Abstract
Numerous reconstructions of tropical hydroclimate in the Pleistocene display substantial variability on precessional timescales. Precessionally-induced insolation variations, with a mean period of $${\sim }21{,}000$$ years, affect the strength of the seasonal cycle, but not annual mean insolation. The existence of variations in annual mean climate on precessional timescales therefore hints at the existence of nonlinear mechanisms that rectify the zero annual mean forcing into a non-zero annual mean response. The aim of this study is to identify these nonlinear rectification mechanisms. The traditional view of precessionally-forced precipitation changes is that tropical precipitation increases with summer insolation. By comparing two simulations with an earth system model (CESM1.0.3) we find that this paradigm is true for continental but not for oceanic changes in precipitation. Focusing on the Atlantic intertropical convergence zone (ITCZ), we find that the continental temperature and precipitation response to precessional forcing are key rectifiers of annual mean precipitation over the ocean. A boundary layer response to temperature changes over northern Africa affects the meridional position of the ITCZ over the North Atlantic in boreal spring and summer, but not in fall and winter. Over the equatorial and South Atlantic, the intensity of precipitation is strongly impacted by diabatic forcing from the continents through an adjustment of the full troposphere. Although the top of atmosphere insolation forcing is seasonally symmetric, continental precipitation changes are largest in boreal summer, thus skewing the annual mean response. These results show that it is important to take into account the seasonality of climatic forcings, even when studying annual mean climate change.
Highlights
Out of Earth’s three orbital parameters—eccentricity, obliquity, and precession—responsible for low-frequency climate variability (Milankovitch 1941), the one with the largest expression in the tropics is precession (Berger 1978; Laskar et al 2004)
It changes the strength of the seasonal cycle in incoming solar radiation at each latitude, and it does so anti-symmetrically across the hemispheres: When the seasonal cycle is stronger in the Northern Hemisphere (‘minimum precession’, similar to e.g. 11 ka) it is weaker in the Southern Hemisphere, and vice versa (‘maximum precession’, similar to e.g. presentday)
CESM1 was developed from the Community Climate System Model (CCSM) version 4 (Gent et al 2011); its enhancements include the incorporation of biogeochemical cycles and atmospheric chemistry (Hurrell et al 2013; Moore et al 2013)
Summary
Out of Earth’s three orbital parameters—eccentricity, obliquity, and precession—responsible for low-frequency climate variability (Milankovitch 1941), the one with the largest expression in the tropics is precession (Berger 1978; Laskar et al 2004). With a period of ∼21,000 years (21 ka), the precessional cycle determines at what time of year the earth is closest to the sun. Many tropical paleoclimate records display substantial precessional variability, much more so than polar records such as the EPICA and NGRIP ice cores (EPICA Community Members 2004; North Greenland Ice Core Project Members 2004). This recorded precessional variability can result from either a physical climate forcing, or from the
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
