| The surface of maximum wind (SMW) concept and algorithm are developed as a way to objectively locate all tropospheric jet streams and upper tropospheric wind maxima in pressure space. 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 all tropospheric jet streams extending into the lower stratosphere. Wind speed data obtained from the SMW are more representative of jet streams than data obtained from nearby isobaric surfaces because the latter contain speed variability associated with variability in the proximity of jet cores. A Northern Hemisphere SMW climatology is developed using NCEP/NCAR Reanalysis data for 1958--2004, including SMW pressure (P˜ ), jet core pressure (P˜core), jet core speed (V˜core), and jet core probability (C˜).; A summer SMW trend indicates significant descent over most the tropics with 47-year changes as large as 100 hPa. In the extratropics, winter P˜ variability is strongly influenced by stratospheric and tropospheric temperature variability linked to the Arctic Oscillation (AO). In the tropics, winter P˜ fluctuates with the El NiNo/Southern Oscillation (ENSO). Winter jet core probabilities (C˜) are increasing over the high latitudes of North America, the Atlantic, and Europe. The high latitude C˜ increases indicate contraction of the polar front jet and are closely linked to the longwave structure and contraction trends of the circumpolar vortex. C˜ variability equatorward of the circumpolar vortex indicates an equatorward shift in the subtropical jet (STJ) and increased meridionality in the STJ over the Atlantic. Positive trends in winter V˜core are as large as 3 m s-1 decade-1and are related to the combined effects of tropospheric warming in the subtropics and tropospheric cooling over the northern oceans. The four Northern Hemisphere teleconnection patterns that contribute most strongly to variability in the probability and speed of jet cores are the AO, the North Atlantic Oscillation (NAO), the Pacific/North American Pattern (PNA), and the ENSO. |
