Predictability of Deformation Features in Arctic Sea Ice

Sea ice deformation localizes along Linear Kinematic Features (LKFs) that are relevant for the air/ocean/sea-ice interaction and for shipping andmarine operations. At high resolution (< 5km) viscous-plastic sea ice models start to resolve LKFs. Here, we study the short-range (up to 10 days) potential predictability of LKFs in Arctic sea ice using ensemble simulations of an ocean/sea-ice model with a grid point separation of 4.5 km. We analyze the sensitivity of predictability to idealized initial perturbations, mimicking the uncertainties in sea ice analyses, and to growing uncertainty of the atmospheric forcing caused by the chaotic nature of the atmosphere. The similarity between pairs of ensemble members is quantified by Pearson correlation and Modified Hausdorff Distance (MHD). In our perfect model experiments, the potential predictability of LKFs, based on the MHD, drops below 0.6 after 4 days in winter. We find that forcing uncertainty (due to limited atmospheric predictability) largely determines LKF predictability on the 10-day time scale, while uncertainties in the initial state impact the potential predictability only within the first 4 days

Antarctic sea ice decline delayed well into the 21st century in a high-resolution climate projection

Despite ongoing global warming and strong sea ice decline in the Arctic, the sea ice extent around the Antarctic continent has not declined during the satellite era since 1979. This is in stark contrast to existing climate models that tend to show a strong negative sea ice trend for the same period; hence the confidence in projected Antarctic sea-ice changes is considered to be low. In the years since 2016, there has been significantly lower Antarctic sea ice extent, which some consider a sign of imminent change; however, others have argued that sea ice extent is expected to regress to the weak decadal trend in the near future. In this presentation, we show results from climate change projections with a new climate model that allows the simulation of mesoscale eddies in dynamically active ocean regions in a computationally efficient way. We find that the high-resolution configuration (HR) favours periods of stable Antarctic sea ice extent in September as observed over the satellite era. Sea ice is not projected to decline well into the 21st century in the HR simulations, which is similar to the delaying effect of, e.g., added glacial melt water in recent studies. The HR ocean configurations simulate an ocean heat transport that responds differently to global warming and is more efficient at moderating the anthropogenic warming of the Southern Ocean. As a consequence, decrease of Antarctic sea ice extent is significantly delayed, in contrast to what existing coarser-resolution climate models predict. Other explanations why current models simulate a non-observed decline of Antarctic sea-ice have been put forward, including the choice of included sea ice physics and underestimated simulated trends in westerly winds. Our results provide an alternative mechanism that might be strong enough to explain the gap between modeled and observed trends alone

Multi-resolution climate modelling with the AWI Climate Model (AWI-CM)

The recently established AWI Climate Model (AWI-CM), a coupled configuration of the Finite Element Sea Ice-Ocean Model (FESOM) with the atmospheric model ECHAM6, uses a novel multi-resolution approach: Its ocean component builds on a finite element dynamical core supporting unstructured triangular surface grids, allowing to distribute the grid points in a flexible manner. This allows to concentrate resolution in dynamically important regions, with a continuous transition zone to the coarser resolution in other areas. The model is an ideal tool to study the influence of explicit resolution of smaller scales in dedicated experiments. The unique – spatially seamless – approach might also be of benefit when it comes to temporally seamless prediction, bridging the gap between numerical weather prediction and climate models. A first benchmark set-up of AWI-CM with moderate resolution in the atmosphere (T63) and 25km in key ocean areas, e.g. around the equator, achieved a similar overall simulation performance in a long control simulation compared to well-established CMIP5 models. In particular, the (isotropically) increased equatorial resolution considerably increased the realism of TIW activity and ENSO-related variability compared to standard resolutions. The potential of AWI-CM is further exploited within the EU project PRIMAVERA in the HighResMIP of CMIP6, where we plan to contribute simulations with eddy-resolving resolutions (1/12° or 9-10 km) in key areas of the global ocean, such as the Gulf Stream-North Atlantic Current region, the Agulhas retroflection zone, or the Arctic basin. First simulations show distinct improvements with respect to the development of deep temperature and salinity biases in the North Atlantic Ocean and an overall improvement of surface biases. At even higher resolutions of 4.5 km locally in the Arctic, linear kinematic features emerge in the simulated sea ice distribution with potentially strong impacts on air-sea fluxes in the coupled system. Although the tested set-ups are computationally very demanding (with numbers of grid points comparable to a regular 0.25° grid), the throughput is high at about 8 simulated years per day because of high scalability. In addition, we are about to finish the development of a finite volume version of the ocean model code (FESOM 2). It is already faster than the original FESOM version by a factor of two to three, which will further enlarge the set of computationally feasible applications

