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

Abstract

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 120E or near Taiwan and 145E) 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. © Copyright 2012 by the American Geophysical Union.