 In the last lecture, we have started talking about what is the basic system responsible for the monsoon. We already talked about the first hypothesis for this system, namely that the monsoon is a gigantic land sea breeze. Now, in this lecture, we will talk about the evolution of the ideas about the basic system. You may recall that in the last lecture, we ended with comments by Simpson, which said that the land sea breeze theory is not tenable because we cannot understand the observed spatial and temporal variability of the monsoon on the basis of land sea breeze theory. We cannot explain it by land sea temperature contrast. So, now, let us look at how the ideas about the basic system have evolved and that is what I propose to do here. And before we do that, let us remind ourselves what is it we are trying to understand, what is it we are trying to explain. And it is of course, the seasonal variation of rainfall and wind. Now, here I have the rainfall for December, January, February and it is over the equatorial region and June, July, August and the monsoon has come over land, there is rain here. Now, the winds also change in direction. As you see, they are in December, January, February from the northeast and they change to southwest here. I want you to also note an important feature of the winds in June, July, August or summer monsoon season, namely that there is a crossing that occurs. These are the southeast trails and they cross over. They cross over into the as southwest winds when they come to the Arabian Sea. This cross equatorial flow, you will realize is very important and many theories have in fact, talked in detail about that as well. Now, so what we are proposing to do in this lecture is to see how the ideas about the basic system responsible for the monsoon have changed over time. Now, as I mentioned, the first hypothesis is that the primary cause of the monsoon is the land ocean contrast and in temperature in the summer and that the basic system is a gigantic land sea breeze. Now, Simpson, who actually pointed out that this land sea breeze model for the monsoon cannot explain the observed variation of the rainfall over India, suggested an alternative hypothesis, an alternative hypothesis to land sea breeze. He suggested that the peculiar distribution of the mountain ranges over the Indian region and regions to its north and east plays an important role in the monsoon rainfall over the Indian region. So, this is a new theory and an important element of that theory is the peculiar topographic distribution. This is the mountains over India. This you see is the huge Tibetan plateau to the north and these are the western guards and there are of course, hills in the northeast region as well. Here you see it as you see it in an atlas. So, you have these Himalayas to the north and you have also high mountains on the Burmese coast and there are the western guards and eastern guards and of course, Vindhya's and Aravlis and so on and so forth. So, this is the peculiar mountain distribution over the country and these are the surface winds which you have seen before. This is the cross equatorial flow and now here it is you can see that the winds are coming almost at right angles to the western guards. So, what is Simpson's theory? Simpson has drawn these streamlines of the flow during the monsoon. They go this way and what he says is that the mountain ranges, these mountain ranges he is talking of the mountain ranges which I have marked as red. These mountain ranges in fact, are equivalent to two sides of a box. You can see on the north and on the east, there are mountain ranges. So, there are two sides of a box into which the air streams through the other two sides. So, these are the other two sides. The air is streaming through the other two sides, but the box is closed to the north as well as to the east. There are no openings in these two sides and the air is flowing from the other two sides. So what he is saying is that the mountain ranges which I had colored red are equivalent to two sides of a box into which the air streams through the other two sides. Now, there are no openings in these two sides. As the air pours in from the other sides, it is clear that within the area shown, the air must rise at least as high as the mountains. This is his logic. So, he is saying that the simple geographical relation of the mountains around India to the main currents necessitates an essential movement of air at least up to 6,000 feet that is the height of the mountains to the east which in reality is more or near 20,000 feet. So, actual ascent of air is very up to very very high levels. He is aware of that and he is saying minimum ascent up to 6,000 feet has to occur simply because that is the height of the walls of that box on the north and east. So, he is saying because there are walls there is forced ascent of air because air that is flowing in has to get out of the box and the only way it can do it is by ascending. So, it is this forced ascent that India it is to this force ascent that India owes its rainfall the essential currents due to India being warm are absolutely insignificant in comparison. So, he is saying the fact that land is warm compared to ocean it will generate some