Yangtze River flooding: July-August 2002
The Yangtze River Valley extends from the Tibetan Plateau to eastern
China and is the third longest river in the world. Approximately one-third
of the population of China lives in this area. Monthly rainfall in the
catchment basin of the Yangtze River (approximated by the red-boxed region
in Fig. 1a-c) is normally largest during March-June with peak totals
exceeding 200 mm during May and June (red curve, Fig.
Seasonal rainfall was above average throughout southern China during
JJA 2002, with the largest surpluses of 300-500 mm observed in the Yangtze
River Valley (Fig. 1b). Local seasonal totals ranged from 700-900 mm
throughout this region (Fig. 1a), exceeding 900 mm in the southwest. In
the East these amounts were 150%-200% of normal, while they exceeded 200%
of normal in parts of the West (Fig. 1c).
For the region as a whole area-averaged rainfall totals during these
three months reached almost 850 mm (blue curve, Fig.
1d), double the climatological mean (red curve). The area-averaged totals exceeded 350 mm
in June, while the totals approached 250 mm and exceeded the 90th
percentile in both July and August (green bars).
This excessive rainfall during July and August resulted primarily from
two prolonged convective episodes. The first occurred during the second
half of July and the second occurred in mid-August, as indicated by daily
precipitation time series’ at three recording stations in the Yangtze
River Valley (Figs. 2a-c). During the July convective episode a daily
extreme total of 225 mm (9 inches) was recorded at Yueyang on 19 July (Fig.
2a). Daily totals at this station remained above 12.5 mm in each of
the subsequent five days, and exceeded 25 mm on three of these days. Both
Changde (Fig. 2b) and Lingling (Fig. 2c) also recorded several days with
totals exceeding 25 mm during the period.
During the second convective episode both Changde and Yueyang recorded
rainfall amounts above 25 mm on six of nine days between 12-20 August.
Lingling also recorded daily totals near 50 mm on three of these days.
For the entire16 July - 31 August period Yueyang recorded 700 mm (27.5
inches) of rain compared to a normal of 200 mm (top panel, Fig.
2a). Both Changde (top panel, Fig. 2b) and Lingling (top panel,
Fig. 2c) recorded 500 mm (20 inches) of rain compared to normals of 175 mm and 190 mm,
b. Atmospheric circulation
The climatological mean monsoon ridge at 200-hPa extends from the
Middle East eastward across southern China during July-August (Fig.
3a). Southern China is situated beneath this ridge axis, which is consistent
with the convective nature of the precipitation that typically impacts the
region during the summer months. At 850-hPa a lee trough is evident in the
climatological mean extending from a low-pressure center over southern
Mongolia southward to Vietnam (Fig. 3b). East of this trough axis a
southerly flow of deep tropical moisture extends from the South China Sea
to northeastern China and Mongolia.
The excessive convective precipitation in the Yangtze River Valley
during July-August 2002 can be linked to three circulation features. The
first is an anomalous mid-latitude wave pattern at upper-levels spanning
the northern and eastern flanks of a suppressed monsoon ridge (Fig.
4a). This pattern featured a strong, persistent trough over central and eastern
China during more than 70% (Fig. 5) of the period in association with a
disappearance of the monsoon ridge from the region. Southeastern China was
situated in the region downstream of this mean trough axis in a region of
the flow known to be dynamically conducive to large-scale ascending
The anomalous wave pattern also featured an enhanced ridge over
northwestern China and Mongolia. This combination of circulation anomalies
is consistent with the historical relationship between large seasonal
rainfall departures in the Yangtze River Valley and the upper-level height
anomalies. For the entire 1979-2002 period the two anomaly fields are
negatively correlated at –0.6 over central China and positively
correlated at 0.5 over Mongolia (Fig. 6a).
The second anomalous circulation feature associated with the Yangtze
River flooding is a trough and strong wind shift line at 850-hPa extending
east-to-west across southern China approximately coincident with the
Yangtze River (Fig. 4b). This circulation acted to confine deep tropical
moisture and convective instability to southeastern China in the area
downstream of the mean upper-level trough axis, and also established the
boundary along which the two major convective episodes were observed.
The two major convective periods are also linked to the interaction
between major mid-latitude cyclogenesis events over central and
southeastern China (Fig. 7a, b) and the convectively unstable air mass
within the quasi-stationary low-level trough axis. Each of these
amplifying troughs (compare Fig. 7c, e and d, f) culminated in a closed
cyclonic circulation to the northwest of the Yangtze River Valley late in
their respective periods (Fig. 7e, f). Bell et al. (1999) also related
enhanced convection in the Yangtze River Valley during JJA1998 to a series
of powerful mid-latitude disturbances moving well south of their normal
position in response to a poorly developed monsoon ridge, combined with a
persistent low-level cyclonic circulation and strong wind shift line
centered over the heart of the Valley.
Nearly all of the annual precipitation in Mongolia occurs during
June-August (red curve, Fig. 1e). This period coincides with a shift in
the jet stream to north of the Tibetan Plateau in association with the
development of the Asian monsoon ridge. In Mongolia the lowest seasonal
rainfall totals are normally observed in the southwest and the largest
totals are observed in the northeast. For the country as a whole
(indicated by blue-boxed region in Fig. 1a-c) area-averaged seasonal
precipitation totals normally reach 125 mm, with 50 mm falling in both
July and August (red curve, Fig. 1e).
Seasonal rainfall totals during JJA 2002 averaged less than 50 mm
across much of the southwest, less than 100 in the central and
northwestern regions, and slightly above 100 mm in the northeast (Fig.
1a). The seasonal totals were less than 50% of normal for much of the
country (Fig. 1c), with deficits exceeding 100 mm in the north-central
part of the country, and ranging from 50-100 mm in the central and
southern regions (Fig. 1b).
This suppressed rainfall occurred during July and August, when
area-average totals reached only 25 mm (blue curve, Fig.
1e) and fell below the 10th percentile (green bars). The exceptionally light
and sporadic rainfall in both months is highlighted by a time series of
daily rainfall totals at Altai (Fig. 2d), situated in west-central
Mongolia, where measurable rain occurred on only six days and totals
reached 10 mm on only two days.
Rainfall in Mongolia is typically related to mid-latitude disturbances
propagating within the cyclonic shear side of the mid-latitude jet stream
along the poleward flank of the Asian monsoon ridge (Fig.
4a). Seasonal variations in this rainfall tend to be associated with quasi-persistent,
large-amplitude circulation anomalies embedded within the mean westerlies.
During July-August 2002 the significantly below-average rainfall in
Mongolia can be related to three factors. The first is a persistent
upper-level ridge through the depth of the troposphere over the western
half of the country (Fig. 4). The second is the previously described
southward shift in major cyclogenesis events to central China (Fig.
7). The third is a complete disappearance of the low-level trough and
southerly flow of moist air (Fig. 4b) that is evident over the southern
portion of the country in the climatological mean (Fig.