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

Abstract

[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) NCEP-NCAR 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.Citation: Strong, C., and R. E. Davis (2006), Variability in the altitude of fast upper tropospheric winds over the Northern Hemisphere during winter,

Winter jet stream trends over the Northern Hemisphere

“…To circumvent these potential difficulties, we use the Surface of Maximum Wind (SMW) analysis frame as described in Strong and Davis (2005, 2006a, 2006b, 2007. At a given observation time the SMW is the quasihorizontal surface that passes through the fastest wind in each column of the atmosphere with a vertical search domain restricted to the upper troposphere and any tropospheric jet streams extending into the lower stratosphere.…”

“…Although mean properties of winter jet streams have been analysed in the context of temperature over the Northern Hemisphere (Koch et al, 2006;Strong and Davis, 2006a), jet core trends have yet to be assessed. Here, we measure the probability and speed of upper tropospheric jet stream cores over the Northern Hemisphere for winters 1958 to 2007, test for jet core trends, and identify the related tropospheric thermal structure changes.…”

Comment on “Historical trends in the jet streams” by Cristina L. Archer and Ken Caldeira

“…The authors presented maps showing regional and seasonal trend variations and concluded that, in general, jet streams have moved poleward and risen to lower pressures in both hemispheres and weakened in the Northern Hemisphere. Previous studies of the boreal winter record also detected poleward jet stream contraction but found that jet stream core speeds increased extensively over the midlatitudes [ Strong and Davis , 2007], and showed that the leading pattern of jet stream pressure variability was an annular seesaw pattern linked to the Arctic Oscillation [ Strong and Davis , 2006, 2008]. Furthermore, Strong and Davis [2007] reported that positive jet stream wind speed trends are robustly indicated by geopotential height trends in three data sets: the ECWMF record since 1958, the NCEP record since 1958, and the satellite‐only portion of the NCEP record (1979–2007).…”

“…Differences from Strong and Davis [2006, 2007, 2008] were not mentioned by Archer and Caldeira [2008], and stem largely from fundamentally different definitions for jet stream properties. Archer and Caldeira [2008] used columns of monthly mean data between 400 and 100 hPa to define jet stream properties as “mass and mass‐flux weighted averages of wind speed, pressure, and latitude” (see their equations (1)–(3)).…”

“… Archer and Caldeira [2008] used columns of monthly mean data between 400 and 100 hPa to define jet stream properties as “mass and mass‐flux weighted averages of wind speed, pressure, and latitude” (see their equations (1)–(3)). Strong and Davis [2006, 2007, 2008] defined jet stream and jet stream core properties using the six‐hourly upper tropospheric surface of maximum wind (SMW) [ Strong and Davis , 2005] together with a minimum wind speed threshold of 25.7 m/s (50 knots) as specified in the AMS Glossary of Meteorology [ Glickman et al , 2000]. A minimum speed threshold is widely recognized as essential to jet stream identification [e.g., Reiter , 1963; Djuric , 1994; Glickman et al , 2000; Koch et al , 2006; Degirmendzic and Wibig , 2007], and Archer and Caldeira [2008] did not apply a minimum speed threshold.…”