Davide Zanchettin

We use hindcasts from a state-of-the-art decadal climate prediction system initialized between 1979 and 2017 to explore the predictability of the Antarctic dipole—that is, the seesaw between sea ice cover in the Weddell and Ross Seas, and discuss its implications for Antarctic sea ice predictability. Our results indicate low forecast skills for the Antarctic dipole in the first hindcast year, with a strong relaxation of March values toward the climatology contrasting with an overestimation of anomalies in September, which we interpret as being linked to a predominance of local drift processes over initialized large-scale dynamics. Forecast skills for the Antarctic dipole and total Antarctic sea ice extent are uncorrelated. Limited predictability of the Antarctic dipole is also found under preconditioning around strong warm and strong cold events of the El Niño-Southern Oscillation. Initialization timing and model drift are reported as potential explanations for the poor predictive skills identified.

The Mediterranean Sea is of great and manifold relevance for global oceanic circulation and climate: Mediterranean waters profoundly affect the salinity of the North Atlantic Ocean and hence the global ocean circulation. Ocean motions are forced fundamentally by the atmosphere. However, direct atmospheric forcing explains just a part of the observed Mediterranean circulation, for example, the former is not able to account for the observed north-south inclination of the sea level, one of the most prominent and persistent features of Mediterranean oceanography. This implies that a significant part of this circulation feature is caused by mechanisms that are all internal, “intrinsic” to the ocean. Yet, no effort has been made so far to disentangle intrinsic oceanic phenomena from atmospherically forced ones in the Mediterranean Sea. Here, we start filling this gap of knowledge. We demonstrate that a conspicuous part of the observed Mediterranean mean state and variability belongs to a skeleton captured for the first time by a multi-centennial ocean simulation without atmospheric forcing. This study paves the way to the identification and comprehension of further observed mean patterns and low-frequency fluctuations in the Mediterranean Sea as the result of intrinsic oceanic processes rather than by a direct effect of the atmospheric forcing and could be extended to other basins where geometry and hydrological structure significantly contribute to shaping the local dynamics.