This website uses cookies, which are small text files that are used to make websites work more effectively. In order to continue using this website, you will need to accept the use of cookies.

Publications

Køltzow, Morten

Flash poster on verification of winter precipitation at Arctic Earth System Modelling workshop

Full Article: https://www.zenodo.org/record/3565685#.XhMrK2XAO70

Webster, Melinda; Gerland, Sebastian; Holland, Marika; Hunke, Elizabeth; Kwok, Ron; Lecomte, Olivier; Massom, Robert; Perovich, Don; Sturm, Matthew

Snow is the most reflective, and also the most insulative, natural material on Earth. Consequently, it is an integral part of the sea-ice and climate systems. However, the spatial and temporal heterogeneities of snow pose challenges for observing, understanding and modelling those systems under anthropogenic warming. Here, we survey the snow–ice system, then provide recommendations for overcoming present challenges. These include: collecting process-oriented observations for model diagnostics and understanding snow–ice feedbacks, and improving our remote sensing capabilities of snow for monitoring large-scale changes in snow on sea ice. These efforts could be achieved through stronger coordination between the observational, remote sensing and modelling communities, and would pay dividends through distinct improvements in predictions of polar environments.

Full Article: https://www.zenodo.org/record/3565646#.XhMq2mXAO70

Ye, Kunhui; Jung, Thomas; Semmler, Tido

Understanding the influence of the Arctic troposphere on the climate at midlatitudes is critical for projecting the impacts of ongoing and anticipated Arctic changes such as Arctic amplification and rapid sea ice decline over the Northern Hemisphere. In this study, we analyze a suite of atmospheric model experiments, with and without atmospheric relaxation toward reanalysis data, to study the impacts of the Arctic troposphere on the midlatitude atmospheric circulation and climate variability. The Arctic troposphere is found to strongly impact the interannual variability of the atmospheric circulation and temperature over the midlatitude continents. The major mechanisms for the impacts of Arctic troposphere include the modulation of the large-scale atmospheric circulation, the associated heat transport over the continents, and the impacts on synoptic variations in the North Atlantic-European sector. The impact of the Arctic troposphere on the intensity of the Siberian High is an important factor for how the Arctic can influence temperature variability in south Siberia and East Asia. The trends in the Arctic troposphere in recent decades are closely linked to the recent winter cooling in Northern Eurasia. These recent cooling trends are not driven by the trends in sea surface temperature/sea ice, tropical atmosphere, and the stratosphere. It is argued that the temperature trend pattern of warm Arctic-cold Eurasia is a manifestation of two possibly independent phenomena and the cooling trend is contributed to by the Arctic troposphere through impacting the large-scale atmospheric circulation, the atmospheric blocking frequency, and the intensity of the Siberian High.

Full Article: https://www.zenodo.org/record/3565642#.XhMqgGXAO70

Fitch, Anna

The vertical velocity probability distribution function (PDF) is analyzed throughout the depth of the loweratmosphere, including the subcloud and cloud layers, in four large-eddy simulation (LES) cases of shallowcumulus and stratocumulus. Double-Gaussian PDF closures are examined to test their ability to represent awide range of turbulence statistics, from stratocumulus cloud layers characterized by Gaussian turbulence toshallow cumulus cloud layers displaying strongly non-Gaussian turbulence statistics. While the majority of themodel closures are found to perform well in the former case, the latter presents a considerable challenge.A new model closure is suggested that accounts for high skewness and kurtosis seen in shallow cumulus cloudlayers. The well-established parabolic relationship between skewness and kurtosis is examined, with results inagreement with previous studies for the subcloud layer. In cumulus cloud layers, however, a modified re-lationship is necessary to improve performance. The new closure significantly improves the estimation of thevertical velocity PDF for shallow cumulus cloud layers, in addition to performing well for stratocumulus. Inparticular, the long updraft tail representing the bulk of cloudy points is much better represented and higher-order moments diagnosed from the PDF are also greatly improved. However, some deficiencies remainowing to fundamental limitations of representing highly non-Gaussian turbulence statistics with a double-Gaussian PDF.

