Globally clustered chimera states in delay--coupled populations

We have identified the existence of globally clustered chimera states in delay coupled oscillator populations and find that these states can breathe periodically, aperiodically and become unstable depending upon the value of coupling delay. We also find that the coupling delay induces frequency suppression in the desynchronized group. We provide numerical evidence and theoretical explanations for the above results and discuss possible applications of the observed phenomena.Comment: Accepted in Phys. Rev. E as a Rapid Communicatio

Is the wind-stress forcing essential for the meridional overturning circulation?

We use a global coupled atmosphere‐ocean sea‐ice model of intermediate complexity to demonstrate that wind‐forcing is a crucial element to sustain meridional overturning flow in the Atlantic. Neglecting wind‐stress in our multi‐century‐long simulations leads to a complete shutdown of the conveyor belt circulation. This result may have tremendous impacts for an assessment of the sensitivity of 2‐d climate models which typically do not capture wind‐driven gyres. It is argued that wind effects may be a key element in determining the fate and length of a collapsed THC state. Possible paleo implications will be discussed

Is the windstress essential for the global meridional overturning circulation

We use a global coupled atmosphere-ocean sea-ice model of intermediate complexity to demonstrate that wind-forcing is a crucial element to sustain meridional overturning flow in the Atlantic. Neglecting wind-stress in our multi-century-long simulations leads to a complete shutdown of the conveyor belt circulation. This result may have tremendous impacts for an assessment of the sensitivity of 2-d climate models which typically do not capture wind-driven gyres. It is argued that wind effects may be a key element in determining the fate and length of a collapsed THC state. Possible paleo implications will be discussed

Sensitivity of the Atlantic meridional overturning circulation to South Atlantic freshwater anomalies

The sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to changes in basin integrated net evaporation is highly dependent on the zonal salinity contrast at the southern border of the Atlantic. Biases in the freshwater budget strongly affect the stability of the AMOC in numerical models. The impact of these biases is investigated, by adding local anomaly patterns in the South Atlantic to the freshwater fluxes at the surface. These anomalies impact the freshwater and salt transport by the different components of the ocean circulation, in particular the basin-scale salt-advection feedback, completely changing the response of the AMOC to arbitrary perturbations. It is found that an appropriate dipole anomaly pattern at the southern border of the Atlantic Ocean can collapse the AMOC entirely even without a further hosing. The results suggest a new view on the stability of the AMOC, controlled by processes in the South Atlantic. <br/

The origin of the "European Medieval Warm Period"

Proxy records and results of a three dimensional climate model show that European summer temperatures roughly a millennium ago were comparable to those of the last 25 years of the 20th century, supporting the existence of a summer "Medieval Warm Period" in Europe. Those two relatively mild periods were separated by a rather cold era, often referred to as the "Little Ice Age". Our modelling results suggest that the warm summer conditions during the early second millennium compared to the climate background state of the 13th–18th century are due to a large extent to the long term cooling induced by changes in land-use in Europe. During the last 200 years, the effect of increasing greenhouse gas concentrations, which was partly levelled off by that of sulphate aerosols, has dominated the climate history over Europe in summer. This induces a clear warming during the last 200 years, allowing summer temperature during the last 25 years to reach back the values simulated for the early second millennium. Volcanic and solar forcing plays a weaker role in this comparison between the last 25 years of the 20th century and the early second millennium. Our hypothesis appears consistent with proxy records but modelling results have to be weighted against the existing uncertainties in the external forcing factors, in particular related to land-use changes, and against the uncertainty of the regional climate sensitivity. Evidence for winter is more equivocal than for summer. The forced response in the model displays a clear temperature maximum at the end of the 20th century. However, the uncertainties are too large to state that this period is the warmest of the past millennium in Europe during winter