Yangtze River flooding: July-August 2002

a. Rainfall

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. 1d).

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 90^th 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, respectively.

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 motion.

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.

 MONGOLIA

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 10^th 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. 3b).