Full Article: https://www.zenodo.org/record/3565638#.XhMp6WXAO70

Ye, Kunhui; Jung, Thomas;

Relaxation experiments with the atmosphere model from European Centre for Medium‐range Weather Forecasts are analyzed to understand influence of lower latitudes (south of about 52 °N) on climate variability and change over the Arctic region. Interannual variability of the Arctic troposphere is impacted strongly by both the tropics and the midlatitudes. In general, the link in winter is stronger than that in summer. Furthermore, the tropics and midlatitudes have different preferred pathways by which they influence the Arctic. Trend analysis suggests that winter surface warming trends over the Arctic are driven strongly by the local sea ice‐atmospheric interaction. Warming at higher altitudes is strongly tied to remote non‐Arctic drivers, with some local amplification. Summer warming trends in northeastern Canada and Greenland are driven strongly by sea surface temperature/sea ice changes and partly by the tropics. The summer warming in northern Europe and western Russia is more strongly driven by the midlatitudes.

Full Article: https://www.zenodo.org/record/3565630#.XhMpt2XAO70

West, Alex; Collins, Mat; Blockley, Ed; Ridley, Jeff; Bodas-Salcedo, Alejandro

A new framework is presented for analysing theproximate causes of model Arctic sea ice biases, demon-strated with the CMIP5 model HadGEM2-ES (Hadley Cen-tre Global Environment Model version 2 – Earth System).In this framework the Arctic sea ice volume is treated as aconsequence of the integrated surface energy balance, viathe volume balance. A simple model allows the local de-pendence of the surface flux on specific model variables tobe described as a function of time and space. When theseare combined with reference datasets, it is possible to esti-mate the surface flux bias induced by the model bias in eachvariable. The method allows the role of the surface albedoand ice thickness–growth feedbacks in sea ice volume bal-ance biases to be quantified along with the roles of modelbias in variables not directly related to the sea ice volume. Itshows biases in the HadGEM2-ES sea ice volume simulationto be due to a bias in spring surface melt onset date, partlycountered by a bias in winter downwelling longwave radia-tion. The framework is applicable in principle to any modeland has the potential to greatly improve understanding of thereasons for ensemble spread in the modelled sea ice state.A secondary finding is that observational uncertainty is thelargest cause of uncertainty in the induced surface flux biascalculation.

Full Article: https://www.zenodo.org/record/3565623#.XhMpGmXAO70

Massonnet, François; Barthélemy, Antoine; Worou, Koffi; Fichefet, Thierry; Vancoppenolle, Martin; Rousset, Clément; Moreno-Chamarro, Eduardo;

The ice thickness distribution (ITD) is one of thecore constituents of modern sea ice models. The ITD ac-counts for the unresolved spatial variability of sea ice thick-ness within each model grid cell. While there is a generalconsensus on the added physical realism brought by theITD, how to discretize it remains an open question. Here,we use the ocean–sea ice general circulation model, Nu-cleus for European Modelling of the Ocean (NEMO) ver-sion 3.6 and Louvain-la-Neuve sea Ice Model (LIM) ver-sion 3 (NEMO3.6-LIM3), forced by atmospheric reanalysesto test how the ITD discretization (number of ice thicknesscategories, positions of the category boundaries) impacts thesimulated mean Arctic and Antarctic sea ice states. We findthat winter ice volumes in both hemispheres increase with thenumber of categories and attribute that increase to a net en-hancement of basal ice growth rates. The range of simulatedmean winter volumes in the various experiments amounts to∼30 % and∼10 % of the reference values (run with fivecategories) in the Arctic and Antarctic, respectively. Thissuggests that the way the ITD is discretized has a signifi-cant influence on the model mean state, all other things beingequal. We also find that the existence of a thick category withlower bounds at∼4 and∼2 m for the Arctic and Antarc-tic, respectively, is a prerequisite for allowing the storageof deformed ice and therefore for fostering thermodynamicgrowth in thinner categories. Our analysis finally suggeststhat increasing the resolution of the ITD without changingthe lower limit of the upper category results in small but notnegligible variations of ice volume and extent. Our study pro-poses for the first time a bi-polar process-based explanationof the origin of mean sea ice state changes when the ITDdiscretization is modified. The sensitivity experiments con-ducted in this study, based on one model, emphasize thatthe choice of category positions, especially of thickest cat-egories, has a primary influence on the simulated mean seaice states while the number of categories and resolution haveonly a secondary influence. It is also found that the currentdefault discretization of the NEMO3.6-LIM3 model is suffi-cient for large-scale present-day climate applications. In allcases, the role of the ITD discretization on the simulatedmean sea ice state has to be appreciated relative to other in-fluences (parameter uncertainty, forcing uncertainty, internalclimate variability).

