, in CAM 5.1 the shallow convection scheme has a realistic plume dilution equation, provides a representation of convective momentum transports, simulates cloud-aerosol indirect
Seasonal characteristics in low versus high obliquity
In this section, an investigation into obliquity-driven changes in a relatively low GHG condition (i.e., OL31Kg vs. OH31Kg) is presented. Given that obliquity can influence meridional insolation gradients and thus seasonality, after exploring large-scale changes over the Afro–Asia–Pacific monsoon region, we explored whether and how stages of dynamical precipitation would be influenced over East Asia–WNP.
Compared with high obliquity, north–south expansions of seasonal precipitation in low
Obliquity-driven seasonal changes in high versus low greenhouse gas concentrations
With a low GHG concentration, obliquity-driven large-scale seasonal changes in monsoon regions are considerable. We therefore identified obliquity-driven changes in present-day GHG concentrations, which are much higher than the values in the 31 ka BP (i.e., OL31K vs. OH31K, Table 1). Because the major contribution of changes in obliquity and GHG coincide with meridional temperature gradients, GHG forcing on obliquity-driven seasonal changes might be reasonably postulated and illuminate the weak
Conclusions
The modeling results of this study suggest that, in low GHG concentrations, the precipitation migration of continental summer monsoons (West Africa, South Asia, and East Asia) closely follows obliquity-driven meridional insolation changes, confirming the traditional paradigm of an obliquity–seasonality relationship. In contrast to high obliquity where the meridional expansion of precipitation can be clearly observed, precipitation (primarily associated with the Afro–Asia summer monsoons) in low
Declaration of Competing Interest
The authors declare that they have no competing interests.
Acknowledgments
This work was supported by the Ministry of Science and Technology, Taiwan, under grants MOST 107–2119–M–001–013– and 108–2628–M–001–006–MY4. We are grateful to the National Center for Atmospheric Research for making the model CESM accessible to the community, the model SPEEDY from the ICTP, and the National Center for High-Performance Computing (NCHC) for supercomputer resources. We also thank the anonymous reviewers for their constructive comments.
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