Klaus Dethloff

Impact of sea ice cover changes on the Northern Hemisphere atmospheric winter circulation

ABSTRACT The response of the Arctic atmosphere to low and high sea ice concentration phases based on European Center for Medium-Range Weather Forecast (ECMWF) Re-Analysis Interim (ERA-Interim) atmospheric data and Hadley Centres sea ice dataset (HadISST1) from 1989 until 2010 has been studied. Time slices of winter atmospheric circulation with high (1990–2000) and low (2001–2010) sea ice concentration in the preceding August/September have been analysed with respect to tropospheric interactions between planetary and baroclinic waves. It is shown that a changed sea ice concentration over the Arctic Ocean impacts differently the development of synoptic and planetary atmospheric circulation systems. During the low ice phase, stronger heat release to the atmosphere over the Arctic Ocean reduces the atmospheric vertical static stability. This leads to an earlier onset of baroclinic instability that further modulates the non-linear interactions between baroclinic wave energy fluxes on time scales of 2.5–6 d and planetary scales of 10–90 d. Our analysis suggests that Arctic sea ice concentration changes exert a remote impact on the large-scale atmospheric circulation during winter, exhibiting a barotropic structure with similar patterns of pressure anomalies at the surface and in the mid-troposphere. These are connected to pronounced planetary wave train changes notably over the North Pacific.

Regional climate model of the Arctic atmosphere

A regional climate model of the whole Arctic using the dynamical package of the High- Resolution Limited Area Model (HIRLAM) and the physical parameterizations of the Hamburg General Circulation Model (ECHAM3) has been applied to simulate the climate of the Arctic north of 65 oN at a 50-km horizontal resolution. The model has been forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses at the lateral boundaries and with climatological or actual observed sea surface temperatures and sea ice cover at the lower boundary. The results of simulating the Arctic climate of the troposphere and lower stratosphere for January 1991 and July 1990 have been described. In both months the model rather closely reproduces the observed monthly mean circulation. While the general spatial patterns of surface air temperature, mean sea level pressure, and geopotential are consistent with the ECMWF analyses, the model shows biases when the results are examined in detail. The largest biases appear during winter in the planetary boundary layer and at the surface. The underestimated vertical heat and humidity transport in the model indicates the necessity of improvements in the parameterizations of vertical transfer due to boundary layer processes. The tropospheric differences between model simulations and analyses decrease with increasing height. The temperature bias in the planetary boundary layer can be reduced by increasing the model sea ice thickness. The use of actual observed sea surface temperatures and sea ice cover leads only to small improvements of the model bias in comparison with climatological sea surface temperatures and sea ice cover. The validation of model computed geopotential, radiative fluxes, surface sensible and latent heat fluxes and clouds against selected station data shows deviations between model simulations and observations due to shortcomings of the model. This first validation indicates that improvements in the physical parameterization packages of radiation and in the description of sea ice thickness and sea ice fraction are necessary to reduce the model bias.

Analysis of a link between fall Arctic sea ice concentration and atmospheric patterns in the following winter

ABSTRACT The impact of anomalous fall Arctic sea ice concentrations (SICs) on atmospheric patterns in the following winter is revisited by analysing results for two time periods: the most recent, satellite-era period (1979–2010) and a longer time-period (1950–2010). On the basis of September SICs for each time-period, an index was constructed which was used to identify anomalous high/low SIC years for both the original, as well as for the linearly detrended sea ice index. Identified years were then used to derive composites for the following winters monthly atmospheric variables. Mid-troposphere geopotential height composites for winter months are in general reminiscent of the North Atlantic Oscillation pattern with high latitude maximum shifted towards the Barents Sea. Also, lower troposphere temperatures indicate the presence of cooler conditions over the continents during low SIC years. However, differences in the composite patterns are significant only for areas with limited spatial extent. While suggested pathways in previously published studies seem reasonable, our results show that these findings are not yet robust enough from a statistical significance perspective. More data (e.g. provided by longer, climate-quality reanalysis datasets) are needed before conclusions of impacts and feedbacks can be drawn with certainty.

