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 August (Fig. 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. 53b).
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 Niņo conditions, and
the below-average 1999 rainy season which occurred during La Niņa 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 Niņo
episodes and enhanced during La Niņa episodes. For example, the monsoon ridge was
particularly suppressed during the 1997 season when strong El Niņo conditions were
present (Fig. 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 Niņa 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 September (Fig. 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.
15).
