Modelling the early-Holocene Laurentide Ice Sheet collapse and abrupt climate change: implications for the 8.2 ka event

Recent research suggested that the deglaciation of an ice saddle connecting three ice domes around Hudson Bay ˜8.5 ka produced a large meltwater pulse. The resulting freshwater input to the North Atlantic was proposed as having caused the most pronounced climate change event of the Holocene, the 8.2 ka event. However, modelling experiments focussing on this saddle collapse meltwater and its climatic impact have not yet been carried out. This thesis aims to establish whether such a meltwater pulse could have forced the 8.2 ka event, and if so, to better constrain the pulse through climate and ice sheet modelling. A series of HadCM3 general circulation model -simulations was performed using idealised freshwater forcing scenarios designed to represent the centennial-length saddle collapse meltwater flux. The simulations demonstrated that the saddle collapse meltwaterwas likely the primary cause of the 8.2 ka event. An appropriate model setup for simulating early-Holocene Laurentide Ice Sheet evolution was then developed using the BISICLES ice sheet model, and an ensemble of simulations of the period 10–7.5 ka was run. An ice saddle collapse is simulated as part of the deglaciation, and the resulting meltwater pulse is in agreement with the timing of North Atlantic surface freshening signals, but is longer and less pronounced than the forcing used in the HadCM3 scenarios that best matched the climate-proxy data. The findings suggest that the BISICLES model setup simulates a dynamically realistic meltwater pulse, but there is a mismatch between the simulated pulse and the forcing necessary for reproducing the 8.2 ka event with HadCM3. Future work should further develop the BISICLES model setup as outlined in the thesis in order to refine the constraints of the meltwater pulse. This could allow for using the 8.2 ka event for assessing the sensitivity of general circulation models to ocean circulation perturbations

Radiation budget of Arctic sea ice during CHINARE2010 -expedition

In this thesis I study the radiation balance and heat budget of a multiyear sea ice floe drifting in the central Arctic ocean. The objectives of the study were to quantify the vertical partitioning of shortwave- and longwave radiation and to quantify the different components of the heat budget of the floe in question, both inside and at its interfaces. The measurements were set up at 88 26.6N, 176 59.88W on 8th of August and carried out for ten days. The measurements were made as a part of the fourth Chinese National Arctic Expedition CHINARE2010. The measurement setup consisted of a net radiometer, four PAR-sensors, a pyrano-albedometer, three spectral radiometers, daily snow pit measurements, weather observations and six ice corings. With the data from these studies I was able to quantify the rate of melting and fluxes of heat both at the surface and at the bottom of the ice. The data allowed for examining the fraction of transmitted and conducted heat but were insufficient for properly quantifying the internal changes and spectral composition of the shortwave radiation at different depths. The surface was observed to be losing heat mainly in the longwave part of the spectrum. The average net radiation on top of the ice on wavelengths between 200 nanometers and 100 micrometers over the period was -25.0 Watts per square meter. The heat fluxes of the shortwave and longwave radiations were of opposite directions and the negative heat flux of the longwave radiation dominated until a distinct change in the radiative conditions on 17th of August. For the remainder of the period these heat fluxes nearly balanced each other and the average net radiation was -2.1 Watts per square meter. The latent and sensible heat fluxes were observed to have a minor role in the surface heat budget with averages of -1.5 Watts per square meter and -0.03 Watts per square meter respectively. The ice was observed to melt primarily at the bottom at a rate of 0.5 cm per day driven by the input of heat from the underlying ocean. Melting at the surface was not apparent until before the last two days of studies, when the upper layer of the snow cover melted. The changes in sea ice and snow cover were visually observed to exhibit significant spatial variability even on a single floe