Influence of changes in topography on Northern Hemisphere atmospheric dynamics during interglacial and glacial times

Merz, Niklaus (2014). Influence of changes in topography on Northern Hemisphere atmospheric dynamics during interglacial and glacial times (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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During the Quaternary Period, i.e., the last 2.6 Ma, the climate on Earth has alternated between warm phases, so-called interglacials, and cold intervals, termed glacials. The resulting glacial–interglacial cycles are observed in many climate archives such as ocean sediments and Antarctic ice cores and express themselves in terms of, e.g., atmospheric and oceanic temperatures, atmospheric composition, and global sea level. Despite the impressive amount of data gathered from paleoclimate reconstructions, profound knowledge about the evolution of atmospheric dynamics over these time scales is still rare as many features of the atmospheric circulation are hardly (if at all) possible to be reconstructed from proxy data. Therefore, estimating the sensitivity of the atmospheric circulation to glacial and interglacial boundary conditions is an important research topic for the climate modelling community.
One key element, which considerably varies between glacial and interglacial states, is the global distribution of land ice. During interglacials, ice sheets are mostly limited to Antarctica and Greenland whereas during glacials substantial parts of North America and Eurasia are covered by massive bodies of ice as well. The atmospheric circulation is found to sensitively react to these changes in ice sheet distribution, mainly because the atmospheric flow is perturbed by the presence of surface topography. However, up to now the impact of the ice sheet topography on atmospheric dynamics has primarily been investigated for present-day and last glacial maximum (LGM) conditions. In this thesis, we extend this analysis to selected periods of the most recent interglacial–glacial cycle using a comprehensive climate model. For each period, we generate a set of sensitivity simulations to estimate the relevance of the ice sheet topography for the atmospheric circulation of that time. Consequently, this setup allows a thorough assessment of the stability of the atmospheric circulation with respect to interglacial–glacial differences but also among interglacial and glacial periods themselves.
The research questions tackled in the individual parts of this thesis are largely motivated by temporal and spatial limitations in observations and reconstructions. Using appropriate climate model simulations, the dynamical understanding of observed signals can be improved and key assumptions can be tested. In the first study, a climate model is used to reassess the observed link between annual mean Greenland accumulation and atmospheric circulation patterns for past climate conditions. The stability in the accumulation–circulation relationship during different climate states is of importance for proving the utility of accumulation records obtained from Greenland ice cores to reconstruct Northern Hemisphere (NH) circulation variability over the past millenia. However, the analysis reveals that the relationship between accumulation variability and large-scale circulation undergoes a distinct seasonal cycle. Therefore, atmospheric patterns based on annual mean accumulation are of limited value. Within a season, local Greenland accumulation variability is indeed linked to consistent circulation patterns, observed for both interglacial and glacial conditions. Hence, it would be possible to deduce reliable reconstructions of seasonal atmospheric variability for the past millenia if accumulation or precipitation records were available resolving single seasons.
A second study investigates processes explaining the remarkable warming during the Eemian interglacial recorded in a recently drilled Greenland ice core. The amplitude of the warming determined for Greenland exceeds other Eemian temperature records of the NH high latitudes suggesting that local feedback processes play a crucial role. One suggested mechanism involves the sensitivity of Greenland’s surface climate to a concurrent retreat of the Greenland ice sheet (GrIS). Making use of a corresponding set of Eemian simulations, it is shown that changes in the GrIS topography indeed have a significant influence on Greenland surface temperatures. The dominant process involves changes in the surface energy balance, as turbulent heat fluxes depend on surface winds, which themselves are controlled to a large extent by the shape of the GrIS. In addition, a reduced GrIS leads to changes in surface albedo in deglaciated areas and regions experiencing snow melt, which eventually causes an increase in local summer temperatures. Overall, the ice sheet–atmosphere interactions can account for a substantial part of the Eemian warming in Greenland, however, the amplitude is highly dependent on the actual GrIS topography, which itself is not known with certainty. Nevertheless, it is shown that interglacial warming in Greenland (as recorded in its ice) can be caused by local topography-related processes and is not necessarily linked to large-scale climate variations.
Besides the control of Greenland temperatures, a third study demonstrates that a reduced Eemian GrIS also crucially affects the local hydrological cycle. The GrIS topography controls where moist air masses are orographically lifted and cause substantial precipitation. In contrast, the general moisture availability and the moisture transport associated with typical weather situations remain unchanged in all Eemian simulations. Eemian precipitation and accumulation records obtained from Greenland ice cores are therefore primarily influenced by the local topography and anomalous low/high precipitation unlikely reflects changes in the large-scale atmospheric circulation. Indeed, the simulated NH flow pattern during the Eemian closely resembles the present-day state and is largely independent of Greenland’s topography.
During glacial climate conditions, the atmospheric flow clearly differs from the interglacial state, in particular in response to the presence of massive continental ice sheets. A corresponding study investigates the influence of glacial and interglacial boundary conditions including different ice sheet topographies on the North Atlantic eddy-driven jet. The behavior of this jet stream is important as it largely determines Europe’s winter climate. Model simulations show that during interglacial times (i.e., the early Holocene and the Eemian) the mean structure of the eddy-driven jet as well as its modes of variability are rather stable. On the contrary, during glacial conditions the jet is strongly enhanced; its winter mean position is shifted southwards and its variability in terms of latitudinal position is reduced. The main control is exerted by the size of the Laurentide Ice Sheet (LIS). The LIS enhances stationary Rossby waves downstream, thus displacing the North Atlantic eddy-driven jet. Furthermore, the dominant role of the LIS height implies that the North Atlantic circulation largely varies within glacial periods as the size of the LIS undergoes substantial fluctuations on millennial time scales.
Finally, an additional chapter is dedicated to outline possible follow-up studies. This includes the analysis of further features of atmospheric dynamics within the simulations performed in the course of this thesis or, alternatively, the generation of additional simulations going beyond the ice sheet experiments, e.g., testing the role of anomalous Arctic sea ice for the Eemian warming. In the appendices, two more studies on glacial atmospheric circulation in the North Atlantic region are added as well as supplementary material on the technical aspects of performing ice sheet sensitivity experiments.

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