1) Seasonal variation of precipitation in Cherapunji India 2) Explain the main reasons why this variation happens especially in the late spring and summer months of April to September.
Look at figures 9.38 and 9.39 and text on adjoining paragraphs about Monsoon development in India and describe:
1) Seasonal variation of precipitation in Cherapunji India
2) Explain the main reasons why this variation happens especially in the late spring and summer months of April to September.
text on adjoining paragraphs:
The arrival of the monsoon is a classic feature of the climate of India, where residents eagerly watch and wait for the arrival of the crop-nurturing rains. The leading edge of the moist air is typically a well-developed boundary that moves north across the nation from June into mid-July. The humid air converges with a drier westerly flow of continental air, causing the humid air to rise; further lifting is provided by hills and mountains. Lifting cools the air to its saturation point, resulting in heavy showers and thunderstorms.
The summer monsoon season of southern and eastern Asia, which lasts from about June through September, leads to long-lasting periods of wet, rainy weather (the wet season) with surface winds blowing from sea to land (see Fig. 9.38b). Although the majority of rain falls during the wet season, it does not rain all the time. In fact, rainy periods lasting between 15 and 40 days are often followed by 3 to 15 days of hot, sunny weather, known as a monsoon break. As the Asian continent gradually cools from late summer into autumn, the summer monsoon flow gradually weakens and the winter monsoon flow begins spreading south. In India, the retreating monsoon typically crosses the nation in September and October at a more gradual pace than the monsoon onset. Often the boundary slows or stalls across far southern India in November and December, which can lead to heavy localized rains.
Many factors help create the monsoon wind system. The latent heat given off during condensation aids in the warming of the air over the continent and strengthens the summer monsoon circulation. Rainfall is enhanced by weak, westward-moving low-pressure areas called monsoon depressions. The formation of these depressions is aided by an upper-level jet stream. Where winds in the jet diverge, surface pressures drop, the monsoon depressions intensify, and surface winds increase. The greater inflow of moist air supplies larger quantities of latent heat, which, in turn, intensifies the summer monsoon circulation.
The strength of the Indian monsoon appears to be related to the reversal of surface air pressure that occurs at irregular intervals about every two to seven years at opposite sides of the tropical South Pacific Ocean. As we will see in Chapter 10, this reversal of pressure (which is known as the Southern Oscillation) is linked to an ocean-warming phenomenon known as El Niño. During an El Niño event, surface water near the equator becomes warmer than average over the central and eastern Pacific. Over the region of warm water we find rising air, huge convective clouds, and heavy rain. Meanwhile, to the west of the warm water (over the region influenced by the summer monsoon), sinking air inhibits cloud formation and convection. Hence, during El Niño years, monsoon rainfall is likely to be deficient. A similar periodic reversal of pressure, called the Indian Ocean Dipole (IOD), takes place over the Indian Ocean. A positive IOD leads to rising air and heavy rain toward the northwest Indian Ocean, and monsoon rainfall over India tends to increase as well. The summer of 2019 brought one of the strongest positive IOD events on record, and India’s monsoon rains were the heaviest in 25 years.
Summer monsoon rains over southern Asia can be truly extreme. The town of Cherrapunji (also known as Sohra), located about 300 km inland on the southern slopes of the Khasi Hills in northeastern India, receives an average of 1176 cm (463 in.) of rainfall each year, most of it between April and October (see Fig. 9.39). The town also holds world records for the heaviest rainfall measured anywhere in a 12-month period—2647 cm (1042 in.) from August 1860 to July 1861—and the heaviest 48-hour rainfall, 249.3 cm (98.15 in.) on June 15–16, 1995.
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