Winter

Near the surface, this is the season of the out-blowing 'winter monsoon', but aloft westerly airflow dominates. This reflects the hemispheric pressure distribution. A shallow layer of cold high-pressure air is centred over the continental interior, but this has disappeared even at 700 mb (see Figure 7.4) where there is a trough over East Asia and zonal circulation over the continent. The upper westerlies split into two currents to the north and south of the high Tibetan (Qinghai-Xizang) Plateau

Easterly And Westerly Jet Stream
10°N 20°N 30°N 40°N 50°N 60°N 70°N 80°N 90°N

Figure 11.17 Distribution of wind velocity (km/hr) and temperature (°C) along the 90°E meridian for January and July, showing the westerly jet streams (JW) and the tropopause. Note the variable intervals in the height and latitudinal scales.

Source: After Pogosyan and Ugarova (1959), courtesy of Meteorologiya Gidrologiya (Moscow).

(Figure 11.17), to reunite again off the east coast of China (Figure 11.18). The plateau, which exceeds 4000 m over a vast area, is a tropospheric cold source in winter, particularly over its western part, although the strength of this source depends on the extent and duration of snow cover (snow-free ground acts as a heat source for the atmosphere in all months). Below 600 mb, the tropospheric heat sink gives rise to a shallow, cold plateau anticyclone, which is best developed in

December and January. The two jet stream branches have been attributed to the disruptive effect of the topographic barrier on the airflow, but this is limited to altitudes below about 4 km. In fact, the northern jet is highly mobile and may be located far from the Tibetan Plateau. Two currents are also observed further west, where there is no obstacle to the flow. The branch over northern India corresponds to a strong latitudinal thermal gradient (from November to April) and it is

Air Flow Over India During Winter

Figure 11.18 The characteristic air circulation over South and East Asia in winter. Solid lines indicate airflow at about 3000 m, and dashed lines at about 600 m. The names refer to the wind systems aloft.

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

Figure 11.18 The characteristic air circulation over South and East Asia in winter. Solid lines indicate airflow at about 3000 m, and dashed lines at about 600 m. The names refer to the wind systems aloft.

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

probable that this factor, combined with the thermal effect of the barrier to the north, is responsible for the anchoring of the southern jet. This branch is the stronger, with an average speed of more than 40 ms-1 at 200 mb, compared with about 20 to 25 m s-1 in the northern jet. Where the two unite over north China and south Japan the average speed exceeds 66 m s-1 (Figure 11.19).

Air subsiding beneath this upper westerly current gives dry out-blowing northerly winds from the subtropical anticyclone over northwest India and Pakistan. The surface wind direction is northwesterly over most of northern India, becoming northeasterly over Burma and Bangladesh and easterly over peninsular India. Equally important is the steering of winter depressions over northern India by the upper jet. The lows, which are not usually frontal, appear to penetrate across the Middle East from the Mediterranean and are important sources of rainfall for northern India and Pakistan (e.g. Kalat, Figure 11.20), especially as it falls when evaporation is at a minimum. The equatorial trough of convergence and precipitation lies between the equator and about latitude 15°S (see Figure 11.16).

Some of these westerly depressions continue eastward, redeveloping in the zone of jet stream confluence about 30°N, 105°E over China, beyond the area of subsidence in the immediate lee of Tibet (see Figure 11.18). It is significant that the mean axis of the winter jet stream over China shows a close correlation with the distribution of winter rainfall (Figure 11.21). Other depressions affecting central and north China travel within the westerlies north of Tibet or are initiated by outbreaks of fresh cP air. In the rear of these depressions are invasions of very cold air (e.g. the buran blizzards of Mongolia and Manchuria). The effect of such cold waves, comparable with the northers in the central and southern United States, is to greatly reduce mean temperatures (Figure 11.22). Winter mean temperatures in less-protected southern China are considerably below those at equivalent latitudes in India; for example, temperatures in Calcutta and Hong Kong (both at approximately 22.5 (N) are 19°C and 16°C in January and 22 °C and 15°C in February, respectively.

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