winds and ascent, but what is more important is this forced ascent due to the mountains. Now, this is the mean average rainfall July rainfall over India and this is of course from the C map which is partly satellite derived and partly based on land gauges, but it has a course resolution compared to the rainfall maps that we see from India Med Department. So, this is the high rainfall on the west coast that you see very very low rainfall here. This is Tamil Nadu coast this is the south eastern coast of the peninsula there we have very low rain very high on the west coast then very high here in the northeast including Cherapunji which gets one of the highest rainfalls and then the rainfall decreases as we go this way this is the main pattern. So, now he actually gives us explanation for why it varies with space the way we have seen. So, the first is the wet area on the west coast that we have seen we have also seen that the main air currents are almost at right angles to the western guards and forced to rise at least 4000 feet. Now, this air is warm and with humidity around 90 percent it therefore, contains much moisture condensation sets in at a height of about 500 feet and during the remainder of its ascent condensation continues giving rise to very heavy rain. See, this is something this is a process that we have seen earlier how forced ascent due to topography in this case western guards can give rise to condensation clouds and rain. Now, the ascent is naturally most sudden in the middle of the range while at the ends where the air can escape around instead of going to the top it is less rapid. So, what he is saying is if we look at the mountain ranges then this is the mountain range of western guards and right in the center it is difficult for the air to go around whereas towards the edges like here the air can kind of skirt around the mountains and go in this manner. So, he is saying that in fact, the strongest ascent he calls it ascent is naturally most sudden in the middle of the range while at the ends where the air can escape around instead of going to the top it is less rapid. So, in fact, right near the central part near Honawar you get highest rain on the west coast. So, this is the explanation for the west coast rainfall and this is something we are all very familiar with. Then he asked for why is it dry over the rest of the peninsula we have seen that as well that compared to the rain here this is the west coast high rain here and compared to that you have a dry hole here the rain is very less on the southeastern part of the peninsula this you have seen. Now, what is his explanation for that he says after the air stream has crossed over the main ridge of the guards it continues to the east, but now instead of rising it descends to the plains right it has already gone over the mountains and now it is descending to the plains. A descending air current is warmed by adiabatic compression this causes the rainfall to seize and the cloud particles to be rapidly evaporated. So, we have descending air current over the plains to the east of the guards and there the rainfall ceases because the cloud particles get rapidly evaporated. So, this is what you see here that you have high rain here and low rain here because the air is descending over this portion here after having ascended over the western guards. Now what about the wet regions on the coast of Burma the air currents over the south of the bay strike the mountain ranges on or near the coast of Burma giving heavy rain where they meet the mountains we can see his idealized figure of mountains again and you will see here see these are the ranges high mountain ranges on the coast of Burma and the air stream that he goes right at right angles to these will naturally have to ascend and that will give very high rainfall over the coast of Burma. So, now we have an explanation for why it rains on the west coast why it is dry over the peninsula and why there is so much rain over the coast of Burma. Now heavy rain at Chirapunji practically saturated air from the bay crosses the deltaic Bengal which at this time is one great swamp because it has already rained a lot there. This air current with the velocity of many miles per hour strikes the Khasi hills at right angles. Now, this is again Simpson's view of the topography simplified view and what you see here are the Khasi hills and so this air which is coming from the bay of Bengal going over what was then called Bengal this whole Bengal region which he claims is almost a swamp now actually encounters Khasi hills at right angles to that air stream. And so this air current with a sufficiently high velocity strikes the Khasi hills at right angles and has to rise at least 4000 feet. So, now you see the situation near Chirapunji where again you have very moist air rising at hitting the mountains at right angles and therefore, rising this is the Khasi hill thing. So, this is why you have very high rain at Chirapunji. Now what happens over the Gangetic plain and the sub montain region to the north now we have seen I think we should just see what the observations are on that side as we mentioned here the highest rainfall is here near the head bay and as we go towards the north west the rainfall