Latitudinal insolation gradients throughout the Holocene II: High frequency variations

Abstract

In a first paper, we presented an in-depth discussion and a computational method (free of the calendar problem) to reckon any kind of latitudinal insolation gradients, LIGs, throughout the Holocene and up to CE 3000. One of the main insights from this exact definition of LIGs is that, unlike what is argued in prior works, during the Holocene, a general classification of LIGs in terms of obliquity signal or climatic precession variations is much more complex, even in summertime. It is especially evident in the Southern Hemisphere, where summer half-year LIGs evolve under the relatively stronger modulation by climatic precession. In this work, the short-term periodicities (high frequency variations in time domain) of all these LIGs are studied by means of the multitaper spectral analysis. The goal is to get more insights on the competing effects between obliquity and climatic precession during the Holocene, and to know how the relative intensity of the obliquity's periodicities is when compared to the main spectral peaks produced by the climatic precession effects at short time scales (e.g., from annual to decadal bands). Our main result is the clarification of the role of the 18.63 yr periodicity originated in the well known retrograding cycle of the Moons’ orbit. We found that this lunar cycle is always present at a 99% significance level in all the analysed LIGs, even in winter with solar cycle included. The conceptual explanation of this persistence is based on the fact that all accurate short-term orbital forcing calculation must include the lunar nodal cycle even in climatic precession variations. We propose to use more specific definitions when short-term orbital variations are taken into account in describing Milanković forcing.