ALBERT

Description: This paper investigates the mechanism which generates decadal modulations in the amplitude of the El Niño-Southern Oscillation phenomenon (ENSO). Our analysis is based on a multicentury present-day climate simulation performed with an ENSO-resolving Coupled General Circulation Model (CGCM). In consistency with observations, it is found that ENSO variance undergoes changes with a time scale of about 10–20 years. This decadal beat is closely linked to the second dominant pattern of tropical (sub)surface temperature variability. The dipole-like characteristic of this mode is generated mainly by the interplay of horizontal, vertical advection and mixing. We suggest a nonlinear mechanism, which is capable of generating decadal tropical climate anomalies as well as decadal ENSO amplitude modulations (DEAMs) without invoking extratropical dynamics. This mechanism is based on the idea of homoclinic orbits. This new paradigm is validated using a low-dimensional ENSO model that is derived empirically from the CGCM simulation.

Description: Numerical modeling shows great potential as a method for investigating and predicting the future development of ice shelves in a warming climate. The quality of ice shelf-ocean models is continuously improving but some limitations remain. For models using a terrain-following vertical coordinate, one such limitation is the enforcement of a minimum water-column thickness beneath ice shelves by adjustment of bottom topography. How this local distortion of bathymetry from reality affects modeled melt rates and cavity circulation is unknown so far. To quantify this effect, simulations with the Finite Element Sea ice–ice shelf–Ocean Model (FESOM) were executed on four different grids with minimum water-column thicknesses of 20 m, 50 m, 100 m and 200 m. While we use a global model grid, modifications of bathymetry are applied only to Filchner–Ronne Ice Shelf. We show that the choice of minimum water-column thickness does not affect the total basal melt rate of this cold-water ice shelf but does in fact impact the distribution of melt rates with significant differences between experiments in the magnitude of melting near the grounding lines.

Description: A set of published paleoclimate proxy records from the northern hemisphere, capturing different climate processes, is used to study glacial–interglacial differences in climate variability at centennial-to-millennial timescales during the past fifty thousand years. These proxy records reveal the existence of distinct oscillatory modes of the climate system. Glacial climate variability is dominated by a single mode, the Dansgaard–Oeschger cycles, composed of stadial and interstadial states. This glacial mode results in well-expressed covariations of the proxies, which are paced by a fundamental 1470-year signal. In contrast, there is no compelling evidence for a dominant and persistent centennial-to-millennial climate cycle during the Holocene. Interglacial climate variations seem to covary less pronounced than those of the last glacial period, suggesting the simultaneous activity of independent climate modes, each characterized by its own natural periods, between approximately 400–3000 years. A conceptual model is introduced to interpret this contrast in covariation at glacial–interglacial timescales. It is assumed that different climate modes can be represented by relaxation oscillators with different natural periods in the centennial-to-millennial band. Interactions among such oscillators may lead to a phase-synchronization and the development of a new climate mode with a joint frequency. We suggest that the coupled state with its synchronized dynamics resembles a glacial whereas the decoupled state represents an interglacial with its reduced covariations of climate fluctuations. The synchronization greatly enhances the frequency stability of the coupled system, and has the potential to reconcile the stability of the glacial 1470-year pacing cycle with an origin within the Earth's climate system.