is decreasing that is very clear here you can see the contours here the heavier rain is here and as we go steadily towards the north west the rainfall has decreased. So, this is what now he is trying to explain why does the rainfall decrease in the Gangetic plain from here to here remember the mountains are here. So, this is where we are trying to find out why the rainfall has decreased. So, now we have so this is the Gangetic plain and this is a stream line idealized picture of the stream line which comes here. So, it is air that has come over the Bay of Bengal and now is easterly over this region. So, this is the air current so the red stream line which I just showed you represents the air flow over the west of the Bay of Bengal this current arrives at the coast of Bengal heavily laden with moisture, but it means no obstruction directly. So, this is a case in which you see there is no obstruction here all the obstructions are here this air that comes across means no obstruction, but it gradually proceeds towards the west. He says this current arrives at the coast of Bengal heavily laden with moisture, but it means no obstruction directly on crossing the coast it is deflected to the west and passes right along the Gangetic plain and reaches as far as Punjab this we have seen. It is however passing into a region from which there is no outlet for air is flowing into the same region from the west and the south and the great Himalayan barrier impedes its motion to the north. So, his picture is as follows you can see that the air is flowing into this region from Arabian Sea as well you see this is the air coming from the Arabian Sea this is the air coming from the Bay of Bengal and so air is coming together here and it has nowhere to go because there is a wall here and there is a wall here this is the way he envisages the picture. So, what we have here is that it is deflected to the west and continues going east it is however passing into a region from which there is no outlet as we have seen because the air is flowing into the same region from the Arabian Sea as westerly flow as well as from the Bay of Bengal as easterly flow and there is the great Himalayan barrier to the north. So, it cannot go north. So, this current of air must therefore rise, but it does not do so suddenly, but gradually over the whole length from the bay to Punjab. So, this is his picture then that over this region there is no over this region then it ascends the air ascends because there is a lot of convergence of air, but it does so gradually it is not like you know going coming to a mountain and suddenly there being a sharp increase and sharp ascent it does so very gradually as it moves here and actually the ascent decreases as you go from Bengal to here. So, this is why the rainfall decreases as you go from here to here this is what his claim is. So, the current of air must therefore rise, but does not do so suddenly, but it rises gradually over the entire region and that is really his explanation for the rainfall over the monsoon zone now. Remember, it depends on there being a current from the Bay of Bengal air current that is to say winds from the Bay of Bengal which go towards the northwest. So, he says since it is now ascending and ascending right from the Bay of Bengal to the northwestern parts he says and the ascent is higher near the bay and slowly decreases as you go towards the northwest he says rainfall is greatest in the stream near the coast of Bengal and decreases slowly in its intensity as it penetrates over northern India. Now this all sounds like a good theory it is a it sounds reasonable, but the catch is that it depends on the winds or what he Simpson calls air currents being the way they are ok. But you know we know very well there are breaks in the monsoon in July when actually there are no easterlies at all. So, it is not we cannot really assume a given pattern for the low level wind and then figure out the rainfall see the wind itself depends on the rainfall. So, during breaks in fact there are no easterlies and this explanation cannot hold. So, this is a rather than saying this explanation does not hold this explanation sort of does not take into account the interaction between rainfall and wind. It assumes that the wind pattern is what it is just like it assumes the mountains to be what they are, but while the mountains remain steady the wind does not ok. Then comes the very interesting question of the dry region in the northwest ok. So, the case of the dry region in the northwest is rather complex and is made up of four factors he says each of which depends on the other and they may be stated as the following. Almost the small amount of moisture which reaches the low pressure area because of its peculiar geographical position. So, we have seen earlier now we are talking of the northwest region here here. So, he is saying relatively small amount of this moisture is reaching here this is one of the reasons because of its geographical location. He says the small amount of moisture which reaches the low pressure area because of its peculiar geographical position is one reason. The dry wind from the west in the upper troposphere now low humidity at the ground caused by high temperature