The tropical upper tropospheric trough (TUTT) in the North Pacific, also known as a mid-oceanic trough, is a semi-permanent feature that extends east-northeast to west-southwest roughly from 35°N in the eastern Pacific to 15°N in the western Pacific, which can be identified in the summertime 200 hPa wind field. Although sometimes the TUTT may promote tropical cyclone (TC) formation, the associated strong westerly vertical wind shear in its eastern flank generally limits the eastward extension of TC activity in the western North Pacific (WNP). In this study, we show that the east-west migration of the TUTT can have important implications on the shift of the mean TC formation on various time scales. Climatologically the TUTT can be identified from 100 hPa to 400 hPa with a relative vorticity maximum between 150 hPa and 200 hPa. In addition to the strong westerly vertical wind shear in its southern flank, the cool-core system is associated with low relative humidity and subsidence to the east of the trough axis. TC formation is enhanced (suppressed) in the eastern portion of the WNP when the TUTT shits eastward (westward) on the interannual time scale. A pronounced westward shift in the TUTT is found in all of the available reanalysis datasets during 1979-2012, suppressing TC genesis in the eastern portion (east of 145ºE) of the western North Pacific basin due to the enhanced vertical wind shear associated with the TUTT shift. As a result, the annual mean TC genesis longitude has significantly shifted westward since 1979. The impact of the global warming on the locations of the TUTT is also investigated by using CMIP5 outputs to infer the possible shift of the mean TC formation location. It is found that large spreads are found in the simulated mean locations of the TUTT in CMIP5 models. Based on the 17 selected climate models, the TUTT would experience an eastward shift by 0.75° and 2.5° longitudes under RCP4.5 and RCP8.5.
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