Interannual Shift of the Tropical Upper-Tropospheric Trough and Its Influence on Tropical Cyclone Formation over the Western North Pacific

Abstract

The east-west migration of the tropical upper-tropospheric trough (TUTT) on the interannual time scale and its influence on tropical cyclone (TC) formation over the western North Pacific (WNP) is investigated in this study. Climatologically, the TUTT can be identified from 100 to 400 hPa with a relative vorticity maximum between 150 and 200 hPa. In addition to the strong westerly vertical wind shear in the south flank of the TUTT, this study shows that the cold-core system is associated with low relative humidity and subsidence to the east of the trough axis. As a result, the TC formation is enhanced (suppressed) in the eastern portion of the WNP when the TUTT shifts eastward (westward) on the interannual time scale. The interannual TUTT shift is closely associated with the SST anomalies in the central and eastern tropical Pacific or ENSO phases. The warm (cold) phase of ENSO corresponds to the eastward (westward) shift of the TUTT. The physical factors found to be responsible for the influence of ENSO on TC formation can be associated with the east-west shift of the TUTT. It is shown that the interannual variations of TC formation in the eastern part of the WNP basin are closely associated with the east-west shift of the TUTT due to the associated environmental conditions that are generally not favorable for TC formation.

Perspective on the northwestward shift of autumn tropical cyclogenesis locations over the western North Pacific from shifting ENSO

“…It is consistent with Wang and Wu (2016) that all the TUTT eastretreat years can be identified as El Niño years. Besides, the WNP MT experiences a southeastward expansion (Fig.…”

“…2a-d, the climatological TUTT, also known as the mid-oceanic trough (e.g., Ferreira and Schubert 1999) or the mid-Pacific trough (Deng et al 2017), often elongates from east-northeast to west-southwest over the tropical oceans. According to Wang and Wu (2016), the WNP TCs often occur to the west of TUTT, mostly over the region of 115°E-160°E, 0°-20°N, due to the strong vertical wind shear to the southeast of the trough (e.g., Kelley and Mock 1982;Fitzpatrick et al 1995).…”

“…The variability of WNP TC/typhoon activity is strongly affected by the El Niño-Southern Oscillation (ENSO) (e.g., Lander 1994;Chan 2000;Wang and Chan 2002;Camargo and Sobel 2005;Camargo et al 2007;Zhao et al 2010;Kim et al 2011;Wang et al 2013;Wang and Wu 2016) and the Pacific Decadal Oscillation (PDO) (e.g., Wang and Liu 2016;Zhao and Wang 2016). Frequent late-season catastrophic super-typhoons thus led us to wonder how the change in the ENSO affects the WNP TCLs during autumn.…”

“…It is well known that the change in sea surface temperature (SST) related to the ENSO shift not only leads to a change in tropical convective activity pattern (e.g., Hu et al 2016b) and then the remote induction of Rossby waves via the perturbation of upper-level divergence (e.g., Sardeshmukh and Hoskins 1988), but also results in changes of large-range atmospheric and oceanic environments related to TC activity (Gray 1979) such as the Walker circulation, the WNP subtropical high, absolute vorticity, relative humidity, vertical wind shear, vertically-integrated moisture transport, and horizontal distribution of local SST (e.g., Chan 2006;Kim et al 2011;Zhang et al 2014;Choi et al 2015;Wang and Wu 2016;Huangfu et al 2017). For example, it has been found by many studies that a southeastward tendency of WNP TCLs in El Niño cases compared to La Niña cases (e.g., Camargo and Sobel 2005;Camargo et al 2007;Wang et al 2013;Zhao and Wang 2016) in both warm and cold phases of the PDO (Zhao and Wang 2016).…”

“…Existing theories also suggest that the east-west shifts of tropical upper-tropospheric trough (TUTT) and MT may induce changes in WNP TCLs (e.g., Lander 1994;Wu et al , 2015Zhao et al 2014b;Huang et al 2016;Wang and Wu 2016;Deng et al 2017;Huangfu et al 2017) on both interannual and interdecadal timescales. It was ever pointed out that the mean TCL showed a westward shift over the WNP mainly due to (1) the pronounced westward shift of TUTT (Wu et al 2015) and/or (2) the interdecadal change of the WNP MT (Huangfu et al 2017).…”