Showing papers by "Reindert J. Haarsma published in 2013"
TL;DR: In this paper, the authors used a very high resolution global climate model with prescribed sea surface temperatures to show that greenhouse warming enhances the occurrence of hurricane-force (> 32.6 m/s-1) storms over western Europe during early autumn (August-October), the majority of which originate as a tropical cyclone.
Abstract: [1] We use a very high resolution global climate model (~25 km grid size) with prescribed sea surface temperatures to show that greenhouse warming enhances the occurrence of hurricane-force (> 32.6 m s–1) storms over western Europe during early autumn (August–October), the majority of which originate as a tropical cyclone. The rise in Atlantic tropical sea surface temperatures extends eastward the breeding ground of tropical cyclones, yielding more frequent and intense hurricanes following pathways directed toward Europe. En route they transform into extratropical depressions and reintensify after merging with the midlatitude baroclinic unstable flow. Our model simulations clearly show that future tropical cyclones are more prone to hit western Europe, and do so earlier in the season, thereby increasing the frequency and impact of hurricane force winds.
119 citations
TL;DR: In this paper, the future changes of atmospheric blocking over the Euro-Atlantic sector, diagnosed from an ensemble of 17 global-climate simulations obtained with the ECHAM5/MPI-OM model, are shown to be largely explainable from the change of the 500hPa mean zonal circulation and its variance.
Abstract: The future changes of atmospheric blocking over the Euro-Atlantic sector, diagnosed from an ensemble of 17 global-climate simulations obtained with the ECHAM5/MPI-OM model, are shown to be largely explainable from the change of the 500 hPa mean zonal circulation and its variance. The reduction of the blocking frequency over the Atlantic and the increased frequency of easterly upper-level flow poleward of 60°N are well explained by the changes of mean zonal circulation. In winter and autumn an additional downstream shift of the frequency maximum is simulated. This is also seen in a subset of the CMIP5 models with RCP8.5. To explain this downstream shift requires the inclusion of the changing variance. It is suggested that the increased downstream variance is caused by the stronger, more eastward extending future jet, which promotes Rossby wave breaking and blocking to occur further downstream. The same relation between jet-strength and central-blocking longitude is found in the variability of the current climate.
45 citations
TL;DR: In this paper, the anthropogenic changes during boreal winter in the thermal and zonal flow structure over Eastern Atlantic and Western Europe (EAWE) have been investigated using an ensemble of CMIP3 and CMIP5 models.
Abstract: The anthropogenic changes during boreal winter in the thermal and zonal flow structure over Eastern Atlantic and Western Europe (EAWE) have been investigated using an ensemble of CMIP3 and CMIP5 models. The ensemble mean change in the zonal wind at 500 hPa over this region is characterized by an eastward extension of the belt of zonal winds. Using the thermal wind relation these wind changes are found to be consistent with the changes in the tropospheric temperature profile. An enhanced warming is simulated in the subtropical upper troposphere and a relative surface cooling in the mid-latitudes. The subtropical upper tropospheric warming is related to the downward branch of the mean meridional circulation, whereas the mid-latitude lower tropospheric relative cooling is linked to the ocean processes that govern changes in its surface temperatures. Inter-model differences in the simulated change of the zonal wind over the EAWE by the CMIP3 and CMIP5 models relate well with differences in the upper tropospheric subtropical warming and the mid-latitude lower tropospheric relative cooling. The simulated change of the zonal wind over the EAWE region by the CMIP3 and CMIP5 models correlates well with changes in the meridional SST gradient. We conclude that uncertainties in the projected changes of the zonal flow over Europe are at least partly due to uncertainties in the response of the North Atlantic Ocean to increased levels of greenhouse gases.
39 citations
TL;DR: In this article, the authors show that large parts of western and central Europe face a 20-50% future reduction in snowfall on Hellmann days (days with daily-mean temperatures below freezing).
Abstract: Large parts of western and central Europe face a 20–50 % future reduction in snowfall on Hellmann days (days with daily-mean temperatures below freezing). This strong reduction occurs in addition to the expected 75 % decrease of the number of Hellmann days near the end of the twenty first century. The result is insensitive to the exact freezing-level threshold, but is in sharp contrast with the winter daily precipitation, which increases under most global warming scenarios. Not only climate model simulations show this. Observational records also reveal that probabilities for precipitation on Hellmann days have been larger in the past. The future reduction is a consequence of the freezing-level threshold becoming a more extreme quantile of the temperature distribution in the future. Only certain circulation types permit these quantiles to be reached, and it is shown that these have intrinsically low precipitation probability.
14 citations
TL;DR: In this paper, the authors quantify the contribution of tropical and extratropical atmospheric forcing mechanisms to the formation of the North Atlantic Oscillation (NAO) pattern, and show that the barotropic vorticity equation can represent the dynamics of both tropical and exarropical forcing.
Abstract: This work is set out to quantify the contribution of tropical and extratropical atmospheric forcing mechanisms to the formation of the North Atlantic Oscillation (NAO) pattern Although the NAO varies on a wide range of time scales, we focus on 10–60 d At these time scales, mechanisms are at play in the atmosphere that can generate the characteristic dipole pattern We focus on the tropical Rossby Wave Source (RWS) and extratropical eddy activity Anomalous tropical and extratropical vorticity forcing associated with the NAO is derived from atmospheric reanalysis data and applied in an idealised barotropic model Also, using winds from composites of the NAO, the vorticity forcing is derived inversely from the barotropic vorticity equation Both types of forcing are imposed in the barotropic model in the tropics and extratropics, respectively An important result is that the tropics dampen the NAO as a result of a negative feedback generated in the extratropics The damping is strongest, about 30%, for the negative phase of the NAO For the positive phase, the damping is about 50% smaller The results show that the barotropic vorticity equation can represent the dynamics of both tropical and extratropical forcing related to the formation of the NAO patterns Keywords: North Atlantic Oscillation, Rossby wave source, eddy activity, tropical forcing (Published: 7 June 2013) Citation: Tellus A 2013, 65 , 19741, http://dxdoiorg/103402/tellusav65i019741
6 citations