 So, now in this set of lectures I am going to discuss the different hypothesis put forth for the basis basic system responsible for the monsoon. So, this is what I am going to consider in the next set of lectures. So, I will first consider the seasonal variation in the pressure and winds at the surface as well as in the lower and upper troposphere associated with the wind and Indian monsoon which has to be understood in terms of the basic system proposed. So, what is the system that we are trying to develop hypothesis for that is what I will look at first. Now, this is something we have seen before this is the seasonal variation of the direction of mean pressure surface wind from July to November and this seasonal variation is what Arabs were concerned about and seasonal variation of the winds you can see this is from the south west in July and from the north east in November. So, these were the south west monsoon and these are the north east monsoon which I will show at the end of this set of lectures to be misnomer. But this is the seasonal variation in the surface wind please notice that there is also a variation in the surface pressure that there is a low pressure belt in July here over what I call the monsoon zone and this low pressure belt shifts southward equatorward in November. Now, if we consider the winds isobars at higher and isobars at higher levels also there is a market change with season. So, now these are the isobars that you see first these are from the re-analysis by the European centre from that we can now generate with the compute to the kind of charts which were we choose to be drawn earlier by hand. So, now what you see here is a low pressure belt over the monsoon zone and this is at 850 HPA which is about 1.5 kilometres above the ground. So, this is at the pressure surface 850 HPA and what you see is pretty much over the surface trough is a trough zone low pressure zone of the surface field over the monsoon zone. Now what are the winds like now this is from IMD Atlas and again you see this has both winds and pressure and actually also temperature, but I am not going to talk much of temperature right now. So, what you see here is again a low pressure belt over the monsoon zone and you can see that south of the low pressure belt are the westerlies and north of the low pressure minimum pressure are easterlies. So you have if this is the line of monsoon trough say minimum pressure then south of it you have westerlies and north of it easterlies as you know from geostrophic this is what would logically happen remember now we are above the planetary boundary layer right. So, there is the main component of wind is geostrophic that is along isobars and therefore you will have westerly winds here and easterly winds here because the high pressure is here low pressure is here geostrophic wind would be this way. So, that the pressure gradient which would be this way from high to low balances the Coriolis force. So, you have westerly winds here and easterly winds here as soon as you come above the boundary layer. Now in the upper troposphere at 200 HPA a high pressure belt is seen exactly more or less over the region where we had a low pressure. So, if we go to 200 HPA and these are the levels of in fact these are levels of the 200 HPA surface. So, there are two ways in which you can show pressure contours either you can show the pressure at a fixed level at the of the atmosphere say 10 kilometers 15 kilometers or whatever or you can take a pressure surface such as 200 HPA and ask the question what is the height of the pressure surface above the sea level and what you find here is that there is a high pressure here which is about at which the height of 200 HPA is more than 12,000 meters. So, this is about 12 kilometers or so and so there is a high pressure belt overlying the low pressure belt. So, if we were to go back see what do we expect see this is a region of cyclonic vorticity if we go to 850 millibar this is a level above the boundary layer this is a region of cyclonic vorticity. So, you expect that the boundary layer will have convergence in this region region of the monsoon zone and air will ascend in this region right, but the air cannot go on ascending it has to eventually descend and for that the troposphere acts as a lead. So, the air that ascends or the air that converges in the boundary layer and ascends has to stop ascending somewhere diverse and then descend in the surrounding region that happens in the upper troposphere. So, you have a cyclonic vorticity here which corresponds to ascent of moisture and when we go to the upper troposphere you have a high pressure zone. So, this high pressure zone is built up because of the ascending air and notice that here the vorticity is actually anti-cyclonic see the wind is stronger here than here. So, the sign of the vorticity is this way so it is clockwise which means it is anti-cyclonic in the northern hemisphere. So, what we can see now is that in fact so what did we see that at 850 millibar 850 HPA I am sorry I keep using the word millibar here at 850 HPA we had cyclonic vorticity and low pressure belt right and then when we went