A.S. von der Heydt

Making sense of palaeoclimate sensitivity

Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in K W−1 m2) of 0.3–1.9 or 0.6–1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2–4.8 K per doubling of atmospheric CO2, which agrees with IPCC estimates.

North Atlantic Multidecadal Climate Variability: An Investigation of Dominant Time Scales and Processes

The issue of multidecadal variability in the North Atlantic has been an important topic of late. It is clear that there are multidecadal variations in several climate variables in the North Atlantic, such as sea surface temperature and sea level height. The details of this variability, in particular the dominant patterns and time scales, are confusing from both an observational as well as a theoretical point of view. After analyzing results from observational datasets and a 500-yr simulation of an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate model, two dominant time scales (20‐30 and 50‐70 yr) of multidecadal variability in the North Atlantic are proposed. The 20‐30-yr variability is characterized by the westward propagation of subsurface temperature anomalies. The hypothesis is that the 20‐30-yr variability is caused by internal variability of the Atlantic Meridional Overturning Circulation (MOC) while the 50‐70-yr variability is related to atmospheric forcing over the Atlantic Ocean and exchange processes between the Atlantic and Arctic Oceans.

A stochastic dynamical systems view of the Atlantic Multidecadal Oscillation

We provide a dynamical systems framework to understand the Atlantic Multidecadal Oscillation and show that this framework is in many ways similar to that of the El Niño/Southern Oscillation. A so-called minimal primitive equation model is used to represent the Atlantic Ocean circulation. Within this minimal model, we identify a normal mode of multidecadal variability that can destabilize the background climate state through a Hopf bifurcation. Next, we argue that noise is setting the amplitude of the sea surface temperature variability associated with this normal mode. The results provide support that a stochastic Hopf bifurcation is involved in the multidecadal variability as observed in the North Atlantic.

On the state dependency of fast feedback processes in (paleo) climate sensitivity

Paleo data have been frequently used to determine the equilibrium (Charney) climate sensitivity Sa, and—if slow feedback processes (e.g., land-ice albedo) are adequately taken into account—they indicate a similar range as estimates based on instrumental data and climate model results. Many studies assume the (fast) feedback processes to be independent of the background climate state, e.g., equally strong during warm and cold periods. Here we assess the dependency of the fast feedback processes on the background climate state using data of the last 800 kyr and a box model of the climate system for interpretation. Applying a new method to account for background state dependency, we find Sa=0.61±0.07 K (W m−2)−1(±1σ) using a reconstruction of Last Glacial Maximum (LGM) cooling of −4.0 K and significantly lower climate sensitivity during glacial climates. Due to uncertainties in reconstructing the LGM temperature anomaly, Sa is estimated in the range Sa = 0.54–0.95 K (W m−2)−1.

Coherent tropical Indo-Pacific interannual climate variability

AbstractA multichannel singular spectrum analysis (MSSA) applied simultaneously to tropical sea surface temperature (SST), zonal wind, and burstiness (zonal wind variability) reveals three significant oscillatory modes. They all show a strong ENSO signal in the eastern Pacific Ocean (PO) but also a substantial SST signal in the western Indian Ocean (IO). A correlation-based analysis shows that the western IO signal contains linearly independent information on ENSO. Of the three Indo-Pacific ENSO modes of the MSSA, one resembles a central Pacific (CP) El Nino, while the others represent eastern Pacific (EP) El Ninos, which either start in the central Pacific and grow eastward (EPe) or start near Peru and grow westward (EPw). A composite analysis shows that EPw El Ninos are preceded by cooling in the western IO about 15 months earlier. Two mechanisms are discussed by which the western IO might influence ENSO. In the atmospheric bridge mechanism, subsidence over the cool western IO in autumn (year 0) leads t...

Does Net E − P Set a Preference for North Atlantic Sinking?

AbstractThe present-day global meridional overturning circulation (MOC) with formation of North Atlantic Deep Water (NADW) and the absence of a deep-water formation in the North Pacific is often considered to be caused by the fact that the North Pacific basin is a net precipitative, while the North Atlantic is a net evaporative basin. In this paper, the authors study the effect of asymmetries in continent geometry and freshwater fluxes on the MOC both in an idealized two-dimensional model and in a global ocean model. This study approaches the problem from a multiple equilibria perspective, where asymmetries in external factors constrain the existence of steady MOC patterns. Both this multiple equilibria perspective and the fact that a realistic global geometry is used add new aspects to the problem. In the global model, it is shown that the Atlantic forced by net precipitation can have a meridional overturning circulation with northern sinking and a sea surface salinity that resembles the present-day sali...

Chaotic and non-chaotic response to quasiperiodic forcing: limits to predictability of ice ages paced by Milankovitch forcing

It is well known that periodic forcing of a nonlinear system, even of a 2D autonomous system, can produce chaotic responses with sensitive dependence on initial conditions if the forcing induces sufficient stretching and folding of the phase space. Quasiperiodic forcing can similarly produce chaotic responses, where the transition to chaos on changing a parameter can bring the system into regions of strange non-chaotic behaviour. Although it is generally acknowledged that the timings of Pleistocene ice ages are at least partly due to Milankovitch forcing (which may be approximated as quasiperiodic, with energy concentrated near a small number of frequencies), the precise details of what can be inferred about the timings of glaciations and deglaciations from the forcing are still unclear. In this article, we perform a quantitative comparison of the response of several low-order nonlinear conceptual models for these ice ages to various types of quasiperiodic forcing. By computing largest Lyapunov exponents and mean periods, we demonstrate that many models can have a chaotic response to quasiperiodic forcing for a range of forcing amplitudes, even though some of the simplest conceptual models do not. These results suggest that pacing of ice ages to forcing may have only limited determinism.