Kerim H. Nisancioglu

Sea Surface Temperature of the mid-Piacenzian Ocean: A Data-Model Comparison

The mid-Piacenzian climate represents the most geologically recent interval of long-term average warmth relative to the last million years, and shares similarities with the climate projected for the end of the 21st century. As such, it represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Here, we present the first systematic comparison of Pliocene sea surface temperature (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) with the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional and dynamic situations where there is discord between the palaeoenvironmental reconstruction and the climate model simulations. These differences have led to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction.

Increased ventilation of Antarctic deep water during the warm mid-Pliocene

The mid-Pliocene warm period is a recent warm geological period that shares similarities with predictions of future climate. It is generally held the mid-Pliocene Atlantic Meridional Overturning Circulation must have been stronger, to explain a weak Atlantic meridional δ13C gradient and large northern high-latitude warming. However, climate models do not simulate such stronger Atlantic Meridional Overturning Circulation, when forced with mid-Pliocene boundary conditions. Proxy reconstructions allow for an alternative scenario that the weak δ13C gradient can be explained by increased ventilation and reduced stratification in the Southern Ocean. Here this alternative scenario is supported by simulations with the Norwegian Earth System Model (NorESM-L), which simulate an intensified and slightly poleward shifted wind field off Antarctica, giving enhanced ventilation and reduced stratification in the Southern Ocean. Our findings challenge the prevailing theory and show how increased Southern Ocean ventilation can reconcile existing model-data discrepancies about Atlantic Meridional Overturning Circulation while explaining fundamental ocean features.

Palaeoclimatology: fresh angle on the polar seesaw.

During the last glacial period, climatic variation in the Northern and Southern Hemispheres was evidently linked. Modelling work points to freshwater discharge into the North Atlantic as a driving factor.

Impact of Fjord Geometry on Grounding Line Stability

Recent and past retreat of marine-terminating glaciers are broadly consistent with observed ocean warming, yet responses vary significantly within regions experiencing similar ocean conditions. We assess how fjord geometry modulates glacier response to a regional ocean warming on decadal to millennial time scales, by using an idealised, numerical model of fast-flowing glaciers including a crevasse-depth calving criterion. Our simulations show that, given identical climate forcing, grounding line responses can differ by tens of kilometres due to variations in channel width. We identify fjord mouths and embayments as vulnerable geometries, showing that glaciers in these fjords are prone to rapid, irreversible retreat, independent of the presence of a fjord sill. This irreversible retreat has relevance for the potential future recovery of marine ice sheets, if the current anthropogenic warming is reduced, or reversed, as well as for the response of marine ice sheets to past climate states; including the warm Bolling-Allerod interstadial, the Younger Dryas cold reversal and the Little Ice Age.

Non-linear retreat of Jakobshavn Isbræ since the Little Ice Age controlled by geometry

Rapid acceleration and retreat of Greenland’s marine-terminating glaciers during the last two decades have initiated questions on the trigger and processes governing observed changes. Destabilization of these glaciers coincides with atmosphere and ocean warming, which broadly has been used to explain the rapid changes. To assess the relative role of external forcing versus fjord geometry, we investigate the retreat of Jakobshavn Isbræ in West Greenland, where margin positions exist since the Little Ice Age maximum in 1850. We use a one-dimensional ice flow model and isolate geometric effects on the retreat 5 using a linear increase in external forcing. We find that the observed retreat of 43 km from 1850 until 2014 can only be simulated when multiple forcing parameters— such as hydrofracturing, submarine melt and frontal buttressing by sea ice—are changed simultaneously. Surface mass balance, in contrast, has a negligible effect. While changing external forcing initiates retreat, fjord geometry controls the retreat pattern. Basal and lateral topography govern shifts from temporary stabilization to rapid retreat, resulting in a highly non-linear glacier 10 response. For example, we simulate a disintegration of a 15 km long floating tongue within one model year, which dislodges the grounding line onto the next pinning point. The retreat pattern loses complexity and becomes linear when we artificially straighten the glacier walls and bed, confirming the topographic controls. For real complex fjord systems such as Jakobshavn Isbræ, geometric pinning points predetermine grounding line stabilization and may therefore be used as a proxy for moraine build-up. Also, we find that after decades of stability and with constant 15 external forcing, grounding lines may retreat rapidly without any trigger. This means that past changes may precondition marine-terminating glaciers to reach tipping-points, and that retreat can occur without additional climate warming. Present-day changes and future projections can therefore not be viewed in isolation of historic retreat.

The interaction between sea ice and salinity-dominated ocean circulation: implications for halocline stability and rapid changes of sea ice cover

Changes in the sea ice cover of the Nordic Seas have been proposed to play a key role for the dramatic temperature excursions associated with the Dansgaard–Oeschger events during the last glacial. In this study, we develop a simple conceptual model to examine how interactions between sea ice and oceanic heat and freshwater transports affect the stability of an upper-ocean halocline in a semi-enclosed basin. The model represents a sea ice covered and salinity stratified Nordic Seas, and consists of a sea ice component and a two-layer ocean. The sea ice thickness depends on the atmospheric energy fluxes as well as the ocean heat flux. We introduce a thickness-dependent sea ice export. Whether sea ice stabilizes or destabilizes against a freshwater perturbation is shown to depend on the representation of the diapycnal flow. In a system where the diapycnal flow increases with density differences, the sea ice acts as a positive feedback on a freshwater perturbation. If the diapycnal flow decreases with density differences, the sea ice acts as a negative feedback. However, both representations lead to a circulation that breaks down when the freshwater input at the surface is small. As a consequence, we get rapid changes in sea ice. In addition to low freshwater forcing, increasing deep-ocean temperatures promote instability and the disappearance of sea ice. Generally, the unstable state is reached before the vertical density difference disappears, and the temperature of the deep ocean do not need to increase as much as previously thought to provoke abrupt changes in sea ice.

Large Changes in Sea Ice Triggered by Small Changes in Atlantic Water Temperature

AbstractThe sensitivity of sea ice to the temperature of inflowing Atlantic water across the Greenland–Scotland Ridge is investigated using an eddy-resolving configuration of the Massachusetts Inst...