to 200 HPA we had a high pressure belt over the region where there was a low pressure belt at lower levels and the vorticity here is anti-cyclonic. So, what we can consider is the following we can consider the tropopause to be the upper lid of the atmosphere because all the action that we are interested in is in the troposphere right beyond the tropopause you know that the temperature starts increasing with height in the stratosphere. So, we can consider the tropopause to be an upper boundary and there instead of having it is not a solid wall like the surface of land so there we could impose the condition of zero wind stress instead of zero wind which was the no slip condition and in simple model sometimes all tropopause is also considered to be a rigid lid like the bottom boundary. But so in any event we expect irrespective of whether you take it as a free boundary with stress free condition wind stress going to zero or a rigid boundary there will be a boundary layer near the tropopause and where frictional effects will be important. So, just like you have a boundary layer near the surface of earth you will have a boundary layer near the tropopause right and now you saw that at the edge of the boundary layer and since we are talking of boundary layer attached to the upper limit of the atmosphere or the tropopause then the edge of the boundary layer means around 200 HP or so the vorticity is anti-cyclonic right. Since the vorticity is anti-cyclonic this boundary layer will be opposite of what we saw at the surface where the vorticity was cyclonic it will be divergence in the boundary layer. So, in fact we have cyclonic vorticity and above the lower boundary layer and convergence in the boundary layer in the upper boundary layer at the edge of the upper boundary layer the vorticity is anti-cyclonic and you expect divergence to occur there and then when you have divergence in the upper level the air that is divergence diverging in the upper layer will descend in the surrounding region region surrounding the region over which ascent is taking place ok. So, now suppose we look at what is the circulation like when we average over 70 to 90 degrees. So, we are looking at a circulation in a north south vertical plane right we are averaging over an entire longitudinal belt 70 degrees to 90 degrees which we can call the Indian longitudes. Remember 90 degrees is sort of the center of the Bay of Bengal it goes very near almost through Calcutta and 70 degrees is just off the west coast of the peninsula and it goes across the Arabian Sea and then through Gujarat and Rajasthan and so on. So, 70 to 90 we can consider our Indian longitudes suppose we take an average over those ok and then ask the question what how is the air moving in this plane meridional vertical plane or north south vertical plane that is what you see here ok. So, this is the equator this is north ok and what you see is that around the monsoon zone is around here 20, 25 or something like that and what you see is convergence of air in the monsoon zone at the lower level ascent and at upper levels you see divergence and the air that is diverging is descending primarily in the winter hemisphere in our summer the winter hemisphere is the southern hemisphere. So, what you see is a very nice circulation here ok. So, you have the monsoon zone here and ascent of air the air that is ascending moves southward into the southern hemisphere and descends here, but note that the ascent is over a rather broad region which goes almost from 10 south to about 25, 30 north ok. So, this is the vertical circulation associated with the monsoon. So, air converging in the low pressure belt in the lower troposphere ascends and diverges in the upper troposphere this is the rising limb of the circulation. The circulation is completed by sinking of the air transported upwards primarily over the region south of the monsoon zone. So, this is the nature of the beast this is what we are trying to understand the theories of. Now, when we talk of seasonal variation it is also important to look at post monsoon season. So, if we look at post monsoon season we saw the what happens at the sea level in few slides earlier this is at 850 HPA. So, this is above the boundary layer remember earlier in the summer monsoon July the monsoon trough was here and the low pressure belt was on the monsoon zone. Now, you see the lowest pressure is here over the bay and we are getting a southward shift of the low pressure zone for October it is more or less around 10 degrees north the monsoon trough has moved to 10 or 12 degrees north or so. So, the seasonality is seen in the seasonal variation in the location of the low pressure belt this is for October. So, in October at 850 HPA remember that is 1.5 kilometers above the ground so it is above the boundary layer the low pressure belt is seen around 10 north from 60 to 90 east and at 200 HPA a high pressure belt is seen. So, again let us just see the 200 HPA in October and what we see is that over the region where there was a low pressure belt now a high pressure belt is seen here. And what you see is the associated winds now so the system which used to be