Spring

The key to change during this transition season is again found in the pattern of the upper airflow. In March, the upper westerlies begin their seasonal migration northward, but whereas the northerly jet strengthens and begins to extend across central China and into Japan, the southerly branch remains positioned south of Tibet, although weakening in intensity.

In April there is weak convection over India, where the circulation is dominated by subsiding air originating

Figure 11.19 Mean 200-mb streamlines and isotachs in knots over Southeast Asia for January and July, based on aircraft reports and sounding data.

Source: From Sadler (1975b) courtesy Dr J. C. Sadler, University of Hawaii.

Figure 11.19 Mean 200-mb streamlines and isotachs in knots over Southeast Asia for January and July, based on aircraft reports and sounding data.

Source: From Sadler (1975b) courtesy Dr J. C. Sadler, University of Hawaii.

along the convective ITCZ trough centred over the equator and following the overhead sun northward over the warm Indian Ocean (see Figure 11.16). The weather over northern India becomes hot, dry and squally in response to the greater solar radiation heating. Mean temperatures in Delhi rise from 23°C in March to 33°C in May. The thermal low-pressure cell (see Chapter 9H.2) now reaches its maximum intensity, but although onshore coastal winds develop, the onset of the monsoon is still a month away and other mechanisms cause only limited precipitation. Some precipitation occurs in the north with 'westerly disturbances', particularly towards the Ganges Delta, where the low-level inflow of warm, humid air is overrun by dry, potentially cold air, triggering squall lines known as nor'westers. In the northwest, where less moisture is available, the convection generates violent squalls and dust storms termed andhis. The mechanism of these storms is not fully known, although high-level divergence in the waves of the subtropical westerly jet stream appears to be essential. The early onset of summer rains in Bengal, Bangladesh, Assam and Burma (e.g. Chittagong, Figure 11.20) is favoured by an orographically produced trough in the upper westerlies, which is located at about 85 to 90°E in May. Low-level convergence of maritime air from the Bay of Bengal, combined with the upper-level divergence ahead of the 300 mb trough, generates thunder squalls. Tropical disturbances in the Bay of Bengal are another source of these early rains. Rain also falls during this season over Sri Lanka and southern India (e.g. Minicoy, Figure 11.20) in response to the northward movement of the equatorial trough.

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