Abstract
Abstract. Palaeo-records from China demonstrate that the East Asian Summer Monsoon (EASM) is dominated by abrupt and large magnitude monsoon shifts on millennial timescales, switching between periods of high and weak monsoon rains. It has been hypothesized that over these timescales, the EASM exhibits two stable states with bifurcation-type tipping points between them. Here we test this hypothesis by looking for early warning signals of past bifurcations in speleothem δ18O records from Sanbao Cave and Hulu Cave, China, spanning the penultimate glacial cycle. We find that although there are increases in both autocorrelation and variance preceding some of the monsoon transitions during this period, it is only immediately prior to the abrupt monsoon shift at the penultimate deglaciation (Termination II) that statistically significant increases are detected. To supplement our data analysis, we produce and analyse multiple model simulations that we derive from these data. We find hysteresis behaviour in our model simulations with transitions directly forced by solar insolation. However, signals of critical slowing down, which occur on the approach to a bifurcation, are only detectable in the model simulations when the change in system stability is sufficiently slow to be detected by the sampling resolution of the data set. This raises the possibility that the early warning "alarms" were missed in the speleothem data over the period 224–150 kyr and it was only at the monsoon termination that the change in the system stability was sufficiently slow to detect early warning signals.
Highlights
The Asian Summer Monsoon directly influences over 60 % of the world’s population (Wu et al, 2012) and yet the drivers of past and future variability remain highly uncertain (Zickfeld et al, 2005; Levermann et al, 2009)
A minimum conceptual model of the East Asian Summer Monsoon developed by Zickfeld et al (2005), stripped down by Levermann et al (2009) and updated by Schewe et al (2012), shows a non-linear solution structure with thresholds for switching a monsoon system between “on” or “off” states that can be defined in terms of atmospheric humidity
To investigate further the dynamical origin of this bimodality we applied non-stationary potential analysis (Kwasniok, 2013, 2015). This showed a bi-stable structure to the East Asian Summer Monsoon (EASM) with hysteresis (Fig. 3b, c), suggesting that abrupt monsoon transitions may involve underlying bifurcations
Summary
The Asian Summer Monsoon directly influences over 60 % of the world’s population (Wu et al, 2012) and yet the drivers of past and future variability remain highly uncertain (Zickfeld et al, 2005; Levermann et al, 2009). Evidence from radiometrically dated East Asian speleothem records of past monsoon behaviour (Yuan et al, 2004) suggests that on millennial timescales, the EASM is driven by a 23 kyr precession cycle (Kutzbach, 1981; Wang et al, 2008), and influenced by feedbacks in sea surface temperatures and changing boundary conditions including Northern Hemisphere ice volume (An, 2000; Sun et al, 2015). 1.4) in comparison to the sinusoidal insolation forcing strongly implies that this response is nonlinear (Fig. 1); whilst Northern Hemisphere summer insolation (NHSI) follows a quasi-sinusoidal cycle, the δ18O profile in speleothems exhibits a step function, suggesting the presence of threshold behaviour in the monsoon system (Schewe et al, 2012).
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 call