here has now moved southward with the season. So, now so in October at 850 HPA the low pressure belt is seen around 10 degrees north from 60 to 90 and at 200 HPA a high pressure belt is seen between 5 and 20 so again you know it is absolutely essential that you have a high pressure region in the upper troposphere overlying a low pressure region. So, as to ensure that the air that is ascending throughout the troposphere can actually diverge and then descend. So, if the vertical cell actually extends through the troposphere you have high pressure in the upper troposphere overlying the low pressure exactly like we saw in July except now the entire system has moved southward. So, it is around 10 north or so is the center is serving 22 or 24 north as it was in July. So, this southward displacement of the system relative to its location in the summer monsoon leads to the variation in the direction of the wind. Thus now you can see the winds here this is the these are the winds that you see here and now you can see that the winds are largely from the north or they are northeasterly here and here the winds are westerly over this region. So, associated with the shift of the low pressure belt we have a shift in the winds as well. Now, what are the hypothesis which can explain the seasonal variation that we have seen in all these plus the more important thing the seasonal variation in rainfall as well. Now, we have so to understand those hypotheses again we have to revise a little bit of the fundamentals we learnt in an earlier lecture that the basic source of energy for the atmospheric circulation is the radiation from the sun. The sun is very hot with temperatures around 6000 k and the wavelength of maximum emission is inversely proportional to the temperature of the radiating body. So, the incoming solar radiation is short wave radiation and since the atmosphere is almost transparent to the incoming short wave radiation it gets absorbed primarily at the surface of the earth be it land or ocean thus although the basic source of energy is the sun the atmosphere is actually heated from below whether it is by land or ocean. The Indian monsoon is associated with seasonal variation of the wind direction and rainfall this is how Indian monsoon was defined by seasonal variation of wind direction originally and of course to us it means seasonal variation of rainfall much more so than the variation of wind. We in the monsoonal regions of the world are concerned primarily with the seasonal variation of rainfall. So, now to understand what leads to the seasonal variation of rainfall we have to remind ourselves that ascent of moisture near the surface is a necessary condition for clouds and hence rainfall. Now it is believed that the monsoon is a response of the tropical atmosphere to a spatial variation of the heating that is to say atmosphere is heating heated from below. So at the bottom boundary if the heating is not uniform but rather varies in space this is what we call differential heating from the bottom and monsoon is believed to be a response of the atmosphere to differential heating that is heating which is not uniform at the bottom boundary but rather varies in space. Now the two major hypothesis for the basic system responsible for the monsoon differ in what is considered to be the most important factor for generating the ascent of surface air. So the two major hypothesis differ in what is the differential heating that generates this kind of monsoonal circulation. The first theory which in fact even today is found in textbooks views the monsoon as a gigantic land sea breeze. This is what we were taught in schools as well that according to the first hypothesis the monsoon is a gigantic land sea breeze arising from land ocean contrast in surface temperatures. Now let us consider first how canonical land sea breeze arises before we consider the monsoon as a gigantic land sea breeze. How do we get land sea breeze? Now land sea breeze is what we all experience when we go near to the coast. How why do we get land sea breeze? Because with the intense incoming radiation in the daytime the land gets hotter than the sea in the vicinity. And why is that given the same amount of radiation land gets hotter than sea because the specific heat capacity of land is different from that of the ocean. So while the heat that is incident on land goes to heat up only a thin layer of the soil for the ocean it heats up a much deeper layer of the water. So this means that given the same kind of radiation incident which is at daytime we will have hotter the land becomes hotter than the ocean around the place relative to the sea the land becomes hotter. Now the associated circulation at the surface would be a breeze from the sea to the land called the land breeze why because the land is hotter than the sea. So there will be a low pressure created on land relative to the sea and air will move towards the low pressure and that is the land sea breeze because that is the air moving from sea to land because the pressure decreases from sea to land. This is the standard land sea breeze. Now what happens at night? At night land cools very fast. So