Early summer

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Generally, during the last week in May the southern branch of the high-level jet begins to break down, becoming intermittent and then gradually shifting northward over the Tibetan Plateau. At 500 mb and below, however, the plateau exerts a blocking effect on the flow and the jet axis there jumps from the south to the north side of the plateau from May to June. Over India, the equatorial trough pushes northward with each weakening of the upper westerlies south of Tibet, but the final burst of the monsoon, with the arrival of the humid, low-level southwesterlies, is not accomplished until the upper-air circulation has switched to its summer pattern (see Figures 11.19 and 11.23). Increased continental convection overcomes the spring subsidence and the return upper-level flow to the south is deflected by the Coriolis force to produce a strengthening easterly jet located at about 10 to 15 °N and a westerly jet to the south of the equator (see Figure 11.16). One theory suggests that this takes place in June as the col between the subtropical anticyclone cells of the western Pacific and the Arabian Sea at the 300-mb level is displaced northwestward from a position about 15 °N, 95 °E in May towards central India. The northwestward movement of the monsoon (see Figure 11.24) is apparently related to the extension over India of the upper tropos-pheric easterlies.

The recognition of the upper airflow has widespread effects in southern Asia. It is directly linked with the Maiyu rains of China (which reach a peak about 10 to 15 June), the onset of the southwest Indian monsoon and the northerly retreat of the upper westerlies over the whole of the Middle East.

Average Monthly Rainfall India

Figure 11.20 Average monthly rainfall (mm) at six stations in the Indian region. The annual total is given after the station name.

Source: Based on 'CLIMAT' normals of the World Meteorological Organization for 1931 to I960.

Figure 11.20 Average monthly rainfall (mm) at six stations in the Indian region. The annual total is given after the station name.

Source: Based on 'CLIMAT' normals of the World Meteorological Organization for 1931 to I960.

It must nevertheless be emphasized that it is still uncertain how far these changes are caused by events in the upper air or indeed whether the onset of the monsoon initiates a readjustment in the upper-air circulation. The presence of the Tibetan Plateau is certainly of importance even if there is no significant barrier effect on the upper airflow. The plateau surface is strongly heated in spring and early summer (Rn is about 180 W m-2 in May) and nearly all of this is transferred via sensible heat to the atmosphere. This results in the formation of a shallow heat low on the plateau, overlain, at about 450 mb, by a warm anticyclone (see Figure 7.1C). The plateau atmospheric boundary layer now extends over an area about twice the size of the plateau surface itself. Easterly airflow on the southern side of the upper anticyclone undoubtedly assists in the northward shift of the subtropical westerly jet stream. At the same time, the pre-monsoonal convective activity over the southeastern rim of the plateau provides a further heat source, by latent heat release, for the upper anticyclone. The seasonal wind reversals over and around the Tibetan Plateau have led Chinese meteorologists to distinguish a 'Plateau Monsoon' system, distinct from that over India.

4 Summer

By mid-July, monsoon air covers most of South and Southeast Asia (see Figure 11.23), and in India the equatorial trough is located at about 25 °N. North of the Tibetan Plateau there is a rather weak upper westerly current with a (subtropical) high-pressure cell over the

Jet Stream Over Asia

Figure 11.21 The mean winter jet stream axis at 12 km over the Far East and the mean winter precipitation over China in cm.

Source: After Mohri and Yeh; from Trewartha (1958). Copyright © Erdkunde. Published by permission.

Figure 11.21 The mean winter jet stream axis at 12 km over the Far East and the mean winter precipitation over China in cm.

Source: After Mohri and Yeh; from Trewartha (1958). Copyright © Erdkunde. Published by permission.

Butterfly Veins

Figure 11.22 Seasonal depression paths and frequencies over China and Japan, together with typical paths of winter cold waves.

Sources: Compiled from various sources, including Tao (1984), Zhang and Lin (1984), Sheng et al. (1986) and Domros and Peng (1988). Reproduced by permission of Springer-Verlag, Berlin.

Figure 11.22 Seasonal depression paths and frequencies over China and Japan, together with typical paths of winter cold waves.

