A document by Erika Henell. Click on the document to view its contents.

For better projections of sea level rise, two things are needed: an improved understanding of the contributing processes and their accurate representation in climate models. A major process is basal melting of ice shelves and glacier tongues by the ocean, which reduces ice sheet stability and increases ice discharge into the ocean. We study marine melting of Greenland’s largest floating ice tongue, the 79° North Glacier, using a high-resolution, 2D-vertical ocean model. While our fjord model is idealized, the results agree with observations of the meltrate and the overturning strength. Our setup is the first application of adaptive vertical coordinates to an ice cavity. Their stratification-zooming allows a vertical resolution finer than 1 m in the entrainment layer of the meltwater plume, which is important for the plume development. In a sensitivity study, we show that the buoyant plume at the ice–ocean interface is responsible for the bulk of basal melting. The melting almost stops when the plume has reached neutral buoyancy. There, the plume detaches from the ice tongue and transports meltwater out of the fjord. The detachment depth depends primarily on the ambient ocean stratification. Our results contribute to the understanding of ice–ocean interactions in glacier cavities. Furthermore, we suggest that our modeling approach with stratification-zooming coordinates will improve the representation of these interactions in global ocean models. Finally, our idealized model topography and forcing are close to a real fjord and completely defined analytically, making the setup an interesting reference case for future model developments.

In the tropical ocean, diurnal heating and the formation of atmospheric convection cells associated with local precipitation events, cold pools and wind bursts, have been shown to impact air-sea exchange and the structure of the ocean surface layer. Here, we use a high-resolution regional ocean model, forced by an atmospheric Large Eddy Simulation (LES) that explicitly resolves these processes in a realistic scenario in the tropical north-east Atlantic Ocean, to study their impact on the ocean surface layer and parameterized air-sea fluxes. We find that the oceanic heat loss is, unexpectedly, reduced in the presence of cold pools by on average 30 W m-2 due to the higher air humidity, weaker mean winds, and increased cloud cover. Our results also show that the total non-solar heat flux is dominated by the diurnal cycle of the trade winds, rather than by diurnal heating. In the ocean surface layer, local wind bursts, rain layers, and cloud shading induce a strong lateral variability in the strength and depth of Diurnal Warm Layers, questioning the local applicability of available bulk parameterizations. From a series of numerical tracer experiments, we identify a new shear-dispersion mechanism, induced by the Diurnal jet, that is reflected in an extreme anisotropy of horizontal dispersion with diffusivities of order 10-100 m2 s-1.

River plumes play an essential role in the transport of terrestrially derived materials (like nutrients, sediments, pollutants, etc.) into the coastal ocean. Quantifying the cross-shore transport in river plumes can help to better understand the contribution of river-borne substances to marine biogeochemical cycles and to parameterize these processes in global ocean models which are usually too coarse to resolve individual rivers. It is known that besides external factors (like runoff, latitude, wind, and tides), also internal estuarine processes like salt mixing affect the exchange flow between an estuary and the coastal ocean. A theoretical framework to separate the plume and the estuary mixing in isohaline coordinates is presented. An idealized coastal ocean model setup resolving the whole plume-estuary continuum is used to validate the theoretical relation and to study the link between the estuarine pre-conditioning and the cross-shore export of river water under different forcing scenarios. It is found that the most effective cross-shore transport of river water happens under moderately upwelling favorable wind conditions and weak tidal forcing. This scenario is characterized by relatively small estuarine mixing, strong stratification, and little interaction between the surface and bottom boundary layers such that a thin layer of buoyant river water can extend far into the ocean. We conclude that reduced estuarine mixing is indicative of an enhanced accumulation of fresh water near the shore, but is not directly related to the cross-shore transport in river plumes.

Realistically approximating the basal melting of ice shelves is critical for reliable climate model projections and the process representations in ice-ocean interaction. In this regard, extensive research attributes the massive thinning of vulnerable ice shelves to basal melting enhancement driven by ocean water warming, focusing mainly on oceanic warm water intrusion into the sub-shelf basins. However, climate models mainly underestimated the impacts of probable small-scale processes at the ice-ocean interface on basal melting by using smooth ice base topographies. This paper provides new insights into how small-scale features on the ice-ocean interface contribute to basal melting enhancement and spatial distribution. We developed a time-dependent, two-dimensional ice-shelf plume model as an optimal tool that allows a high-resolution representation of basal topography and with the unique ability to provide valuable information from the mixed boundary layer between ocean and ice shelves. In an exemplary case study for the floating ice tongue of the 79◦ North Glacier, systematic sensitive analyses were performed with the developed model. Our results show that the sub-km-scale basal channels with realistic dimensions increase the mean basal melt rate and generate extreme and sizeable lateral variability of melting at the grounding line. This mechanism is not reproducible with the tuning of drag coefficient. Besides, it reveals that the subglacial discharge in the channels has contradicting effects of reducing the melt rate by refreshing the sea water and increasing the freezing point while increasing the melt rate due to high water speed. However, the latter was dominant in our experiments.