which prevents condensation and as the air rises below the level of the upper wind. So, there is no condensation there are no deep clouds ok and this because of this the temperature of land is high and because of that there is low humidity at the ground ok. So, this is how all these factors are interconnected and the high temperature caused by the absence of cloud and rain is what he lists as the fourth important thing ok. So, it is difficult to this is my comment on Simpson's explanation. So, difficult to take what he has given as an explanation of the lack of rainfall over the northwestern part rather all the factors are consistent with the region being what is called a heat low and I will be talking about these heat lows these are low pressure regions which have no rainfall. I will be talking about these dynamical systems a little later in the lecture as well. So, again this is a matter of things being consistent that there is no cloud and rain the temperature is high ok. So, the humidity is low and so on and so forth and there is a lot of dry air coming into the region. So, now according to Simpson what how what is the basic system responsible for the monsoon what is the primary cause of the monsoon. So, although he has short down the Land C Breeze theory of the monsoon he still says that the primary cause of the monsoon is the difference of temperature over land and sea ok. So, that he still maintains is the important thing the relatively high temperature over land in the northern hemisphere during the summer tends to lower the pressure there as the pressure falls over land air motion results and the air motion is acted on by rotation of the earth which again modifies the original pressure. So, he is bringing in a lot of interactions here and the result of all these interactions each of which affects the others is a close low pressure system over the whole of Asia and North Africa with the lowest pressure in the northwest of India. The pressure distribution in the final system bears no relationship to the actual temperature in the different parts. This is an important point to recognize that the finally pressure and temperature patterns are not by any means identical. Then he goes on to say the air over the north Indian ocean including the Arabian sea and the Bay of Bengal takes part in the general motion set up by this huge pressure system which inverse low pressure extending right from Africa to India and high pressure in the southern hemisphere and this huge pressure gradient this huge pressure system sets up the air currents or the winds ok. So, the air takes part in this general motion set up by this pressure gradient as the pressure falls over land in the northern hemisphere the pressure rises over the ocean in the southern hemisphere. So, we have high pressure in the southern hemisphere a huge low pressure belt in the northern hemisphere the resulting air motion affected by the earth's rotation and the distribution of land in the southern hemisphere modifies the pressure distribution. The final result being a closed high pressure system in the south of the Indian ocean. So, he is again talking of how the pressure gradient is set up the air over south Indian ocean moves under the influence of this pressure distribution and the rotation of the earth towards the northwest as far as the equator. So, these are the south easterly trades going from the high pressure region to the south of the equator to the equator ok. At the equator the air from the south is caught up in the circulation around the low pressure system in the northern hemisphere and moves towards the northeast now this is because the Coriolis force changes sign as the air crosses the equator instead of pushing any wind to its right now to its left in the southern hemisphere now it is pushing it to its right. So, this is why we get south westerly flow and the southwest air motion over the north Indian ocean is therefore, a continuation of the southeast air motion over the south Indian ocean. This is why I pointed out to you this important cross equatorial flow which contributes moist air to the monsoon. So, this has been again emphasized and it has been shown that in fact the air that is crossing as the southwest monsoon over the Arabian sea emnates from the southern hemisphere as southeast trades this is what Simpson has pointed out. So, in consequence the air which reaches the Indian area has travelled for 4000 mines over the ocean and is therefore, highly charged with aqueous vapor. The southwest air current over the north Indian ocean which is impaled forward by forces extending over the whole region of its motion is directed towards the high mountains of India which are so arranged as to form a barrier to the north and the east. So, now you have a the scenario that Simpson has built up that you have a pressure gradient set up with high in the southern hemisphere low in the African Asian belt in the northern hemisphere air motion results from this pressure gradient and there is rotation of the earth and in consequence of all this you get southwest flow and the air is finally directed into this box as Simpson envisaged it with a wall in the north and wall in the east. So, in consequence the air actually which