Marta Kasper

Arctic influence on subseasonal midlatitude prediction

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.

Using NWP to assess the influence of the Arctic atmosphere on midlatitude weather and climate

The influence of the Arctic atmosphere on Northern Hemisphere midlatitude tropospheric weather and climate is explored by comparing the skill of two sets of 14-day weather forecast experiments using the ECMWF model with and without relaxation of the Arctic atmosphere towards ERA-Interim reanalysis data during the integration. Two pathways are identified along which the Arctic influences midlatitude weather: a 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 discernible 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 midlatitude weather over the continents. Specifically, an anomalous northerly flow from the Kara Sea towards West Asia leads to cold surface temperature anomalies not only over West Asia but also over Eastern and Central Europe. Finally, the results of this study are discussed in the light of potential midlatitude benefits of improved Arctic prediction capabilities.摘要本文采用ECMWF模式进行了两组14天天气预报试验, 两组试验的区别是积分过程中是否采用趋向ERA再分析数据的北极大气松弛系数, 通过比较来分析北极大气对北半球中纬度对流层天气和气候的影响, 并确定了两条北极影响中纬度天气的路径: 主要路径在亚洲和东欧, 次要路径在北美. 总的来说, 北半球冬季(夏季)静止行星波幅度最大(最弱)的情况下, 关联最强(最弱);北大西洋和北太平洋地区的北极影响很弱, 这与西南气流是一致的. 对气流依赖性的分析表明, 北极异常气流加强了北极对中纬度大陆地区的影响, 具体来说, 喀拉海向西亚的偏北异常气流导致了西亚及东欧和中欧地区的异常低温. 最后, 从改进北极预测能力以利于中纬度的角度讨论了本文的研究结果.