Variability in the altitude of fast upper tropospheric winds over the Northern Hemisphere during winter

[1] The surface of maximum wind (SMW) is used as a frame for examining spatial and temporal variability in the vertical position of fast upper tropospheric winds over the Northern Hemisphere during winter. At a given observation time in a gridded data set, the SMW is defined as the surface passing through the fastest analyzed wind above each grid node, with a vertical search domain restricted to the upper troposphere and any tropospheric jet streams extending into the lower stratosphere. Documenting how SMW pressure varies spatially and temporally guides the operational and climatological analysis of tropospheric jet streams and enables more informed interpretation of isobaric wind speed signals (i.e., isobaric speed variability can be caused by both jet core pressure changes and jet core speed changes). Using six-hourly Northern Hemisphere (NH) NCEPNCAR reanalysis data from 1958 to 2004, the mean three-dimensional structure of the NH SMW is mapped for winter, and vertical cross sections are used to show the position of the SMW relative to the mean isotachs, the mean lapse-rate tropopause, and the mean dynamic tropopause. The Arctic Oscillation (AO) and El Niño/Southern Oscillation (ENSO) are identified as the leading contributors to SMW pressure variability over the NH using principal components analysis. During the AO positive phase, mean SMW pressures are decreased at high latitudes and increased at middle and subtropical latitudes. The SMW response to the AO is elliptical with the centers of pressure change displaced more equatorward over Asia and the Atlantic. During the warm phase of ENSO, eastward expansion and strengthening of the Pacific jet stream is associated with SMW pressures up to 27 hPa lower near 30 N. The relatively low SMW pressures of the east Pacific SMW are flanked, during the warm phase of ENSO, by slower upper tropospheric winds and higher SMW pressures near the equator and the Gulf of Alaska.