crosses the north Indian ocean in fact comes into the box which is formed by the mountains there. So, the air is caught in a trap from which there is no escape, but for rising this is Simpson's thing. So, Simpson concludes that we have seen that the heavy rain characterizing the monsoon is caused by the inflow of large quantities of damp warm air into the region on account of the peculiar distribution of mountain ranges it is forced to ascend and in consequence deposit its moisture as rain. So, he is saying that mountain ranges are primarily responsible are a major driving are a major element in making it possible for the large rainfall over India which we call the monsoon. But he further remarked that we have seen that the actual temperature over the Indian land area is not directly responsible for the existential currents which cause the rain. He is saying the actual temperature is not responsible for the rain it is not responsible for the ascending currents which are related to the rain on the contrary in the regions where the rainfall is prevented the temperature is abnormally high that is in the north western part of India while the greatest rainfall is accompanied by low temperature. So, this is Simpson's theory of the monsoon. Now, so although Simpson argued against considering the monsoon as a gigantic land sea breeze the basic system that he proposed is also special to the monsoonal region. He claims that it is because of the special configuration of the mountain ranges that we get ascent of air here and therefore, in fun. So, it is a system which is special to the monsoonal region resulting from an interaction of the air motion set up by land ocean contrast with the special topography of the region. So, this is what Simpson has argued for. Now, so all along we are thinking the thinking so far has been that the monsoon is the basic system responsible for the monsoon is a special system which occurs on over the monsoonal region. Whereas, however, been a different school of thought in which the monsoon is considered to be the seasonal manifestation of the seasonal variation of the tropical circulation and rainfall in response to the seasonal variation of the incoming radiation. So, thus the basic system responsible for the monsoon rainfall is assumed to be the same as that associated with the rainfall over the rest of the tropics that is ITCG, Alaa, Charney or the equatorial trop, Alaa, Reel. So, the second school of thought says the basic system that gives us rainfall over monsoonal region is not different from the ITCG which gives rainfall over the tropical pacific. But the amplitude of the seasonal variation in the location of the system is exceptionally large over monsoonal region. Now, the implications for the variability of the monsoon from these two high two kinds of schools of thought if you wish are entirely different because if the second hypothesis is true if the system is the same then some of the feedbacks which lead to the variability of the ITCG over the pacific should also operate over the monsoonal region. Whereas, if it is a special system one has to work out what leads to the variability by looking at the special system itself. It appears very interestingly now in fact several years I would say almost 50 years, 40 years before Simpson the idea that the monsoon is associated with the upper air and so over the Indian region of the rain belt which is present around the year over the equatorial Indian Ocean was first suggested by Blanford in his classic treatise. It is amazing how many of the so called new ideas that we think have come only in the modern era can be traced to this genius Blanford and which he has in fact expounded on in his very classic treatise this is rainfall of India monographed by Henry H. Blanford who was the meteorological reporter to the government of India and this is published as an Indian meteorological memoir 1886. So, this is a very classic memoir and in that how does he talk about the basic system responsible for the monsoon. So, he begins with the spring months he says during the spring months on the Bay of Bengal and the Arabian Sea the winds are light frequently alternating with calms and somewhat variable though chiefly from the south west in the former and northerly and in May westerly on the latter sea. So, he is saying during spring months the winds are weak and variable the change that ensures at the end of May or in June remember that is when the onset of monsoon over Kerala occurs this is what he is talking about the change that ensures at the end of May or in June when the surrounding seas are swept by a strong monsoon current and heavy continued rain sets in on the coast of India is very marked and has long been recognized in popular language as the burst of the monsoon. So, this is the onset over Kerala which has been popularly called burst of the monsoon for well over a century now. Then he talks of what is the essential cause the essential cause of the change change from the spring conditions where you have light and variable winds and hardly any rain over India to what happens during the burst of the monsoon. The essential cause of the change appears to be that during the spring months the sea winds which feed the