What is monsoon?What are the different hypotheses for the origin of monsoon?(

Although precise definition of monsoon is lacking but it can be used to connote a wind which blows with consistency & regularity during a part of the year & is absent or blows from another direction for the rest of the year which is due to the heat received by different parts of the Earth.

       In Genetic sense, monsoon is associated with following features:-

1) A system with seasonal marked wind shifts due to differential heating of land & sea;

2) winds that are largely confined to tropics between 20(degrees)N-20(degrees)S;

3) Summer monsoon may be thought of a South-East trade winds of southern hemisphere which on crossing the equator are deflected to the right by the Earth's rotation & approach the land from south-west.

              In the recent years hypotheses for the development of monsoon is changing very rapidly. More investigations have been made to study the monsoon system in detail in the recent years. Many information data has been collected to throw light on the genesis of monsoon.However,the mechanism of monsoon in a world till today spears to be complex in nature & yet to be fully explained.

The different hypotheses on monsoon are as follows:-

1) Convectional system:-According to this hypothesis monsoon is of thermal origin formed due to extensive heating of land & sea surface, & monsoon wind is a convectional system of giant scale. Pressure gradient between landmass & sea is the driving force. In this situation pressure & potential temperature surfaces don’t coincide & available potential energy is converted into kinetic energy, therefore, intensity of monsoon depends on the efficiency of conversion clouds modulate the radiational balance between land & sea. Potential energy formed due to differential heating is converted into kinetic energy of monsoon winds.

2)Jefferey's Model:-It is one of the earliest mathematical treatment of monsoon circulation. He indicated a reversal in wind direction at an altitude of 2.1 Km. its computation coincides with the absence of monsoon circulation. The wins of lower levels are westerly winds while above 6 Km. they become easterly. However, the theories’ shortcomings are:-

a)To try & get over the difficulties arising out of the lock of Geostrophic balance at low latitudes;&

b)It's necessary to learn more about differential heating.

3)Radon technique:-In 1960's this technique was employed, to locate the origin of monsoon. The output of radon is more to be observed over the land than over the seas/oceans.& viceversa.An experiment in 1960 by the soviet research vessel indicated that monsoon wind is entirely of land origin, since experts have found low radon content in S.E.Trade winds, a few degrees south of the equator. Over the Arabian sea the radon concentration is often 10 times larger.

                   If the monsoon air is entirely of land origin ,with high radon content, then it won’t have sufficient time to pick up an adequate moisture amount during its travel across the Arabian sea.The vertical moisture column is only 1 Km. at 65 degrees East while it is 6 Km. at 75 degrees East,when it strikes the Malabar coast. This is'nt possible by high radon content airmass of land origin.

3)Heat engine/Thermal engine concept:-It was proposed by P.Koteswaram.He suggested that easterly jet stream results from the thermal effect of the elevated Tibetan plateau.Re-radiation from this plateau warm uo the mid-troposphere & setting up a strong solenoidal field in upper troposphere for driving a clockwise circulation.During it's rise ,the air spreads out southwards & gradually sinks over the Equatorial region of Indian ocean.The sinking air is deflected to the right by the easterly rotation & moves towards India as return current from south westerly direction.The theory is confronted with 2 questions:-

a)Is indian monsoon thermally driven by heat source over the Tibetan plateau?

b)Whether there is descending moto of air over the Equatorial regions of Indian ocean.

4)Diabetic source & sinks hypotheses:-In 1960's an attempt to infer monsoon’s origin by inferring the diabetic heating rates from the mean vertical motion over Indian region. The processing of diferent data revealed an interesting fact & was found to maintain a semi-monsoon over India:-

a)Warming at thr rate of 3.2 degree celcius per day over N.E.India;

b)Diabetic cooling over N.W.India at the rate of 2.4 degree Celsius per day.

                                                                                                                                                                       

       The table indicates ascending air over north-east India & descending air over north-west india.The energy is provided by the moisture laden air supplying latent heat on a macro-scale. It is observed that the winter & summer monsoons were associated wit diabetic heat sources & sinks oriented from east-south-east to west-north-west direction. The source-sink pattern tended to move north-westwards in the progression from the winter to summer monsoon & it was linked with sun's declination we have learnt about diabetic sources & sinks as a consequence of MONEX.

5)Wave energetics hypothesis:-Here origin of monsoon is believed to be due to wave energetics.Several computations have appeared on recent years. The main flow of energy is from ultra-long waves to zonal motions in upper troposphere. The smaller waves receive energy from zonal motion & larger quasi-stationery waves. The peak occurs in transfer process around wave number-8 which occurs mostly in upper troposphere.

            The substantial amount of conversion of available potential energy to kinetic energy derived through east-west overturnings.Upper tropospheric anticyclone which anchor over the monsoon regime is representative of wave number-1 & 2 & is in an area where major conversions of potential energy to kinetic energy has occured.

           This is accomplished by the easterly Jet stream over India with it's core around 150mb. & it's location varying with different phases of monsoon (summer).It's understood that diabetic heat sources & sinks display south-east to north-west orientation. Cellular motions like Walker's cell & Hadley's cell also help in conversion of energy. A seventh potential energy is converted into kinetic energy in a Walker cell. These has been observed by Krishnamurthy et al.

7)Upper tropospheric vorticity budget hypothesis:-Krishnamurthy has computed mean velocity potential & stream function for the upper troposphere.seasonal mean vorticity was minimum when divergence was maximum over Tibet because observations revealed mean vorticity & divergence fields were found to be out of phase by half circle.Discrepencies has occurred over the computed & observed divergences.

If large scale divergence has strong fluctuations with time then divergence & vorticity fluctuations could possibly provide required dampening. Upper air convergence will result in surface divergence while upper tropospheric divergence will bring surface air convergence .Hence, on-shore/off-shore winds is individually controlled by mechanism.