wind goes from the land to the sea at night because the sea is warmer and the low pressure would be there. So it is in the reverse direction. Now in the standard land sea breeze what happens to the air that goes to land in the daytime it rises over land over the hot land and then the circulation is completed by divergence of this air and sinking over the cooler sea. So the associated circulation at the surface in the daytime is a breeze from sea to the land which would lead to the ascent of air over the hot land, wind from the land to the sea at higher levels and sinking over the cooler sea. So you get a circulation completed in that at night it reverses. So this is the kind of thing that we get that in the daytime we have at low level circulation from the sea to the land because the land is hotter and the pressure is lower and ascent of air over land then it diverges and because ocean is cooler than land it descends over there. So if land is maintained hotter than ocean this is what we get and this is called the land sea breeze in the daytime it is from sea to land and exactly opposite happens at night which is from land to sea. So the impact of continuous differential heating over land and ocean would be this. Now according to the first hypothesis the primary cause of the monsoon is the differential heating between the continental and oceanic regions. Now in the summer land gets hotter than the surrounding seas for the same incident solar radiation because of the difference in heat capacities of land and ocean. Thus within the same latitudinal belt air over land is much hotter than that over the ocean and you can see that here this is the surface monthly surface temperature climatology this is for July and this is for January. So July means that the sun is overhead in the northern hemisphere and you see the land brighter shades or higher temperatures you can see that the land is much hotter than the ocean. And this is southern hemispheric summer in January where you can see that the Australian continent is much hotter than the surrounding ocean. So it is clearly seen that maximum heating or temperature the surface is in the summer hemisphere. So first of all note that highest temperatures occur in the summer hemisphere which is the northern hemisphere in July and the southern hemisphere in January. And within the same latitudinal belt land gets much hotter than the ocean. So this is the famous land ocean contrast which is supposed to be responsible for the monsoon according to the first hypothesis. And it is amazing that we have a very long history of theorizing about the monsoon over more than 300 years ago. Edmund Halley who is better known for the Halley comet in fact published a paper entitled an historical account of trade winds and monsoons observable in the seas between and near the tropics with an attempt to assign the physical cause of the said winds is very interesting. We are discussing in these lectures physical cause of the monsoon and this is exactly the problem that was addressed by Halley way back in 1686. This was published in the philosophical transactions of the Royal Society of London. Now what did he suggest in this? He suggested that the primary cause of the monsoon was the differential heating between ocean and land. This is what we were taught in school. Now Halley and many scientists after him considered the monsoon to be a gigantic land sea breeze in which ascent of air and hence clouds and rainfall over the heated land is generated by the land ocean temperature contrast. So we remember we have talked about how ascent of surface air in a moist surface moist moist surface air in a tropical atmosphere can lead to clouds and rainfall. So what leads to the ascent was suggested by Halley as the land ocean contrast. So the contrast between land and ocean in the temperatures or in the heating of the atmosphere from below leads to a gigantic air sea breeze sea air breeze land sea breeze in which there is ascent of air over land because now we are not thinking of day and night but rather with season. So in the summer in our summer land is much hotter than the sea around it and therefore we get a land sea breeze but which is a gigantic because we are the contrast is also on the scale of continents and oceans. So over the heated land we get ascent of air which will give clouds and rainfall was the assumption and this is so the land ocean temperature contrast was supposed to be the primary driving force of the monsoon. Now in Halley's time people were not aware of the impact of rotation of the earth Coriolis came later and Coriolis force was included in this discussion. So this is the continent we are looking at and this is the land ocean contrast but we should also mention that we have a very unusual topographic scenario in our country. We have Tibetan plateau to the north which extends through half the troposphere. So not only do we have heating of the subcontinent relative to the ocean around but we also have an what people call an elevated heat source because this mountain is rising through half the troposphere. So it is a very very strong heating that is generated. So people