storms of that season are the relatively damp but by no means saturated air of the surrounding seas. So, during the spring he says the air that comes that generates the storms around India is actually simply the sea wind which feed the damp but by no means saturated air of the surrounding seas. So, this is a local air sea interaction kind of thing. So, these are local winds that generate the storms or if saturated at least not so to any considerable vertical height above the sea. So, what he is saying is that the water vapor in the air is constrained to the lower layers of the air in the spring lower layers of the air in the spring but in more southerly latitude that is to say in our spring itself over the equatorial region in the neighborhood of the equator is a belt of the atmosphere corresponding to the doldrums of the Atlantic into which the southeast trades of the south Indian ocean for a study study supply of almost saturated air. So, he is saying in spring itself there is a rainy belt and I believe this is a picture this is a satellite picture in spring and what you see is that there is an equatorial band here and this is the band which is what Blanford refers to as a rainy belt and into this air is flowing moisture laden air is flowing from the southeast here these are the southeast trades this is what he is talking about. So, he says that in the more southerly latitudes there is a band which corresponds to doldrums in the Atlantic which is very similar to the ITCZ what we call now the ITCZ over the Atlantic into which southeast trades of the south Indian ocean for a study supply of almost saturated air. So, at all times of the year more or less convection goes on in this belt. So, this is what we refer to as the ITCZ or the equatorial trough. So, the ITCZ or the equatorial trough is located over the equator in our spring that is April May and he says that in fact at all times of the year some convection goes on here there is always some rain here as proved by its raininess in all the seasons and consequently it affords a great reservoir of air which up to an indefinite height above the sea surface borders at all times on a state of saturation. Now, remember he is talking in 1886 where we did not have upper air measurements and so he says that there is moisture up to an indefinite height above the sea surface. Now, we know up to what height it is there, but it is up to a so he is talking of a very very deep moist layer. So, what he is describing is an ITCZ over the equatorial region and the critical characteristics of the ITCZ in terms of the moisture moist air being water vapor layer extending to very very high levels that is what he is referring to. So, now it appears to be he continues this is Blanford again talking he has described what is happening over the equatorial region in the spring and then he says now it appears to be the eventual rush of this air towards the region of low pressure developed over India gradually during the summer months during the spring months sorry that constitutes the burst of the monsoon. So, he has a very clear notion that the burst of the monsoon involves this movement of this moist air in the ITCZ over this region. So, he is saying the coming of the ITCZ over our region constitutes the burst of the monsoon one started and this is a very important point he makes one started the energy of its movement is sustained by the condensation of its own copious water vapor by the latent heat so set free. So, this is a very important point he is making that once you have a tropical convergence zone going then the it is sustained by the latent heat of condensation within its own region and the energy of convection furnished by the heated atmosphere of the planes. So, he is talking of the you know the land ocean contrast that has been often emphasized. So, the energy of convection furnished by this kind of a gradient of temperature between land and ocean furnished by the heated atmosphere over the planes he says as compared to that over the ocean is competent only to keep up the febler in drought that obtains during the spring months. So, actually as Simpson had pointed out in May India is much hotter than in July. So, actually this land ocean contrast is very very high in May. So, there is considerable energy of convection furnished by this land ocean contrast, but all it can do is to generate a feeble in drought that obtains during the spring months and indeed even this in drought is maintained partly by the moderate condensation of cumulus and local thunderstorm fed by the vapor it brings. So, even this energy is not the total energy for that feeble in drought part of the energy also comes from our pre monsoon thundershowers this is what Blanford is saying. So, then he actually puts it very beautifully I think the solar heat directly absorbed by the dry land atmosphere or taken up from the heated ground bears much the same relation to the general air movement as the pull on the trigger does to the propulsion of the rifle ball. I think this is an extremely important simile or analogy because he has expressed very beautifully I think what is the role played by the land sea contrast. He says solar heat directly absorbed by the dry land atmosphere or taken