Delayed Antarctic sea-ice decline in high-resolution climate change simulations

Despite global warming and Arctic sea-ice loss, on average the Antarctic sea-ice extent has not declined since 1979 when satellite data became available. In contrast, climate model simulations tend to exhibit strong negative sea-ice trends for the same period. This Antarctic sea-ice paradox leads to low confidence in 21st-century sea-ice projections. Here we present multi-resolution climate change projections that account for Southern Ocean mesoscale eddies. The high-resolution configuration simulates stable September Antarctic sea-ice extent that is not projected to decline until the mid-21st century. We argue that one reason for this finding is a more realistic ocean circulation that increases the equatorward heat transport response to global warming. As a result, the ocean becomes more efficient at moderating the anthropogenic warming around Antarctica and hence at delaying sea-ice decline. Our study suggests that explicitly simulating Southern Ocean eddies is necessary for providing Antarctic sea-ice projections with higher confidence

IceBird 2018 summer Campaign - Sea ice thickness measurements with Polar 6 from Station Nord and Alert

Arctic sea ice extent and thickness have undergone dramatic changes in the past decades: Summer sea ice extent has declined at an annual rate of approximately 12.7 % per decade over the satellite record (1978 – present, [5]) and its mean thickness has decreased by 0.58 m +/- 0.07 m per decade over the period 2000 - 2012 [3]. The thinning of sea ice is accompanied by an increase of ice drift velocity [8], deformation [7] and a decrease of net ice growth rates. Climate model simulations indicate that ice extent and thickness will further decline through the 21st century in response to atmospheric greenhouse gas increases. However, the mass balance of Arctic sea ice is not only determined by changes in the energy balance of the coupled ice-ocean-atmosphere system but also by the increasing influence of dynamic effects. One aspect of the mass balance of Arctic sea ice are changes of ice volume export rates through Fram Strait and the decline of thick and old multi-year ice North of Ellesmere Island. Thickness surveys carried out north of Greenland and Fram Strait give insight into composition and properties of Arctic sea ice in general and how it changes over time. An extensive data set of ground-based and airborne electromagnetic ice thickness measurements were collected between 2001 and 2017 during several aircraft (PAMARCMIP, TIFAX) and Polarstern campaigns. The first aim of the IceBird 2018 summer campaign is to complement earlier measurements made north of Svalbard, Greenland and in Fram Strait. Sea ice thickness information will be used to examine the connection between thickness variability, ice age and source area. Together with satellite based information on sea ice motion, data will be used to quantify sea ice outflow through Fram Strait in summer. These estimates shall improve the understanding of interannual variability in summer sea ice outflow and complement existing winter volume flux calculations. A second objective is to extent sea ice thickness measurements to the Lincoln Sea where we will study thinning of sea ice due to reduction of old multi-year ice in this area. Like the measurements planned over the Fram Strait area, the surveys are a continuation of earlier aircraft campaigns made north of Alert and shall improve understanding of ice mass balance changes in the Arctic

Ocean Model Formulation Influences Transient Climate Response

The transient climate response (TCR) is 20% higher in the Alfred Wegener Institute Climate Model (AWI-CM) compared to the Max Planck Institute Earth System Model (MPI-ESM) whereas the equilibrium climate sensitivity (ECS) is by up to 10% higher in AWI-CM. These results are largely independent of the two considered model resolutions for each model. The two coupled CMIP6 models share the same atmosphere-land component ECHAM6.3 developed at the Max Planck Institute for Meteorology (MPI-M). However, ECHAM6.3 is coupled to two different ocean models, namely the MPIOM sea ice-ocean model developed at MPI-M and the FESOM sea ice-ocean model developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). A reason for the different TCR is related to ocean heat uptake in response to greenhouse gas forcing. Specifically, AWI-CM simulations show stronger surface heating than MPI-ESM simulations while the latter accumulate more heat in the deeper ocean. The vertically integrated ocean heat content is increasing slower in AWI-CM model configurations compared to MPI-ESM model configurations in the high latitudes. Weaker vertical mixing in AWI-CM model configurations compared to MPI-ESM model configurations seems to be key for these differences. The strongest difference in vertical ocean mixing occurs inside the Weddell and Ross Gyres and the northern North Atlantic. Over the North Atlantic, these differences materialize in a lack of a warming hole in AWI-CM model configurations and the presence of a warming hole in MPI-ESM model configurations. All these differences occur largely independent of the considered model resolutions

ASIMBO 2018 - Sea ice thickness measurements with Polar 6 from Station Nord and Alert

Aim of the ASIMBO 2018 campaign is to complement earlier ice thickness measurements made by plane or helicoter north of Svalbard, Greenland and in Fram Strait. Sea ice thickness information will be used to examine the connection between thickness variability, ice age and source area. Together with satellite based information on sea ice motion, data will be used to quantify sea ice outflow through Fram Strait in summer. These estimates shall improve the understanding of interannual variability in summer sea ice outflow and complement existing winter volume flux calculations. A second objective is to extent sea ice thickness measurements to the Lincoln Sea where we will study thinning of sea ice due to reduction of old multi-year ice in this area. Like the measurements planned over the Fram Strait area, the surveys are a continuation of earlier aircraft campaigns made north of Alert and shall improve understanding of ice mass balance changes in the Arctic