The tenth IMSC, Beijing, China, 2007 - International Meetings on ...

ways were considered. Four zones, from the north to the south in the east half of <strong>China</strong>, were
divided according to the different features of climatic jumps. A significant jump signal in
summer rainfall happened in the late 1970s and early 1980s, almost simultaneous over these
four zones. Increasing jumps are found over the Northeast and Yangtze-Huai area, while
decreasing jumps over North <strong>China</strong> and the South to Yangtze River. In North <strong>China</strong>, another
decreasing jump happened in the mid-1960s. <strong>The</strong> step-wise decreases of rainfall in North
<strong>China</strong> have caused severe droughts with widespread concerns.
For North <strong>China</strong>, rainfall amount was categorized into six grades with fixed intervals:
rainstorm, heavy rain, moderate rain, light rain, trace rain and no-rain. <strong>The</strong> number of each
grade rainfall days in summer was examined. <strong>The</strong> larger the grade is, the more similarity is
between the changes of the number of the grade rainfall days and the overall rainfall amount.
Particularly, there was a decreasing jump in the 1960s, clearly for the series of total rainfall
amount and the number of rainfall days for rainstorm but unclearly for others; the decreasing
jump in the 1980s appeared in all series except that of light rain.
For relative extremes defined against a climatological seasonal cycle at each station, the
extreme rainfall frequency and strength in North <strong>China</strong> also experienced two decreasing jumps,
a weak one in the mid 1960s and a strong one in 1979-1980. However, the proportion of the
extreme rainfall amount to the annual rainfall amount exhibited different trends, without
significant jumps. <strong>The</strong> increasing proportion of extremes since the 1980s suggested more
extreme rainfall events under an overall dryer background in recent decades than before. <strong>The</strong>
interannual variability of the frequency and that of the extreme strength are similar. Changes in
atmospheric circulation such as the East Asian summer monsoon were analyzed to explain the
typical drying process in North <strong>China</strong>.
Water vapour transport from the tropical Atlantic and summer rainfall over tropical southern
Africa
Nicolas Vigaud
UCT, Dept of Oceanography
nvigaud@ocean.uct.ac.za
Yves Richard
Universite of Bourgogne, Centre de Recherche en Climatologie
Mathieu Rouault
UCT, Dept of Oceanography
Nicolas Fauchereau
Universite of Bourgogne, Centre de Recherche en Climatologie
An EOF analysis of the onshore flow of moisture along the west coast of southern Africa
using NCEP-DOE AMIP II Re-analyses suggests two dominant modes of variability that are
linked to (a) variations within the South Atlantic anticyclone (b) the intensity of the flow that
penetrates from the tropical Atlantic. <strong>The</strong> second mode, referred as the Equatorial Westerly
mode, contributes the most to moisture input from the Atlantic onto the subcontinent at tropical
latitudes.
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