up from the heated ground bears much the same relation to the air movement that is the strong monsoon winds as the pull on the trigger does to the propulsion of the rifle ball. It determines the disturbance of atmospheric equilibrium, but it does not furnish the energy of the resulting air stream. So, just like the energy is provided by what is in the bullet itself or the rifle ball itself the trigger does not provide the energy what the trigger does is to provide the perturbation. So, similarly he says the land sea contrast provides the perturbation or a disturbance to the atmospheric equilibrium, but actually it does not furnish the energy of the resulting air stream that energy comes from the latent heat release. This is what Blanford has said and he says so long as this supply is small and limited to the shallow stratum of air immediately fed by the evaporating surface beneath it, so long is the air movement feeble and interrupted. So, he is now talking of the spring situation, he is saying so that the supply of moisture is limited to the shallow layer above the sea and is fed by the directly by the local evaporation over the sea itself. Then the air movement is feeble and interrupted it is only when the barotropic I am sorry it is only when the barometric gradient from south to north has become sufficiently great to tap the great reservoir of latent energy supplied by the evaporation of the southeast trade zone that the air current becomes strong and sustained constituting the summer monsoon. So, he this is again a very very important point that only when the barotropic barometric gradient from south to north this is something Simpson also talked about has become sufficiently great to tap the great reservoir of latent energy. So, he is saying that pressure gradient has to be great, but once that has become great then it is the latent heat tapping by the winds that lead to the monsoon. This is what he is saying sufficiently great to tap the great reservoir of latent energy supplied by the evaporation of the southeast trade zone that the air current becomes strong and sustained constituting the summer monsoon. So, strong and sustained wind can occur only when this in a great reservoir of latent energies tapped and he says sustained to long after the heated land surface has been in a great measure quenched and cooled by the rainfall. So, he is now talking of in a sense he has before Simpson pointed out the same thing again that this monsoon is sustained even when the land surface cools because of the rainfall. So, it is amazing that more than a century back Blanford has suggested that the Indian summer monsoon is associated with the appearance over the Indian region of the TCG over the equatorial Indian ocean. He called it by another name, but the physical system he described was in fact the intertropical conversion zone as Charney had conceived it. So, this is a very very interesting thing that this kind of an understanding of the basic system geniuses like Blanford had long before in fact more than one century back and now recent ideas how have how has the monsoon been talked about in recent literature. So, unfortunately what has happened is that although the view that the most fundamental driving mechanism by of the monsoon is the differential heating of land and ocean continued to be supported for a long time. So, the thing that was originally proposed by Haley actually continued to be supported for a long long time namely if you look at a important chapter the elementary monsoon in a book entitled the monsoons that came out in 87 where Peter Webster has tried to explain how monsoon arises. What he talks about also is a gigantic land sea breeze of course with modifications in the sense he puts in how the deep clouds arise over land and so on and so forth, but the basic idea remains that it is a gigantic land sea breeze and the primary cause is the land sea temperature contrast. So, this idea has continued to be in literature for a long time considering the monsoon as a manifestation of the of the tropical atmosphere to the seasonal variation of the incoming radiation began to get support in the 70s I would say in the late 60s and early 70s. So, this monsoon as a gigantic land sea breeze land sea temperature contrast being the primary cause of the monsoon and so on or monsoon being attributed to a very special system over the monsoonal regions has continued to hold sway in text books as well as in monographs and you know books specialized books on the subject even to this day I must say, but an alternative school of thought namely considering the monsoon as a manifestation of the response of the tropical atmosphere to the seasonal variation of the incoming radiation began to get support. Now, this was with Reel who was again another giant in the field of tropical metrology he is the one who formulated the concept of equatorial trough, but even his perspective of the basic system responsible for the monsoon appears to have changed between 54 and 69. So, let me just see Reel wrote two books both of them are very important one in 54 called tropical metrology and one in 79 called climate and weather in the tropics both are very good books and describe in great detail the system