Sources: Compiled from various sources, including Tao (1984), Zhang and Lin (1984), Sheng et al. (1986) and Domros and Peng (1988). Reproduced by permission of Springer-Verlag, Berlin.

Figure 11.23 The characteristic air circulation over South and East Asia in summer. Solid lines indicate airflow at about 6000 m and dashed lines at about 600 m. Note that the low-level flow is very uniform between about 600 and 3000 m.

Sources: After Thompson (1951), Flohn (1968), Frost and Stephenson (1965), and others.

East Asian Monsoon Domroes And Peng
Figure 11.24 Mean onset date of the summer monsoon over South and East Asia.

Source: After Tao Shi-yan and Chen Longxun. From Domrös and Peng (1988). Reproduced by permission of Professor Tao Shi-yan and the Chinese Geographical Society, and Springer-Verlag.

Air Circulation The Middle East

Figure 11.23 The characteristic air circulation over South and East Asia in summer. Solid lines indicate airflow at about 6000 m and dashed lines at about 600 m. Note that the low-level flow is very uniform between about 600 and 3000 m.

Sources: After Thompson (1951), Flohn (1968), Frost and Stephenson (1965), and others.

Westerly Jet Tebet

Figure 11.25 The easterly tropical jet stream. (A) The location of the easterly jet streams at 200 mb on 25 July 1955. Streamlines are shown in solid lines and isotachs (wind speed) dashed. Wind speeds are given in knots (westerly components positive, easterly negative). (B) The average July rainfall (shaded areas receive more than 25 cm) in relation to the location of the easterly jet streams.

Figure 11.25 The easterly tropical jet stream. (A) The location of the easterly jet streams at 200 mb on 25 July 1955. Streamlines are shown in solid lines and isotachs (wind speed) dashed. Wind speeds are given in knots (westerly components positive, easterly negative). (B) The average July rainfall (shaded areas receive more than 25 cm) in relation to the location of the easterly jet streams.

Source: From Koteswaram (1958).

Figure 11.26 The percentage contribution of the monsoon rainfall (June to September) to the annual total.

o plateau. The southwest monsoon in South Asia is overlain by strong upper easterlies (see Figure 11.19) with a pronounced jet at 150 mb (about 15 km), which extends westward across South Arabia and Africa (Figure 11.25). No easterly jets have been observed so far over the tropical Atlantic or Pacific. The jet is related to a steep lateral temperature gradient, with the upper air getting progressively colder to the south.

An important characteristic of the tropical easterly jet is the location of the main belt of summer rainfall on the right (i.e. north) side of the axis upstream of the wind maximum and on the left side downstream, except for areas where the orographic effect is predominant (see Figure 11.25). The mean jet maximum is located at about 15 °N, 50 to 80 °E.

Sources: After Rao and Ramamoorthy, in Indian Meteorological Department (1960); and Ananthakrishnan and Rajagopalachari, in Hutchings (1964).

Figure 11.26 The percentage contribution of the monsoon rainfall (June to September) to the annual total.

The monsoon current does not give rise to a simple pattern of weather over India, despite the fact that much of the country receives 80 per cent or more of its annual precipitation during the monsoon season (Figure 11.26). In the northwest, a thin wedge of monsoon air is overlain by subsiding continental air. The inversion prevents convection and consequently little or no rain falls in the summer months in the arid northwest of the

Isobars Show Northeast Monsoon

Figure 11.27 Monsoon depressions of 12:00 GMT, 4 July 1957.

(A) shows the height (in tens of metres) of the 500 mb surface;

(B), the sea-level isobars. The broken line in (B) represents the equatorial trough, and precipitation areas are shown by the oblique shading.

Figure 11.27 Monsoon depressions of 12:00 GMT, 4 July 1957.

(A) shows the height (in tens of metres) of the 500 mb surface;

(B), the sea-level isobars. The broken line in (B) represents the equatorial trough, and precipitation areas are shown by the oblique shading.

Source: Based on the IGY charts of the Deutscher Wetterdienst.

Source: Based on the IGY charts of the Deutscher Wetterdienst.