Recent Greenland Accumulation Estimated from Regional Climate Model Simulations and Ice Core Analysis

Abstract The accumulation defined as “precipitation minus evaporation” over Greenland has been simulated with the high-resolution limited-area regional climate model HIRHAM4 applied over an Arctic integration domain. This simulation is compared with a revised estimate of annual accumulation rate distribution over Greenland taking into account information from a new set of ice core analyses, based on surface sample collections from the North Greenland Traverse. The region with accumulation rates below 150 mm yr−1 in central-northwest Greenland is much larger than previously assumed and extends about 500 km farther to the south. It is demonstrated that good agreement between modeled and observed regional precipitation and accumulation patterns exists, particularly concerning the location and the values of very low accumulation in the middle of Greenland. The accumulation rates in the northern part of Greenland are reduced in comparison to previous estimates. These minima are connected with a prevailing bloc...

A dynamical link between the Arctic and the global climate system

Received 16 November 2005; revised 12 December 2005; accepted 15 December 2005; published 1 February 2006. [1] By means of simulations with a global coupled AOGCM it is shown that changes in the polar energy sink region can exert a strong influence on the mid- and high-latitude climate by modulating the strength of the mid-latitude westerlies and storm tracks. It is found, that a more realistic sea-ice and snow albedo treatment changes the ice-albedo feedback and the radiative exchange between the atmosphere and the ocean-sea-ice system. The planetary wave energy fluxes in the middle troposphere of mid-latitudes between 30 and 50 Na re redistributed, which induces perturbations in the zonal and meridional planetary wave trains from the tropics over the mid-latitudes into the Arctic. It is shown, that the improved parameterization of Arctic sea-ice and snow albedo can trigger changes in the Arctic and North Atlantic Oscillation pattern with strong implications for the European climate. Citation: Dethloff, K., et al. (2006), A dynamical link between the Arctic and the global climate system, Geophys. Res. Lett., 33, L03703, doi:10.1029/2005GL025245.

Stratospheric response to Arctic sea ice retreat and associated planetary wave propagation changes

The stratospheric response to the observed Arctic sea ice retreat is analysed based on European Centre for Medium-Range Weather Forecast (ECMWF) Re-Analysis Interim (ERA-Interim) atmospheric data from 1979–2012. It is shown that changes in August/September sea ice concentration impact on tropospheric and stratospheric geopotential heights in the following winter. During low ice phases a negative tropospheric Arctic Oscillation pattern is found, which is connected to a weakened stratospheric polar vortex and warmer stratospheric temperatures. Furthermore, the analysis reveals enhanced upward EP fluxes due to planetary waves for low ice conditions. Strong stratospheric anomalies in the Atlantic/European region are associated with a weaker polar vortex. Low ice periods are connected with additional tropospheric wave energy excitation in the Pacific/North America region and influence the stratosphere through three-dimensional planetary wave propagation.

Volcanic perturbation of the atmosphere in both polar regions: 1991–1994

Long-term measurements by sunphotometers of the spectral dependence of aerosol optical depth are reported for several sites in the Arctic and Antarctic for the period January 1991 through December 1994. In the Antarctic a pronounced increase of atmospheric turbidity was observed at the end of September 1991. The observed wavelength dependence in aerosol optical depth indicated that the increase was due to the presence of fresh and therefore small stratospheric aerosol particles associated with the eruption of Cerro Hudson in August 1991. After the breakdown of the polar vortex in mid-November 1991 we measured a second significant increase of the aerosol optical depth. At this time the 1.0-μm aerosol optical depth was approximately 0.12 or about 10 times background levels. This second incrcase is shown to be the result of the influx of Mount Pinatubo aerosols. A similar perturbation of the aerosol optical depth was observed in the Arctic with the return of sunlight in March 1992. However, the increased loading of the Arctic stratosphere by the Pinatubo aerosols was already evident at high northern latitudes in satellite measurements at the end of 1991. Stratospheric Aerosol and Gas Experiment II stratospheric 1.0-μm optical depth measurements show that meridional transport of Pinatubo aerosol from equatorial to middle and higher latitudes is greatest in the winter/spring hemisphere. This observation explains the observed seasonal trend of aerosol optical depth during the posteruption. A significant decrease of the perturbation by Mount Pinatubo aerosol was observed in both polar regions by the end of 1994. The measured 1.0-μm aerosol optical depths at this time were only 0.04 ; these values exceed the background level by about 0.01-0.02. Therefore the aerosol optical depth values are still slightly higher than during undisturbed conditions. In addition, we show that the occurrence of volcanic aerosols caused changes in the spectral dependence of the aerosol optical depth in the Arctic and the Antarctic. These variations, including the changes in the aerosol size distribution, derived from the aerosol optical depth, are discussed in comparison to undisturbed conditions.

