Eigil Kaas

A 10,000-Year Record of Arctic Ocean Sea-Ice Variability-View from the Beach

Sea-ice coverage near northern Greenland and in the western Arctic Ocean varied in opposition over much of the Holocene. We present a sea-ice record from northern Greenland covering the past 10,000 years. Multiyear sea ice reached a minimum between ~8500 and 6000 years ago, when the limit of year-round sea ice at the coast of Greenland was located ~1000 kilometers to the north of its present position. The subsequent increase in multiyear sea ice culminated during the past 2500 years and is linked to an increase in ice export from the western Arctic and higher variability of ice-drift routes. When the ice was at its minimum in northern Greenland, it greatly increased at Ellesmere Island to the west. The lack of uniformity in past sea-ice changes, which is probably related to large-scale atmospheric anomalies such as the Arctic Oscillation, is not well reproduced in models. This needs to be further explored, as it is likely to have an impact on predictions of future sea-ice distribution.

Anomalously mild Younger Dryas summer conditions in southern Greenland

The first late-glacial lake sediments found in Greenland were analyzed with respect to a variety of environmental variables. The analyzed sequence covers the time span between 14400 and 10500 calendar yr B.P., and the data imply that the conditions in southernmost Greenland during the Younger Dryas stadial, 12800-11550 calendar yr B.P., were characterized by an arid climate with cold winters and mild summers, preceded by humid conditions with cooler summers. Climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and Fohn effects. It shows that regional and local variations of Younger Dryas summer conditions in the North Atlantic region may have been larger than previously found from proxy data and modeling experiments.

The Relationship between a Zonal Index and Blocking Activity

Abstract The degree to which Northern Hemisphere blocking activity is controlled by variations in zonal mean conditions is investigated. A set of Northern Hemisphere winter season 500-hPa analyzed fields is examined for blocks using an objective index defined solely from the eddy fields. With this blocking index, it is shown that in most regions enhanced blocking activity is associated with relatively strong zonally averaged winds around 30°N and weak winds around 50°–60°N. Also, the preferred zonal positions of blocks are related to the state of the zonal mean flow. A similar analysis is carried out using data from a perpetual January GCM simulation and the same relationship between blocking activity and zonally averaged conditions is found to be valid to an even stronger degree for these data. To investigate whether anomalous zonal mean flows are actually controlling the associated level of blocking activity, two experiments with the GCM are performed. In one experiment the zonal mean state of the GCM i...

A Mass-Conservative Semi-Implicit Semi-Lagrangian Limited-Area Shallow-Water Model on the Sphere

A locally mass conservative shallow-water model using a two-time-level, semi-implicit, semi-Lagrangian integration scheme is presented. The momentum equations are solved with the traditional semi-Lagrangian gridpoint form. The explicit continuity equation is solved using a cell-integrated semi-Lagrangian scheme, and the semi-implicit part is designed such that the resulting elliptic equation is on the same form as for the traditional semi-Lagrangian gridpoint system. The accuracy of the model is assessed by running standard test cases adapted to a limited-area domain. The accuracy and efficiency of the new model is comparable to traditional semi-Lagrangian methods.

Finite-Volume Methods in Meteorology

Recent developments in finite-volume methods provide the basis for new dynamical cores that conserve exactly integral invariants, globally as well as locally, and, especially, for the design of exact mass conserving tracer transport models. The new technologies are reviewed and the perspectives for the future are discussed.

Reduction of systematic errors by empirical model correction: impact on seasonal prediction skill

Recent studies indicate that the atmospheric response to anomalies in the lower boundary conditions, e.g. sea surface temperatures, is strongly dependent on the atmospheric background flow. Since all general circulation models have long-term systematic errors it is therefore possible that the skill in seasonal prediction is improved by reducing the systematic errors of the model. In this study sensitivity experiments along this line are made with an empirically corrected dynamical model for which the systematic errors are reduced substantially and the dynamical variability has become more realistic than for the original model. As a measure of seasonal prediction skill, correlation of temporal anomalies between modelled and observed data has been determined. The corrected model shows improved skill in the Southern Hemisphere in general—on average a 20–30% improvement for the Southern Hemisphere compared with the original model. In the Northern Hemisphere skill is improved in some areas, but in other areas the skill of the original model is better. On average there is no improvement for the Northern Hemisphere. Also, pattern correlations have been determined for the following areas: the Northern Hemisphere, the Southern Hemisphere, the tropics and Europe. The general picture is that the two model versions are very similar in the Northern Hemisphere and in the tropics. For Europe the results of the two models are rather different, but no model can be said to be better than the other. In the Southern Hemisphere it is again seen that the correlations are higher for the corrected model than for the original model.

