1.14 Lateral Interaction with Mid-latitude Flow

The large-scale kinetic energy spectra of mid-latitude weather disturbances in the troposphere and lower stratosphere are more continuous than that in the tropics. The spectra reveals that only certain types of the mid-latitude disturbances can propagate and can exit in the tropics. On the other hand, there are trapped waves in the tropics, such as Kelvin waves and Yanai-Maruyama waves, that have maximum amplitudes within the tropics (Gill, 1982).

1.14.1 Monsoon

The precise definition of planetary-scale monsoon is a much debated issue. The word "monsoon" derives from the Arabic word "mausin", meaning "season". In general, the term monsoon means a dramatic change in persistent wind direction according to season. Monsoons are important to TC forecasting because they affect mean flow patterns of tropospheric wave disturbances, and affect TC formation, motion and structure (Ramage, 1971).

Monsoons can be viewed as three-dimensional circulations associated with the global distribution of land and sea. For example, Asian monsoon has often been compared to a giant land-sea breeze. The seasonal variations of monsoon usually can be seen as the change of amplitude of long waves. Some common features in both summer and winter monsoons are:

a. Lower tropospheric winds flow from a high-pressure area into a low-pressure area.

b. The low-pressure area is usually formed as a belt-shaped zone, and is called the monsoon trough. Monsoon troughs are typically associated with low-level convergence and cyclonic vorticity, and cloudiness.

c. The wind speeds between the high-pressure area and the monsoon trough are usually concentrated into a jet pattern. This is referred to as a monsoon jet, monsoon surge, a low-level jet, or a cross equatorial jet. Features associated with the jet typically have local characteristics.

d. The monsoonal circulation has vertical structure. The rising air along the monsoon trough diverges out of an upper-tropospheric anticyclone along an upper-level jet-like flow.

Monsoon indices have been developed to determine which areas of the world actually experience monsoons. These indices typically overestimate the monsoonal areas, including such regions as the Russian Arctic and the Gulf of Alaska. Ramage (1971) and Hastenrath (1994) listed four criteria which define a region as monsoonal:

(a). the prevailing wind direction shifts by at least 120 degrees between January and July,

(b). the mean frequency of prevailing wind directions in January and July exceeds 40%,

(c). the mean resultant winds in January and July exceed 3 m/s, and

(d). fewer than one cyclone-anticyclone alternation occurs every two years in either January or July in a 5° latitude-longitude rectangle.

1.14.3 Indian-Asian Southwest Summer Monsoon

The Indian-Asian summer monsoon is the most widely studied. During the winter, the temperature typically decreases poleward of the equator. With the heating of the continents during summer, especially the Tibetan plateau, the temperature gradient is reversed. The region of highest temperature is now located near the southern base of the Himalayas. The winter northeasterly flow (Fig. 2.6) reverses to southwesterly, lasting from May to October. The surface wind chart for July ( Fig. 2.8) is typical of the monsoonal flow pattern. The southeasterlies wind, trade wind, of the South Indian Ocean turn southerly, then southwesterly north of the equator. A speed maximum, often referred to as the Somalia jet, exists in the Arabian Sea near the Gulf of Aden.

The influence of the Indian-Asian monsoon extends from eastern Africa well into the western North Pacific near 150°E during August (see Fig. 2.4). As mentioned in the general circulation section, the monsoon trough of the western North Pacific is poorly depicted on a mean chart due to its migratory nature.

Aloft, a large anticyclone is situated over the Tibetan Plateau. The easterlies to the south serve as the return branch of the surface monsoonal flow. This results in large, persistent vertical shear, making tropical cyclogenesis very rare during these months. During the period 1975-1990, there were an average of 0.5 TCs in June, 0.0 in July, 0.1 in August, and 0.2 in September.

