Abrupt Climate Transitions in an unforced Integration of CESM (Community Earth System Model)

Abstract:

In a pre-industrial control simulation of CESM changes of Greenland temperature show an asymmetric pattern that is a slow decrease at centennial timescales and an abrupt increase at decadal timescales, resembling the pattern of Dansgaard-Oeschger-cycles. A strong influence of equatorial Pacific SST anomalies on the pressure state above the North Atlantic and hence the wind forcing of the Labrador Sea gyre is found. The cooling is triggered by a stochastic anomalous pressure pattern over the North Atlantic that weakens the wind forcing of the Labrador gyre, slowing down and advecting less warm and saline subtropical waters to high latitudes. Then sea ice growth commences in the Labrador Sea due to locally reduced warm water transport. The sea ice anomaly above the Labrador Sea enhances the anomalous atmospheric circulation and eventually causes a cold core anticyclonic high pressure anomaly to persist during the entire 300 years that the cold North Atlantic phase lasts. Furthermore the decreased salinity transport towards the Labrador Sea induces a positive feedback that weakens the gyre circulation further and causes a reduced deep water convection. This leads to a reduced Atlantic meridional overturning circulation of about 4 Sverdrup, and eventually a reduced northward heat transport. After a 30 year period of prevailing cold SST anomalies in the east Pacific the disturbed atmospheric circulation above the North Atlantic slowly disappears, the Labrador Sea gyre circulation and deep water convection resume, melting the additional sea ice away and eventually causing an abrupt increase of Greenland temperatures. First ocean-sea ice only simulations forced with the anomalous atmospheric fluxes extracted from the original simulation reproduce the climate transition and the changed ocean circulation , even if starting from background ocean conditions that differ from the original ones.