realize that Tibetan plateau would also have an impact and so the kind of schematic of the monsoon driven by land ocean contrast is what you see here that land heats faster than ocean this is land this is the same solar radiation but land heats faster than ocean then there is a topography this is the Tibetan topography which provides elevated heat source and so land heating faster than ocean means lower pressure gets created here and that leads to this is the gigantic land sea breeze. So that leads to moisture from the ocean being sucked in towards the land and that leading to clouds and rain okay heated over the land over the land rises forms convective clouds releases latent heat and you get copious rainfall over land and this is supposed to be the monsoon this is the first model of the monsoon. So as I mentioned before now this land sea gigantic land sea breeze theory was modified a bit by Haley who took into account what Coriolis force would do. So what would happen is rather than simply the wind going from high pressure to low pressure a Coriolis force would make an impact on this wind and because that is important for special scales of thousands of kilometers characterizing the monsoon. So finally from this first hypothesis monsoon is considered to be the response of an atmosphere on a rotating earth to land ocean contrast in surface temperature or heating okay and this implies that the system responsible for the monsoon is special to the monsoonal region right because it is a system that gets generated during the summer over land okay and it reverses during our winter it disappears there is no more rainfall in our winter. So this is if we believe that land ocean contrast is the primary force of the monsoon then the system responsible for the monsoon is very special to the monsoon region okay. Now as I said this theory is even today mentioned in many many textbooks and in important reviews like two I have quoted this is a paper by Webster on elementary monsoon in a book edited by Fine and Stephens in 87 and many books of which I give just one example which gives introduction to circulating atmosphere which has something about the monsoon referred to the monsoon as being generated by land ocean contrast in temperature okay. So what is the hypothesis then according to this hypothesis the low pressure over the land mass over of the monsoon trough which is this low pressure here remember this is the low pressure belt here okay is generated by the heating of the land relative to the ocean. So heating of the subcontinent and of course the Tibetan plateau here leads to the low pressure over here this is what. So seasonality of the winds and the rainfall characteristic of the monsoon are believed to be generated by the land ocean contrast. In order to understand the proposed mechanism we first let us consider a simple case of a fluid subject to differential heating at the bottom boundary okay. So to just understand what happens when you have say atmosphere heated from below just consider the simplest possible scenario in which we forget about the rotation of the earth okay and we consider what happens to a fluid which is heated from below. Now this is a very classic problem this is the response of a fluid to differential heating at the bottom boundary. So you have response of a heating fluid heated from below and this is called the classic Benant convection problem or the so called porridge problem why is it called the porridge problem or in India you could call it the sambar problem is that if the heat is too much the porridge or the sambar will get burnt unless it is stirred okay. So this is why it is called the porridge problem and now I will go on to say what does one mean by heat being too much okay. The nature of the response of the fluid depends on the value of the non dimensional Rayleigh number okay which is a measure of the forcing by the imposed temperature gradient vis-a-vis the viscosity and conductivity of the fluid. We know very well that water when heated from below does not really get burnt because it just convex whereas porridge or the thick sambar heated from below can get burnt. So it has to do with the viscosity of the fluid as well and Rayleigh number is a non dimensional number which determines how large is the heating vis-a-vis the viscosity and conductivity of the fluid. So when viscous effects dominate as for a thick sambar or porridge the Rayleigh number is small and there is no convection that is to say there is no convection of the fluid the fluid is sitting at the bottom as it is and burning because there is too much heat there okay the sambar gets burnt over the hot part of the bottom plate. As the Rayleigh number increases the fluid begins to convict and the classic example is that if you heat water then the hot water at the bottom rises to the top and there is regular convection maintained in the fluid and there is no burning and so on. So what how the fluid reacts depends on the Rayleigh number. Now this actually the equations of this can be readily solved at this point I am not going to bother you with the equations but show you just the solution. So the circulation and