detailed description of winter monsoon

It is important to consider the winter monsoon for 2 reasons:-

a) It corresponds to the night cycle of the sea breeze & like the summer monsoon is an important component of the global monsoon.

b) It has the capacity to generate seasonal rainfall over the southern part of Indian peninsula, Sri Lanka, Malaysia & Indonesia.

The onset of winter monsoon is not clearly defined & at times there is no distinction between withdrawal of summer monsoon & the onset of winter monsoon. There is a period of transition of about a week for the winter rains to st in Tamil Nadu.Tamil Nadu experiences north-east monsoon from October to December. Intense convection over Indonesia generates precipitation & releases of latent heat subsequently. This is an ascend corresponding to the rising limb of the Hadley's cell. the ascending air spreads to the north & south which isn't possible for the ascending air over Tibet during the summer monsoon because the northwards air movement was checked by the Himalayas.

               The northward moving air of the ascended heat by the rising limb of the Hadley's cell descends over the intense anticyclonic region over Siberia & China. The southward branch of the moving air is associated with return current that forms Australian monsoons.Sub-tropical westerly jet stream is a characteristics feature of the winter monsoon. During the winter monsoons the descending branch of the Walker's cell lie over the Pacific but this isn’t well documented so far.

                       Rains during the winter monsoon occurs in spells, accompanied by strong surges of cold air that emanates from a large anticyclone results in development over Siberia & China. Occasionally ,outbreak of cold air from the results in development of a family of low pressure systems over the equatorial regions of the South China sea & western Pacific  which moves slowly westwards bringing  widespread rain over  Sarawak & east coastal Malaysia. Sometimes these low pressure systems intensify  into depressions with heavy rains often causing severe floods.

              Meteorologists wonder the cause of intensification of these low pressure systems. The interaction of low pressure systems & outbreak of cold air from Asian anticyclone is not well understood. Many scientists have tried to introduce the concept of instability at the interface of the two air masses having different meteorological characteristics.

                       The first air mass is extension of Asian Anticyclone while the second one is generated by the northern near-equatorial trough. although measurements of upper winds suggests that on occasions necessary conditions for intensifying an incipient disturbance are satisfied but the scientists haven’t been able to show that these conditions are self sufficient for disturbance formation.

NORMAL WEATHER CONDITIONS DURING WINTER MONSOONS:-

Storms & depressions:-Like Malaysia Indonesia, peninsular India receives rain in winter du to westward passage of storms & depressions. The vortices which bring rains are remnants of low pressure systems that effect Malaysia & crossover to the Bay of Bengal & progress towards Tamil Nadu. the tracks of these storms 7 depressions lie along 5(degrees)N - 15(degrees)N. Storms with tracks south of 15(degrees)N, enter the Arabian sea after traversing the peninsular India. As the storms enter over the Arabian sea area it intensifies & a good number of them recurves towards northern sector of the west coastal India. The dependence of winter rain on number of cyclonic storms 7 depressions is very pronounced in southern Indian peninsula, which is more true for coastal districts than in the interior parts of peninsular India. After the occurrence of the highest seasonal rains between October to December of about 75 cm along south-eastern coastal Tamil Nadu 7 southern adjoining parts of Andhra Pradesh is followed by gradual decrease in precipitation except for a small area in north-east of Kerala where precipitation is caused due to western Ghats.

Normal pressure:-

During the north-east monsoons ,a large system of low pressure over the central parts of Bay of Bengal which often extends to Indian peninsula by the way of a trough & into central sectors of the Arabian sea as low pressure system. there is a progressive shift from normal wind pattern occur whenever there is a depressions or tropical cyclone in bay of Bengal. General orientation of low pressure system & fluctuations in it's intensity govern rainfall. Rain in southern peninsula of India is good when trough is well defined & seasonal low over the Bay of Bengal.

Normal winds:-

the mean upper winds over the peninsular India is from north to south or from north-east in lower levels of the atmosphere upto 1.5 Km over the north-easterly flow is the easterly winds, above 3 Km wind flows in southerly direction & is a part of circulation around a large anticyclonic cell over Bay of Bengal. there is no easterly jet stream over peninsular India. Wide variations from mean winds on daily weather charts are linked with passage of depressions or Tropical cyclones. Any distinct relationship between location of anticyclonic cell over the middle & upper reaches of the atmosphere & rainfall over peninsular India couldn’t be established.

Normal temperature distribution:-

Warmest areas in Tamil Nadu are located approximately near the 10(degrees)N & temperature decreases northwards. the northward temperature gradient increases steadily with the height up to 300 mb at which level the temperature drops by as much as 8(degrees)Celsius as we move north from 20(degrees)N to 30 (degrees)N.