[1] The stability of the glacial thermohaline circulation (THC) with respect to North Atlantic freshwater input is examined using a global ocean general circulation model. It is found that the quasi-equilibrium hysteresis behaviour is much less pronounced under glacial conditions than under present-day conditions, and the existence of multiple equilibria requires an anomalous freshwater inflow. The results may help to assess the effect of iceberg invasions and meltwater events, suggesting that the THC is prone to instability during a deglaciation phase when the Atlantic meridional overturning is weakened. Under full glacial conditions, however, the THC is mono-stable and even extreme freshwater pulses are unable to exert a persistentn effect on the conveyor. INDEX TERMS: 4255

Five initialization and ensemble generation methods are investigated with respect to their impact on the prediction skill of the German decadal prediction system "Mittelfristige Klimaprognose" (MiKlip). Among the tested methods, three tackle aspects of model-consistent initialization using the ensemble Kalman filter, the filtered anomaly initialization, and the initialization method by partially coupled spin-up (MODINI). The remaining two methods alter the ensemble generation: the ensemble dispersion filter corrects each ensemble member with the ensemble mean during model integration. And the bred vectors perturb the climate state using the fastest growing modes. The new methods are compared against the latest MiKlip system in the low-resolution configuration (Preop-LR), which uses lagging the climate state by a few days for ensemble generation and nudging toward ocean and atmosphere reanalyses for initialization. Results show that the tested methods provide an added value for the prediction skill as compared to Preop-LR in that they improve prediction skill over the eastern and central Pacific and different regions in the North Atlantic Ocean. In this respect, the ensemble Kalman filter and filtered anomaly initialization show the most distinct improvements over Preop-LR for surface temperatures and upper ocean heat content, followed by the bred vectors, the ensemble dispersion filter, and MODINI. However, no single method exists that is superior to the others with respect to all metrics considered. In particular, all methods affect the Atlantic Meridional Overturning Circulation in different ways, both with respect to the basin-wide long-term mean and variability and with respect to the temporal evolution at the 26 • N latitude.

During the Last Glacial Maximum, the Earth's orography and oceanic heat transport contribute to a cooling in the North Atlantic. By using an atmospheric general circulation model of intermediate complexity, we investigate the sensitivity of the atmospheric temperature and circulation during glacial climate, focussing on the impact of the orography and different oceanic heat transports. The results show a strong dependence of the glacial Northern Hemisphere circulation pattern to the changed orography. The blocking effect of the elevated orography due to the Laurentide Ice Sheet over the North American continent forced a deflection of westerlies, their enhancement and a southward displacement over the Atlantic. Independently, the glacial climate is influenced by the oceanic heat transport. The reduced oceanic heat transport on the glacial climate shows a 20-40% contribution for the total cooling relative to the present-day climate in the North Atlantic and polar regions. Finally, we find that the altered orography in the Northern Hemisphere and different oceanic heat transports result in a changed hydrological cycle, a reduction of the Hadley circulation and a southward shift of the Intertropical Convergence Zone in the boreal winter during glacial times. r

Different reconstructions of glacial sea-surface temperatures (SST) are used to force a hybrid coupled atmosphere-ocean model. The resulting glacial states differ in global salinity and temperature distributions, and consequently in the strength of the thermohaline circulation. Stability analysis of the Atlantic Ocean circulation, by means of freshwater-flux hysteresis maps, reveals mono-stability for each glacial background state, which appears to be a robust feature of the glacial ocean. We show that this behaviour is directly linked to the hydrological cycle. A monotonic relation between the freshwater input necessary for reaching the off-mode and the hydrological budget in the Atlantic catchment area, accounts for the sensitivity of the ocean's circulation. The most sensitive part of the hydrological balance appears to be in the tropical and subtropical regions suggesting that the 'Achilles heel' of the global conveyor belt circulation is not restricted to the northern North Atlantic where convection occurs.

Different sea surface temperature (SST) reconstructions for the Last Glacial Maximum are applied to a hybrid-coupled climate model. The resulting oceanic states are perturbed by North Atlantic meltwater inputs in order to simulate the effect of Heinrich Events on the Atlantic thermohaline circulation (THC) and SST. The experiments show that both the Atlantic SST signature of the meltwater event and the time span of THC recovery strongly depend on the climatic background state. Data-model comparison reveals that the overall spatial signature of SST anomalies is captured much better in the glacial meltwater experiments than in an analogous experiment under present-day conditions. In particular, a breakdown of the modern THC would induce a much stronger temperature drop in high northern latitudes than did Heinrich Events during the ice age. Moreover, our results suggest that the present-day circulation can settle into a stable 'off' mode, whereas the glacial THC was mono-stable. Mono-stability may serve as an explanation for the recovery of the THC after Heinrich Event shutdowns during the Last Glaciation. r