1) Indian Summer Monsoon
The Indian subcontinent lies along the western flank of
the Southeast Asian summer monsoon system, and receives most of its
annual rainfall during June–September. Climatologically, two rainfall
maxima with totals exceeding 1200 mm are observed over India and the
surrounding Indian Ocean during the monsoon season (Fig.
53a). The first maximum is centered over the Bay of Bengal and
extends northwestward into eastern and central India. The second maximum
is located along the west coast of India on the windward side of the
Western Ghats Mountains. A relative minimum in rainfall of less than 500
mm is found in the climatological mean between these two regions in the
southeastern state of Tamil Nadu. Relatively low climatological rainfall
totals of 400–600 mm are also observed across northern India, while the
lowest totals (200–400 mm) are observed in northwestern India in the
vicinity of the Great Indian Desert.
During the 1999 monsoon season, precipitation was
below-average for the Indian subcontinent as a whole (Fig.
53b), with much of the rainfall deficit observed during July and
53c). The largest precipitation deficits were observed along the
west coast, in the northwest and in the south. However, the percent of
normal seasonal mean precipitation (not shown) varied considerably along
the west coast, with approximately 1/3 of normal rainfall recorded in
Surat (north of Bombay) and 125 % of normal rainfall observed in
Mangalore (south of Bombay). Elsewhere, above-average rainfall was
observed over portions of northeastern India, with departures exceeding
200 mm in some locations (Fig.
Also during 1999, eastern India was affected in late
October by two major Tropical Cyclones, which moved inland from the Bay
of Bengal with maximum sustained winds of up to 300 kph (190 mph). The
largest impacts from these cyclones were felt in the state of Orissa,
located near 20°N in eastern India. Approximately 10 million people were
left homeless from these cyclones, with millions also losing their crops
and their access to clean water and health services. The human death
toll from these cyclones was estimated to be over 10,000, and the cattle
death toll was estimated at 175,000.
Historically, above- (below-) average Indian monsoon
rainfall has been generally associated with the cold (warm) phase of the
ENSO. However, Krishna Kumar et al. (1999) note that this relationship
has become markedly less clear since the 1980s. Recent examples of this
unclear relationship between Indian rainfall and the ENSO cycle include
the above-average 1997 rainy season which occurred during strong El Niño
conditions, and the below-average 1999 rainy season which occurred
during La Niña conditions.
In contrast to Indian rainfall alone, the overall
India/ Asia monsoon circulation does exhibit a fairly strong
relationship with the ENSO cycle. Perhaps one of the most important
components of this circulation is the upper-level monsoon ridge, which
reflects the overall strength of the entire southeast Asian monsoon
complex and not simply rainfall over India. This monsoon ridge tends to
be suppressed during significant El Niño episodes and enhanced during La
Niña episodes. For example, the monsoon ridge was particularly
suppressed during the 1997 season when strong El Niño conditions were
54a). In contrast, it was enhanced and extended westward during both
the 1998 (Fig.
54b) and 1999 monsoon seasons (Fig.
54c) in association with La Niña conditions. These coherent
variations in the Asian monsoon ridge were also evident in the
subtropics of both hemispheres extending from the eastern Pacific
eastward to Australasia. As noted in section
3e(2), this large-scale anomaly pattern represents the leading mode
of atmospheric variability on both interannual and interdecadal time
scales (Mo and Kousky 1993).
2) Central China Rainfall
The largest rainfall totals across central China
[indicated by the red, boxed region in Fig.
53b] typically occur between April and September (black curve, Fig.
53d), with an area-averaged peak total approaching 190 mm in June.
Overall, rainfall in this region was well above average during
April–September 1999 (Fig.
53b), with above-normal totals observed in every month except
53d). The increased rainfall was observed primarily across the
eastern and southern portions of central China, with the largest
anomalies exceeding 400 mm in the east. This above-average rainfall was
associated with a much larger area of increased precipitation that
covered the western tropical and subtropical North Pacific, the entire
South China Sea, Indonesia and the eastern half of the Equatorial Indian
Ocean (see section 3, Fig.