Full Article: https://www.zenodo.org/record/3565614#.XhMo3WXAO70

Thomas Oudar

Understanding the mid-latitude atmospheric circulation response to CO2 forcing is challenging and complex due to the strong internal variability and the multiple potential CO2-induced effects. While a significant poleward shift of the jet is projected in summer, changes remain uncertain in winter. In this study, we investigate the boreal winter extratropical jet response to an abrupt quadrupling of atmospheric CO2 in the CMIP6-generation global climate model CNRM-CM6-1. First, we show that the model performs better than the former generation CNRM-CM5 model in representing the atmospheric dynamics in the northern extratropics. Then, when atmospheric CO2 is quadrupled, CNRM-CM6-1 exhibits a strengthening and upward shift of the jet. A poleward shift is identified and robust in the Pacific in boreal winter. In the Atlantic, the jet response rather exhibits a squeezing, especially at the eastern part of the basin. It is found that changes are more robust across the Northern Hemisphere in early-winter than in late-winter season. Finally, the circulation response is broken down into individual contributions of various drivers. The uniform global mean component of the SST warming is found to explain most of the total atmospheric response to a quadrupling of CO2, with relatively smaller contributions from faster CO2 effects, the SST pattern change and the Arctic sea ice decline. The cloud radiative effect contribution is also assessed and found to be rather weak in the CNRM-CM6-1 model. This study highlights that long experiments are required to isolate the wintertime circulation response from the internal variability, and that idealized experimental setups are helpful to disentangle the physical drivers.

Full Article: https://www.zenodo.org/record/3565595#.XhMYZmXAO70

Eade, Rosie

The winter of 1962/63 was the coldest in the UK in over a century while the mildest winter occurred in 1988/89. For countries to be resilient against the impacts of large weather variations in the future, it is important to understand the likelihood of seeing such extreme fluctuations in addition to future climate change. In Europe and North America, these fluctuations are related to a combination of year-to-year variability and low-frequency variability of the North Atlantic Oscillation (NAO). The NAO is now a significant source of predictability for seasonal forecasts in these regions, however the signal-to-noise ratio of the ensemble mean to total variability in these ensemble predictions has been shown to be anomalously small, which means the real world is more predictable than our climate models suggest. Here we provide a new evaluation of the ability of climate models to reproduce longer-term variability and extreme trends like those seen between the 1960s and 1990s, with a focus on the NAO. We also investigate relationships with other large scale changes such as the reduction in Arctic Sea Ice over recent decades.

(PAMIP section: Research funded as part of APPLICATE project, Grant number 727862)

Full Article: https://www.zenodo.org/record/3565550#.XhMX7WXAO70

Eade, Rosemary; Smith, Doug; Hermanson, Leon; Dunstone, Nick

Climate model studies have shown a wide range of responses in the northern hemisphere to reduced sea ice loss in the Arctic. The most robust is the thermodynamic response with local warming in the region of the sea ice loss. Many studies also find a shift in the tropospheric jet towards the equator and a tendency towards the negative phase of the North Atlantic Oscillation. However, these mid-latitude dynamic responses seem somewhat dependent on the extent and location of sea ice loss as well as model differences.

Full Article: https://www.zenodo.org/record/3565538#.XhMXgWXAO70