Tropical cyclones in enhanced resolution CMIP5 experiments

This study investigates the possible effects of global warming on tropical cyclone (TC) activity. The study is conducted using the coupled ocean–atmosphere global climate model EC-Earth configured at a relatively high resolution (T159 with 62 vertical levels), which is integrated following the CMIP5 protocol. By considering the late twentieth century (1979–2009) in the historical simulation and the twenty-first century end (2070–2100) in the RCP4.5 and RCP8.5 scenarios, significant future annual mean frequency decreases are found globally and in both hemispheres, accompanied by significant mean lifetime decreases and significant intensity increases, the latter being found through several different measures (but with caveats). In addition, the relatively novel aspect of simulating TCs of the past (1900–1930) is studied to further assess the robustness of the climate change results. These results suggest that TCs in the early twentieth century were more frequent in the southern hemisphere and dissipated more energy in the southern hemisphere and the South Indian Ocean. Although some model biases are present and the coarse model resolution prevents intense TCs in being simulated, reasonable TC simulation skill for other metrics (e.g., TC genesis, frequency of occurrence) is found when validated against present day observations. Thus the model displays an acceptable ability to connect TC climatology with the larger scale circulation.

Southward Intertropical Convergence Zone Shifts and Implications for an Atmospheric Bipolar Seesaw

AbstractIn this study, southward intertropical convergence zone (ITCZ) shifts are investigated in three different scenarios: Northern Hemispheric cooling, Southern Hemispheric warming, and a bipolar seesaw-like forcing that combines the latter two. The experiments demonstrate the mutual effects that northern- and southern-high-latitude forcings exert on tropical precipitation, suggesting a time-scale-dependent dominance of northern versus southern forcings. In accordance with this, two-phase tropical precipitation shifts are suggested, involving a fast component dominated by the high-northern-latitude forcing and a slower component due to the southern-high-latitude forcing. The results may thus be useful for the future understanding and interpretation of high-resolution tropical paleoprecipitation proxies and their relation to high-latitude records (e.g., ice core data). The experiments also show that Southern Ocean warming has a global impact, affecting both the tropics and northern extratropics, as seen...

Using analysis increments to estimate atmospheric heating rates following volcanic eruptions

By using the analysis increments of the temperatures from atmospheric re-analyses it is possible to roughly estimate the stratospheric radiative heating rates from volcanic eruption. The method is applied to the 30 hPa temperature from the 15 years of the ECMWF-reanalysis. After the influence from the Quasi Biennial Oscillation is subtracted maximum values of the zonal mean radiative heating rates related to El Chichon and Pinatubo volcanic eruptions are estimated to 0.30 K/day and 0.25 K/day, respectively. The estimated heating rates are not solely due to aerosol heating but includes the effects of ozone reduction and geostrophic adjustment as well.

Sea ice thickness and recent Arctic warming

The climatic impact of increased Arctic sea ice loss has received growing attention in the last years. However, little focus has been set on the role of sea ice thickness, although it strongly determines surface heat fluxes. Here ensembles of simulations using the EC-Earth atmospheric model (Integrated Forecast System) are performed and analyzed to quantify the atmospheric impacts of Arctic sea ice thickness change since 1982 as revealed by the sea ice model assimilation Global Ice-Ocean Modeling and Assimilation System. Results show that the recent sea ice thinning has significantly affected the Arctic climate, while remote atmospheric responses are less pronounced owing to a high internal atmospheric variability. Locally, the sea ice thinning results in enhancement of near-surface warming of about 1°C per decade in winter, which is most pronounced over marginal sea ice areas with thin ice. This leads to an increase of the Arctic amplification factor by 37%.