Of all of the phenomena associated with the Indian-Asian monsoon, the onset is probably the most important and interesting. The interest lies in the suddenness of the onset, which can typically be determined to the day using rainfall as the indicator. The onset typically occurs during May. But the actual date of the onset varies throughout the region. Table 2.1 provides the typical onset date for various regions.

1.14.4 Southeast Asian Winter Monsoon

The Southeast Asian winter monsoon ( Fig 2.6) owes its existence in large part to the Tibetan plateau. The east-west orientation of the Himalayas blocks the synoptic scale exchanges of cold polar air with warm tropical air. The only avenue of exchange is east of the Himalayas, over southeast Asia. Consequently, cold air from the Siberian anticyclone flows southward across eastern China, over the South China Sea, and into Southeast Asia. Numerical modeling studies show that if the Himalayan mountains were "removed", the Siberian anticyclone would no longer be a semi- permanent feature. Rather, the region would resemble North America with transitory cyclones and anticyclones.

The onset of the northeast monsoon is far more distinct than the summer monsoon. Cold air begins to penetrate North Vietnam during late August and September. By the end of October, the entire Indochina Peninsula is covered by the winter northeasterlies. The monsoon typically lasts through March.

During the northeast monsoon season, the South China Sea region is characterized by strong vertical shear. The low-level monsoon northeasterlies are overlaid with strong westerlies from India. This environment makes tropical cyclogenesis rare. More importantly, TCs moving into this region are typically sheared, with the low-level circulation steered to the southwest.

As with the summer southwest monsoon, the winter monsoon, while persistent, experiences surges and lulls. Surges occur when an upper-level trough develops over northern China and moves off the east coast 24 hours later. Lulls are associated with surface bubble highs moving off the east coast of China, creating easterly or southeasterly flow over the South China Sea.

1.14.5 Northwest Australian Summer Monsoon

Like its Indian counterpart, the Australian summer monsoon (Fig. 2.2) is largely driven by the strong heat lows which form over Northern Australia. However, since Australia lacks a mountain range like the Tibetan Plateau, the monsoonal flow is neither as strong nor as steady as the Indian monsoon.

The onset of the monsoonal flow typically occurs during January. As the southeast Asian winter monsoon strengthens, its flow penetrates deeper and deeper into the tropical latitudes. By January, the flow crosses the equator near Indonesia and turns westerly. These equatorial westerlies flow across New Guinea, the Solomon Islands, and Northern Australia. The season lasts until April when easterly trades return ( Fig 2.3), covering the entire area south of about 10°S.

As with the Indian monsoon, a semi-permanent anticyclone is located over the surface heat lows. Upper-level southeasterlies to the north of this anticyclone cross the equator, providing the return flow of the Hadley circulation. This reversal in flow (upper-level southeasterlies over low-level northwesterlies) results in a large vertical shear similar to the Indian summer monsoon. However, in this case, the shear region is only located equatorward of 10°S. Thus, unlike the Indian monsoon season, tropical cyclogenesis still occurs during the Australian monsoon season, but is confined to south of 10°S.

1.14.6 African Monsoon

The magnitude and the thickness (less than 6 km) of the air layers of monsoonal circulations over Africa are smaller and shallower than that over Asia. In the West Africa, a large continental area north of the equator to about 15°N, there is a difference between the two lower tropospheric monsoon winds: northeasterly trade winds in January and southerlies to southwesterlies in July. The air masses which the two monsoon winds bring are different. The northeasterly trade winds prevail to an elevation of about 3000 m and bring dry, stable, and often dusty air, these winds are called the "harmattan". The southwesterly monsoon winds are warm and humid.

In the East Africa, the continent stretches on both sides of the equator, these two monsoon winds differ only in direction and the air masses which they bring are similar. In January the ITCZ is located at about 15°S and most of East Africa is under the influence of northeasterly winds, which become northwesterlies south of the equator. In July the ITCZ is situated at about 15°N and most of East Africa is under the influence of southeasterly and southerly winds.

Section 1.13 Section 1.15

Chapter 2