the temperature distribution driven by differential heating obtained by numerically solving the governing equations for two values of Rayleigh number are shown in the next slide and what you see here is the temperature in colors and blue to red means hotter and hotter and the fluid is heated from below and it is heated only over this part of the plate okay. It is heated only over red part of the plate and this is the numerical solution to what happens in a steady state it is in fact being cooled on all sides on the sides here so that on the whole the fluid does not get heated up okay there is equilibrium possible. So we have heating from here and what this leads to is ascent of air or whatever fluid it is okay. So low level you get a low pressure region created here fluid moves towards the low pressure region imagine if you like that this is land and this is sea. So the fluid is moving air is moving towards the land rising diverging and in this case descending in the surrounding air where this bottom temperature is colder okay. So we have the kind of circulation that we described qualitatively as land sea breeze here. Now what happens as Rayleigh number increases then the ascending limb becomes narrower and the descending limb becomes broader okay. So as the heating increases visa with the viscous effects and we know that the atmosphere we have seen in the earlier lecture that atmosphere is not very viscous and viscous effects are felt mainly in the boundary layers. So as we go towards the inviscid limit then we see that we get a very very narrow ascending region and a relatively broad descending region. So this is now the basic response of a fluid heated from below remember we are not worried here about the rotation of the fluid okay. So as I mentioned that convection is characterized by rising above the heated region and sinking elsewhere and circulation is completed by fluid converging to the low pressure over the heated region at low level and diverging at the higher levels. So the strength of the circulation increases with the magnitude of differential heating. So keeping the viscosity same if we increase the differential heating which is what implies that Rayleigh number is higher the strength of the circulation is higher stronger ascent here and you see this is the circulation that is driven that you have convergence at low levels air ascending divergence at high levels and air descending in the surrounding region okay. So this is the way the circulation is completed and the strength of the circulation increases with the magnitude of the differential heating. So for very large Rayleigh number the rising zone is much narrower than the region over which it sinks okay. Now so let us go back to the atmosphere we expect that such a strong ascent of moisture forced by differential heating can lead to clouds and rainfall right because once we have strong ascent driven by this differential heating and if the ascent is above the lifting condensation level we can get clouds and rainfall okay. So if the monsoon is indeed a gigantic land sea breeze the variation of the summer monsoon rainfall in space and time should be governed by the variation of the temperature difference between land and ocean okay. So the first hypothesis which is the one we were taught and which is the one which is quoted extensively in many many textbooks namely that the monsoon is driven by land ocean contrast in temperatures this is the way this is the fluid mechanics of that the ascent driven by differential heating and the stronger the differential heating the narrower the ascent and narrower the ascending region and stronger the ascent and more clouds and rainfall. So monsoon is then envisaged as a gigantic land sea breeze which is a response to continents being much much hotter than the oceans okay. Now one thing we have to realize is that if we believe that we have understood what is the physical system responsible for the monsoon we should be able to understand and explain the variability of the monsoon in space and time you remember in the second lecture we talked about how rainfall varies in space and time we saw how rainfall varies within a season from dry spells to wet spells and how it varies in space also for example in the monsoon zone you get much more high rain over the Bay of Bengal and as we go north westwards towards Rajasthan the rainfall decreases right. So we should be able to explain the space time variation of monsoon and particularly rainfall from the understanding of the basic physics system physics of the monsoon or the basic system responsible for the monsoon right this is an absolute requirement. Now what happens is with this first hypothesis which actually gives primacy to land ocean temperature contrast observations of the space time variations of the monsoon over the Indian region are not consistent with this expectation right. What was the expectation that if the monsoon is indeed a gigantic land sea breeze variation in monsoon rainfall will be governed by variation in temperature difference between land and ocean right. For example if the temperature difference decreases what happens if the temperature