systems in the tropics in terms of what is observed and what is the physics and so on. Now, what you see here is what appeared in Reel's 54 books and this is the location of the equatorial trough now in January in solid and dashed in July and what you see is that the equatorial trough in fact is mostly in the southern hemisphere in this part in January, but here this is over the Atlantic and so on is straddles the equator as even north of the equator in January in July it shifts only a little bit over this region and remains in the northern hemisphere in this part of the world this is the African region you know 0 to 40 he is he claims that the equatorial trough actually shifts in July, but over the Asian monsoon region these are the Asian monsoon longitude in fact he puts a blank there he does not show any equatorial trough suggesting that perhaps something else is happening over the Asian monsoon region this was in 54, but a lot more information came in the after the mid 60s with the advent of meteorological satellites and lot of papers were written Chania and Elyson's CIS theory came out and so on and so forth and with that Reel appears to have changed his perspective and in the 79 book in fact he draws equatorial trough as a continuous line across the Asian monsoon suggesting that he now believes that the monsoon is also can also be contributed to the equatorial trough coming on to that region so this is a very interesting thing as I mentioned so Reel's perspective about the basic system responsible for the monsoon appears to have changed between 54 and 69 whereas in 54 the equatorial trough in the northern hemispheric summer was shown to be absent over the longitudes of the Asian monsoon 69 book he depicts the equatorial trough over these longitudes as well suggesting that he considered it to be the basic system responsible for the monsoon although even in that book he did not state it in that way however the system responsible for the monsoon was believed to be distinct from the equatorial trough or ITCG associated with the rainfall over the Pacific and the Atlantic oceans even with the first analysis of satellite derived cloudiness by Sadler so this is Sadler's depiction of this is in fact climatological cloud cover this is entirely derived by analysis of satellite imagery which came with the first meteorological satellites and what you see here is the situation for July and this is India here so this is the eastern hemisphere and this is the western hemisphere if you wish and this is what he calls monsoonal region here and he calls the ITCG over the Pacific here and here is the Atlantic and the East Pacific and over both those regions he attributes the high cloudiness to ITCG so Sadler also clearly hinting that you know this is the monsoonal system right here this is centered about here this is India and this is our monsoon rainfall decreasing as we go here which is reflected in the clouds so this is the heart of the Asian monsoon system and that is depicted not as ITCG but monsoonal even by Sadler so actually while there was a suggestion though nobody had put it in so many words that monsoon is could be considered as a manifestation of the seasonal migration of the ITCG there were objections raised to this concept by Murakami again in 87 same year that Webster published his elementary monsoon and what he said was the following if these are the he is looking at OLR climatology and these are the low OLR regions here in January and this is for July now what you see is that in January the latitudinal extent of this low OLR region is say about 10 15 or so but if you go to July over the same longitudes you see the latitudinal extent of the cloud band is huge it is more like 30 40 degrees in latitude here so what he is saying is somehow the band low OLR band flares up over this region occupies a huge region in comparison which say this is the ITCG over the Pacific so in comparison with the ITCG over the Pacific or the ITCG over the Atlantic both of which you see here have typical latitudinal extent of 10 degrees there is a huge thing here so how can this be the same how can the system responsible for this be the same as the system which is responsible for this now so he says considering the mean OLR distributions for January and July he says over the Indian ocean of persistent bell shaped distribution of low OLR values can be seen mainly during the December to February season along the area connecting the northern Madagascar and Sumatra the lack of similar distribution during the summer indicates that the ITCG over the Indian ocean has changed its existence drastically from winter to summer in other words Murakami suggested that the system over the Indian ocean in the northern summer is different because the latitudinal extent of the low OLR region over the Indian longitudes during the summer monsoon is much larger than that found elsewhere in the tropics such as the Pacific where it is associated with the ITCG I am going to stop at this point because now as far as we are concerned the question is still open as to whether the migration of the ITCG is what leads to the monsoon over the Asian region so we will have to look at that and that is what we will look at in the next lecture.