Temperature‐related trends in the vertical position of the summer upper tropospheric surface of maximum wind over the Northern Hemisphere

2006

J. Geophys. Res.

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[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,

Section: Introductionsupporting

confidence: 68%

mentioning

confidence: 99%

See 1 more Smart Citation

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

Strong

2006

J. Geophys. Res.

Self Cite

“…Here the latitude of the subtropical jet is defined as the latitude of the most equatorward local maximum in the zonal‐mean zonal wind field in the upper troposphere and lower stratosphere (UTLS) in each hemisphere, capturing the meridional position of the subtropical jet core. We use an algorithm similar to what was used by Strong and Davis [] to find the surface of maximum wind, the pressure of the maximum zonal‐mean zonal wind in the UTLS, in each grid point's column. First, each zonal‐mean zonal wind column is scanned from the surface to 50hPa above the tropopause to find the maximum column wind speed.…”

Section: Methods and Diagnosticsmentioning

confidence: 99%

Seasonal to multidecadal variability of the width of the tropical belt

Birner

2013

JGR Atmospheres

An expansion of the tropical belt has been extensively reported in observations, reanalyses, and climate model simulations, but there is a great deal of uncertainty in estimates of the rate of widening as different diagnostics give a wide range of results. This study critically examines robust diagnostics for the width of the tropical belt to explore their seasonality, interannual variability, and multidecadal trends. The width based on the latitudes of the maximum tropospheric dry bulk static stability, measuring the difference in potential temperature between the tropopause and the surface, is found to be closely coupled to the width based on the subtropical jet cores on all time scales. In contrast, the tropical belt width and Northern Hemisphere edge latitudes based on the latitudes at which the vertically averaged stream function vanishes, a measure of the Hadley circulation's poleward edges, lag those of the other diagnostics by approximately 1month. The tropical belt width varies by up to 10° latitude among the diagnostics, with trends in the tropical belt width ranging from −0.5 to 2.0° per decade over the 1979–2012 period. Nevertheless, in agreement with previous studies, nearly all diagnostics exhibit a widening trend, although the stream function diagnostic exhibits a significantly stronger widening than either the jet or dry bulk stability diagnostics. Finally, GPS radio occultation observations are used to assess the ability of the reanalyses to reproduce the tropical belt width, finding that they better situate the latitudes of maximum bulk stability versus those of the subtropical jets.

“…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.…”

Section: The Surface Of Maximum Windmentioning

confidence: 99%

Winter jet stream trends over the Northern Hemisphere

Strong

Quart J Royal Meteoro Soc

2007

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Trends in the speed and probability of winter jet stream cores over the Northern Hemisphere were measured for 1958-2007, and related changes in the thermal structure of the troposphere were identified. Eddy-driven jet (EDJ) core speeds and probabilities increased over the midlatitudes (40-60°N), with changes as large as 15% (speed) and 30% (probability). These increasing trends are collocated with increases in baroclinicity driven by a spatially heterogeneous pattern of height change consisting of large-scale warming with cooling centres embedded poleward of 60°N. The cooling centres reduced high-latitude baroclinicity, making jet cores poleward of 60°N less frequent and weaker. Over the west and central Pacific, subtropical jet stream (STJ) core probabilities remained relatively constant while core speeds increased by as much as 1.75 m/s decade −1 in association with Hadley cell intensification. The STJ shifted poleward over the east Pacific and Middle East, and an equatorward shift and intensification of the STJ were found over the Atlantic basin-contributing to an increased separation of the EDJ and STJ.