ALBERT

Description: We have conducted a series of atmosphere-only and coupled model experiments on time scales from weather to climate and with different methods to address the question how the large scale circulation of the Northern mid-latitudes is affected by the shrinking Arctic sea ice as well as by the overlying atmosphere. A major pathway has been found from the Barents Sea / Kara Sea area to Eastern Europe and Northern Asia and a secondary one from the Canadian Arctic into North America. In contrast, the atmosphere above ocean areas is less affected by the Arctic. A recurring response feature to declined Arctic sea ice is the slowdown and southward shift of the jet stream with less cyclone activity north of it leading to around 0.5 K colder conditions over some limited regions of North America and North Siberia in winter consistent with a negative Arctic Oscillation index. This happens despite the tendency of less intense cold advection due to the warmer Arctic in cases of anomalous northerly flow. It should be noted that for robust responses large ensemble simulations are needed due to low signal-to-noise ratio. In this respect it has been proven helpful to perform simulations in a Numerical Weather Prediction setting as the short simulation time enables us to easily run ensembles of several hundreds of realizations. Furthermore, in such a setting the initial response to a suddenly changed Arctic sea ice cover can be studied giving us hints how anomalies in the atmosphere develop. Coupled model simulations hint at no discernable influence of shrinking Arctic sea ice on the ocean on time scales of a year while on decadal to centennial time scales the ocean starts to react with possible feedbacks to the atmosphere. Due to less and thinner sea ice cover the momentum flux into the ocean increases which spins up the Beaufort Gyre. This response propagates towards the North Atlantic as an increased outflow through the Fram Strait, which drives increased volume transport into the Barents Sea, thus fostering the Atlantification of the basin. The response is not confined to the interior of the Arctic and our results suggest that it may reach as far south as the North Atlantic Current as a combined response to the dynamical ocean adjustment triggered within the Arctic and, secondarily, to the atmospheric weakening of the westerly winds.

Description: The influence of the Arctic atmosphere on Northern Hemisphere mid-latitude tropospheric weather and climate is explored by comparing the skill of two sets of 14-day weather forecast experiments with the ECMWF model with and without relaxation of the Arctic atmosphere towards ERA-Interim reanalysis data during the course of the integration. Two pathways are identified along which the Arctic influences mid-latitude weather, one pronounced one over Asia and Eastern Europe and a secondary one over North America. In general, linkages are found to be strongest (weakest) during boreal winter (summer) when the amplitude of stationary planetary waves over the Northern Hemisphere is strongest (weakest). No discernable Arctic impact is found over the North Atlantic and North Pacific region, which is consistent with predominantly southwesterly flow. An analysis of the flow-dependence of the linkages shows that anomalous northerly flow conditions increase the Arctic influence on mid-latitude weather over the continents. Specifically, an anomalous northerly flow from Kara Sea towards Western Asia leads to cold surface temperature anomalies not only over Western Asia but also over Eastern and Central Europe. Finally, the results of this study are discussed in the light of potential mid-latitude benefits of improved Arctic prediction capabilities.

Description: Would improved prediction capabilities over the Antarctic lead to improved forecast skill in southern midlatitudes? Or more generally speaking, how large is the influence of the Antarctic atmosphere on the weather and climate of the southern mid-latitudes? To answer these questions we assess the skill of two sets of 14-day forecasts with the Integrated Forecast System of the European Centre for Medium-Range Weather Forecasts with and without relaxation towards the Interim reanalysis of the ECMWF over the Antarctic south of 75 ° S. Due to the relaxation both the mean absolute error and the root mean square error decrease by 2 to 5% averaged over the southern mid-latitudes with the larger values in winter. Over southern South America and the South Atlantic error reductions are slightly larger and amount to around 5 to 6%. No dependency of the error reductions of the El Niño Southern Oscillation or the Antarctic Oscillation could be found although error reductions averaged over the whole southern mid-latitudes tend to be larger in situations with decreased westerly flow in the mid-latitudes. In weather situations with anomalous meridional flow from Antarctica to southern South America improvements are most pronounced in the latter area which implies that this is the major pathway for Antarctic influence on southern mid-latitude weather and climate.

Description: Forecast experiments with the European Centre for Medium-Range Weather Forecasts model with and without relaxation of the Arctic troposphere toward reanalysis data are carried out in order to explore the influence that improved Arctic forecasts during wintertime would have on the skill of medium-range and extended-range prediction of 500 hPa geopotential height in the Northern Hemisphere midlatitudes. It turns out that the largest midlatitude improvements are found over eastern Europe, northern Asia, and North America; no discernible impact is found over the North Atlantic and North Pacific, where midlatitude and tropical dynamics appear to be more important. The strength of the linkage between the Arctic and the midlatitudes is found to be flow dependent, with anomalous northerly wind leading to a stronger Arctic influence. Finally, the results are discussed in the context of the possible impact of Arctic sea ice decline on midlatitude weather and climate.