Objective extraction and analysis of statistical features of Dansgaard–Oeschger events

Johannes Lohmann,Peter D Ditlevsen

doi:10.5194/cp-15-1771-2019

Abstract

Abstract. The strongest mode of centennial to millennial climate variability in the paleoclimatic record is represented by Dansgaard–Oeschger (DO) cycles. Despite decades of research, their dynamics and physical mechanisms remain poorly understood. Valuable insights can be obtained by studying high-resolution Greenland ice core proxies, such as the NGRIP δ18O record. However, conventional statistical analysis is complicated by the high noise level, the cause of which is partly due to glaciological effects unrelated to climate and which is furthermore changing over time. We remove the high-frequency noise and extract the most robust features of the DO cycles, such as rapid warming and interstadial cooling rates, by fitting a consistent piecewise linear model to Greenland ice core records. With statistical hypothesis tests we aim to obtain an empirical understanding of what controls the amplitudes and durations of the DO cycles. To this end, we investigate distributions and correlations between different features, as well as modulations in time by external climate factors, such as CO2 and insolation. Our analysis suggests different mechanisms underlying warming and cooling transitions due to contrasting distributions and external influences of the stadial and interstadial durations, as well as the fact that the interstadial durations can be predicted to some degree by linear cooling rates already shortly after interstadial onset.

Highlights

Different physical mechanism(s) underlying Dansgaard– Oeschger (DO) events have been proposed in the literature
Assuming the sawtooth shape of the events, we develop an algorithm for fitting the sawtooth shape to the entire NGRIP δ18O record of the last glacial, similar to ramp-fitting a jump in a noisy record
This work presents a statistical analysis of DO event features based on best-fit parameters of a piecewise linear waveform to the NGRIP δ18O record

Summary

Introduction

Different physical mechanism(s) underlying Dansgaard– Oeschger (DO) events have been proposed in the literature Most of these are characterized by changes between different modes of operation of the Atlantic Meridional Overturning Circulation (AMOC) that accompany the warm and cold phases of a DO cycle. On the other hand, unforced millennial-scale oscillations involving the AMOC have been reported in comprehensive climate models (Vettoretti and Peltier, 2016; Klockmann et al, 2018) In these oscillations, sea ice variability in ocean convection areas plays an important role, which has been proposed previously (Li et al, 2010; Dokken et al, 2013; Petersen et al, 2013) and is supported by recent proxy records (Sadatzki et al, 2019). Another scenario underlying DO cycles might be spontaneous climate transitions due to extremes in the chaotic atmospheric dynamics (Drijfhout et al, 2013; Kleppin et al, 2015)

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