Have steering flows in the western North Pacific and the South China Sea changed over the last 50 years?

Long-term trends in steering flows over the western North Pacific (WNP) and the South China Sea (SCS) are examined during the peak typhoon season. A nonparametric and robust trend detection method is employed. Both the NCEP and ERA reanalysis data sets suggest a statistically significant decreasing trend in steering flows in the subtropical region of the western WNP ( between 120 degrees E or near Taiwan and 145 degrees E) and the northern SCS during 1958-2001. Over this period, the decrease in the WNP is quite large with a magnitude of 1.1 m s(-1) given that the background mean steering flow is only 3.26 m s(-1). This decrease corresponds approximately to one third of the mean flow. When the data are extended from 1958 to 2009 the long-term decrease in steering flows in the aforementioned subtropical region are still significant, although more modest at a rate of 0.7 m s(-1). Time series of translational speeds averaged over the same subtropical region also exhibit a slow-down of storms' motion over the last 52 years. This is consistent with the weakening of easterly steering flows analyzed from independent data sets. Results of this study imply a longer life span for tropical cyclones and a greater tendency for storms along prevailing typhoon tracks to recurve. Citation: Chu, P.-S., J.-H. Kim, and Y. Ruan Chen (2012), Have steering flows in the western North Pacific and the South China Sea changed over the last 50 years?, Geophys. Res. Lett., 39, L10704, doi: 10.1029/2012GL051709.