Monsoon Trough Charts

Figure 11.29 The location of the monsoon trough in its normal position during an active summer monsoon phase (solid) and during breaks in the monsoon (dashed).* Areas 1 to 4 indicate four successive daily areas of heavy rain ([more] 50 mm/day) during the period 7 to 10 July 1973 as a monsoon depression moved west along the Ganges valley. Areas of lighter rainfall were much more extensive.

Figure 11.29 The location of the monsoon trough in its normal position during an active summer monsoon phase (solid) and during breaks in the monsoon (dashed).* Areas 1 to 4 indicate four successive daily areas of heavy rain ([more] 50 mm/day) during the period 7 to 10 July 1973 as a monsoon depression moved west along the Ganges valley. Areas of lighter rainfall were much more extensive.

Source: *After Das (1987). Copyright © 1987. Reproduced by permission of John Wiley & Sons, Inc.

Figure 11.28 The normal track of monsoon depressions, together with a typical depression pressure distribution (mb).

Source: After Das (1987). Copyright © 1987. Reproduced by permission of John Wiley & Sons, Inc.

subcontinent (e.g. Bikaner and Kalat, Figure 11.20). This is similar to the Sahel zone in West Africa, discussed below.

Around the head of the Bay of Bengal and along the Ganges valley the main weather mechanisms in summer are the 'monsoon depressions' (Figure 11.27), which usually move westward or northwestward across India, steered by the upper easterlies (Figure 11.28), mainly in July and August. On average, they occur about twice a month, apparently when an upper trough becomes superimposed over a surface disturbance in the Bay of Bengal. Monsoon depressions have cold cores, are generally without fronts and are some 1000 to 1250 km across, with a cyclonic circulation up to about 8 km, and a typical lifetime of two to five days. They produce average daily rainfalls of 1200 to 2000 mm, occurring mainly as convective rains in the southwest quadrant of the depression. The main rain areas typically lie south of the equatorial or monsoon trough (Figure 11.29) (in the southwest quadrant of the monsoon depressions, resembling an inverted mid-latitude depression). Figure 11.30 shows the extent and magnitude of a particularly severe monsoon depression. Such storms occur mainly in two zones: (1) the Ganges valley east of 76 °E; (2) a belt across central India at around 21°N, at its widest covering 6° of latitude. Monsoon depressions also tend

Severe Midlatitude Climate
Figure 11.30 Rainfall (mm) produced in three days over a 50,000-km2area of central India northeast of Nagpur by a severe, westward-moving monsoon depression, during September 1926.

Source: Dhar and Nandargi (1993). Copyright © John Wiley & Sons Ltd. Reproduced with permission.

to occur on the windward coasts and mountains of India, Burma and Malaya. Without such disturbances, the distribution of monsoon rains would be controlled to a much larger degree by orography.

A key part of the southwest monsoonal flow occurs in the form of a 15 to 45 m s-1 jet stream at a level of only 1000 to 1500 m. This jet, strongest during active periods of the Indian monsoon, flows northwestward from Madagascar (Figure 11.31) and crosses the equator from the south over East Africa, where its core is often marked by a streak of cloud (similar to that shown in Plate 14) and where it may bring excessive local rainfall. The jet is displaced northward and strengthens from February to July; by May it has become constricted against the Abyssinian Highlands, it accelerates still more and is deflected eastward across the Arabian Sea towards the west coast of the Indian peninsula.

This low-level jet, unique in the trade wind belt, flows offshore from the Horn of Africa, bringing in cool waters and contributing to a temperature inversion that is also produced by dry upper air originating over Arabia or East Africa and by subsidence due to the convergent upper easterlies. The flow from the southwest over the Indian Ocean is relatively dry near the equator and near shore, apart from a shallow, moist layer near the base. Downwind towards India, however, there is a strong temperature and moisture interaction between the ocean surface and the low-level jet flow. Hence, deep convection builds up and convective instability is released, especially as the airflow slows down and converges near the west coast of India and as it is forced up over the Western Ghats. A portion of this southwest monsoon airflow is deflected by the Western Ghats to form 100km diameter offshore vortices lasting two to three days

Axis The Low Level Somali Jet

Figure 11.31 The mean monthly positions (A) and the mean July velocity (m s-1) (B) of the low-level (1 km) Somali jet stream over the Indian Ocean.