A discontinuous Galerkin method for the shallow water equations in spherical triangular coordinates

A global model of the atmosphere is presented governed by the shallow water equations and discretized by a Runge-Kutta discontinuous Galerkin method on an unstructured triangular grid. The shallow water equations on the sphere, a two-dimensional surface in R^3, are locally represented in terms of spherical triangular coordinates, the appropriate local coordinate mappings on triangles. On every triangular grid element, this leads to a two-dimensional representation of tangential momentum and therefore only two discrete momentum equations. The discontinuous Galerkin method consists of an integral formulation which requires both area (elements) and line (element faces) integrals. Here, we use a Rusanov numerical flux to resolve the discontinuous fluxes at the element faces. A strong stability-preserving third-order Runge-Kutta method is applied for the time discretization. The polynomial space of order k on each curved triangle of the grid is characterized by a Lagrange basis and requires high-order quadature rules for the integration over elements and element faces. For the presented method no mass matrix inversion is necessary, except in a preprocessing step. The validation of the atmospheric model has been done considering standard tests from Williamson et al. [D.L. Williamson, J.B. Drake, J.J. Hack, R. Jakob, P.N. Swarztrauber, A standard test set for numerical approximations to the shallow water equations in spherical geometry, J. Comput. Phys. 102 (1992) 211-224], unsteady analytical solutions of the nonlinear shallow water equations and a barotropic instability caused by an initial perturbation of a jet stream. A convergence rate of O(@Dx^k^+^1) was observed in the model experiments. Furthermore, a numerical experiment is presented, for which the third-order time-integration method limits the model error. Thus, the time step @Dt is restricted by both the CFL-condition and accuracy demands. Conservation of mass was shown up to machine precision and energy conservation converges for both increasing grid resolution and increasing polynomial order k.

Winter Weather Patterns over Northern Eurasia and Arctic Sea Ice Loss

AbstractUsing NCEP–NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) winter daily (1 December–28 February) data for the period 1979–2012, this paper reveals the leading pattern of winter daily 850-hPa wind variability over northern Eurasia from a dynamic perspective. The results show that the leading pattern accounts for 18% of the total anomalous kinetic energy and consists of two subpatterns: the dipole and the tripole wind patterns. The dipole wind pattern does not exhibit any apparent trend. The tripole wind pattern, however, has displayed significant trends since the late 1980s. The negative phase of the tripole wind pattern corresponds to an anomalous anticyclone over northern Eurasia during winter, as well as two anomalous cyclones occurring over southern Europe and in the mid- to high latitudes of East Asia. These anomalous cyclones in turn lead to enhanced winter precipitation in these two regions, as well as negative surface temperature anomalies over the mid- to high latitudes of Asia. The...

Decadal climate variability in a coupled atmosphere-ocean climate model of moderate complexity

In this study we determined characteristic temporal modes of atmospheric variability at the decadal and interdecadal timescales. This was done on the basis of 1000 year long integrations of a global coupled atmosphere-ocean climate model of moderate complexity including the troposphere, stratosphere, and mesosphere. The applied model resolves explicitely the basic features of the large-scale long-term atmospheric and oceanic variables. The synoptic-scale processes are described in terms of autocorrelation and crosscorrelation functions. The paper includes an extended description and validation of the model as well as the results of analyses of two 1000 year long model integrations. One model run has been performed with the fully coupled model of the atmosphere-ocean system. The performed time-frequency analyses of atmospheric fields reveal strong decadal and interdecadal modes with periods of about 9, 18, and 30 years. To quantify the influence of the ocean on atmospheric variations an additional run with seasonally varying prescribed sea surface temperatures has been carried out, which is characterized by strong decadal modes with periods of about 9 years. The comparison of both runs suggests that decadal variability can be understood as an inherent atmospheric mode due to the nonlinear dynamics of the large-scale atmospheric circulation patterns whereas interdecadal climate variability has to be regarded as coupled atmosphere-ocean modes.