| dc.description.abstract | Previous studies had shown that roughly more than 80% of tropical cyclones/ tropical
depressions(TC/TDs) develop in the monsoon gyres over the western North Pacific and the
activity of the monsoon gyres is associated with MJO (30-60 days oscillation) development.
In addition, MJO also affects the track of TC/TDs, more recurving (straight moving) TC/TDs
occur during MJO low (high). When the recurving TC/TDs go northward, they will be
accelerated by the mid-latitude westerlies. Thus, one may wonder: Whether or not the fast
moving TC/TDs are always recurving? Using the 1979-2012 JTWC TC/TD best track dataset,
we found that the top 10% fast moving TC/TDs are recurving, while the last 10% TC/TDs
which have slow moving speed cluster in the South China Sea. Based upon this preliminary
result, two major issues are raised: (1)Why the slow moving TC/TDs do not go northward and
eventually embed in the mid-latitude westerlies, but the fast moving TC/TDs do? (2)Why the
initial speed is significantly higher for fast moving TC/TDs than those slow ones immediately
after they are formed?
In order to answer these two concerns, the NCEP-DOE Reanalysis-2 data are used for the
composite analysis to examine the environmental differences between the fast and slow
moving types of TC/TDs circulations. It appears that an anomaly low located between south
of Japan and east of Taiwan is conducive to both the further development and northward
migration of the fast moving TC/TDs. In contrast, the anomaly low is confined to the South
China Sea and west of Philippines with an anomaly high toward its northeast side for the
slow moving TC/TDs situation so that the slow moving TC/TDs are limited in this south region.
To better understand the dynamic mechanism causing the initial speed differences between
the fast and slow moving TC/TDs, the vorticity budget analysis is applied to evaluate the
contribution of each term in the equation from two days before to two days after TC/TDs
genesis. It is interesting that the time series of vorticity tendency for the fast moving TC/TDs
show significant increase whereas the slow moving ones decrease gradually during these
five-day initial stages. After careful investigation, it turns out that the advection term, not
only the horizontal distribution but also the time variation, is similar to tendency term.
Apparently, the differences in initial speed between fast and slow moving TC/TDs are
dominated by the advection effect. | en_US |