Source: After Findlater (1971), reproduced by permission of the Controller of Her Majesty's Stationery Office.

Figure 11.31 The mean monthly positions (A) and the mean July velocity (m s-1) (B) of the low-level (1 km) Somali jet stream over the Indian Ocean.

Source: After Findlater (1971), reproduced by permission of the Controller of Her Majesty's Stationery Office.

and capable of bringing 100 mm of rain in twenty-four hours along the western coastal belt of the peninsula. At Mangalore (13°N), there are on average twenty-five rain-days per month in June, twenty-eight in July and twenty-five in August. The monthly rainfall averages are 980, 1060 and 580 mm, respectively, accounting for 75 per cent of the annual total. In the lee of the Ghats, amounts are much reduced and there are semi-arid areas receiving less than 64 cm per year.

In southern India, excluding the southeast, there is a marked tendency for less rainfall when the equatorial trough is furthest north. Figure 11.20 shows a maximum at Minicoy in June, with a secondary peak in October as the equatorial trough and its associated disturbances withdraw southwards. This double peak occurs in much of interior peninsular India south of about 20°N and in western Sri Lanka, although autumn is the wettest period.

There is a variable pulse alternating between active and break periods in the May to September summer monsoon flow (see Figure 11.16) which, particularly at times of its strongest expression (e.g. 1971), produces periodic rainfall (Figure 11.32). During active periods the convective monsoon trough is located in a southerly position, giving heavy rain over north and central India and the west coast (see Figure 11.16). Consequently, there is a strong upper-level outflow to the south, which strengthens both the easterly jet north of the equator and the westerly jet to the south over the Indian Ocean. The other upper-air outflow to the north fuels the weaker westerly jet there. Convective activity moves east from the Indian Ocean to the cooler eastern Pacific with an irregular periodicity (on average forty to fifty days for the most marked waves), finding maximum expression at the 850-mb level and clearly being connected with the Walker circulation. After the passage of an active convective wave there is a more stable break in the summer monsoon when the ITCZ shifts to the south. The easterly jet now weakens and subsiding air is forced to rise by the Himalayas along a break trough located above the foothills (see Figure 11.16), which replaces the monsoon trough during break periods. This circulation brings rain to the foothills of the Himalayas and the Brahmaputra valley at a time of generally low rainfall elsewhere. The shift of the ITCZ to the south of the subcontinent is associated with a similar movement and strengthening of the westerly jet to the north, weakening the Tibetan anticyclone or displacing it northeastward. The lack of rain over much of the subcontinent during break periods may be due in part to the eastward extension across India of the subtropical high-pressure cell centred over Arabia at this time.

It is important to realize that the monsoon rains are highly variable from year to year, emphasizing the role played by disturbances in generating rainfall within the generally moist southwesterly airflow. Droughts occur with some regularity in the Indian subcontinent: between 1890 and 1975 there were nine years of extreme drought (Figure 11.33) and at least five others of significant drought. These droughts are brought about by a combination of a late burst of the summer monsoon and an increase in the number and length of the break periods. Breaks are most common in August to September, lasting on average for five days, but they may occur at any time during the summer and can last up to three weeks.

The strong surface heat source over the Tibetan Plateau, which is most effective during the day, gives rise to a 50 to 85 per cent frequency of deep cumulonimbus

Figure 11.32 Mean daily rainfall (mm) along the west coast of India during the period 16 May to 30 September 1971, showing a pronounced burst of the monsoon followed by active periods and breaks of a periodic nature. All years do not exhibit these features as clearly.

Source: After Webster (1987b). Copyright © 1987. Reproduced by permission of John Wiley & Sons, Inc.

Figure 11.32 Mean daily rainfall (mm) along the west coast of India during the period 16 May to 30 September 1971, showing a pronounced burst of the monsoon followed by active periods and breaks of a periodic nature. All years do not exhibit these features as clearly.

Source: After Webster (1987b). Copyright © 1987. Reproduced by permission of John Wiley & Sons, Inc.

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