difference decreases we go to a case which we have seen earlier where the heating becomes less intense right then we expect to get less rain because the ascent is weaker this is from simple minded fluid mechanical theory right. So then we expect the variability to be determined by the land temperature contrast ok. But in fact the observations are inconsistent with this expectation ok and this was first pointed out very eloquently by Simpson in the quarterly journal of Royal Met Society as early as in 1921 ok. In a major review paper called the Southwest Monsoon he says I believe very few educated people would have any difficulties in giving an answer to the question what is the cause of the monsoon ok. They would refer to the high temperature over the land compared with that over the surrounding seas. So this is the main land sea contrast of temperatures would speak of ascending currents of air causing an in draft of sea air towards the interior of the country right because the land is hotter you the air would ascend which he calls ascending currents of air. And this of course is possible because the moist sea air at low levels is coming towards the land in the land sea breeze and it is that air that is ascending. So they would speak of ascending currents of air causing an in draft of sea air towards the interior of the country. It is only when one points out that India is much hotter in May before the monsoon sets in it is only when one points out that India is much hotter in May before the monsoon sets in then in July when it is at its height right. So what he is now saying is in May if the temperature contrast is much higher which it is because the temperatures of land are very very high in May they are of the order of 40 degrees in many places whereas in July the land is much cooler right. So one would expect that there should be more rain in May than in July right. So it is only when one points out that India is much hotter in May before the monsoon sets in before the rainfall begins then in July when it is at its height right. So when you look at temporal variation you have less land ocean contrast in temperature when you have rainfall or draws attention to the fact that the hottest part of India the northwest which is part of Rajasthan gets no rain at all during the monsoon. So if we look at the spatial variation then temperature is highest temperature is highest over Rajasthan in the northwest as you go along the monsoon trough towards the bay of Bengal the temperature decreases. So the land the temperature contrast with ocean is maximum vis-a-vis Rajasthan or the northwestern part whereas it is minimum over the region to the eastern part eastern part of the monsoon trough. But there is no rain at all over Rajasthan and plenty of rain near the bay of Bengal you know over the eastern part Orissa Bengal and so on and so forth. So this again is contrary to an expectation that the variability in space will be determined by variability of land ocean land sea temperature contrast and then he makes a further point or shows by statistics that the average temperature is much greater in years of bad rains than in years of good rains that they begin to doubt whether they know the real cause of the monsoon. Now this fact that the average temperature is much greater in years of bad rains than in years of good rains was something that was stated by Simpson but he did not actually show data for it but I will show you some data this was done by Kothawale and Rup Kumar and what they did was they made averages or composites of all droughts and all excess monsoon seasons okay and they looked at surface temperature anomaly remember anomaly is just the temperature minus the average for the Indian region and did it for surface as well as at higher levels okay and what do they find first look at the surface okay and this is monthly now this is January to December and these are seasons okay. So now notice that for each case hatched corresponds to excess monsoon here and deficit rainfall or droughts are solid years okay. So now when you come here let us first look at JJAS you find that droughts are the solid bars in droughts the surface temperature anomaly is positive whereas in excess rainfall years it is negative okay. So this is exactly what Simpson pointed out that when you have less rainfall the land ocean temperature contrast is more because the surface temperature anomaly is positive land is hotter than average whereas land is colder than average when the rain is high okay. So this is absolutely opposite of what one would expect if the rainfall was governed by land ocean contrast in fact you see the same story for the individual months here this is June July August and September. So you see exactly the same thing all the solids are above positive anomaly all the negatives hatched are negative which means land is colder for excess rain it is interesting that as you go to higher levels like 700 HPA this is 3 kilometers here cloud is already beginning to have an impact on the temperatures. So there you find that all the hatched things are above that is to say at 700 millibars if you have more rain you have higher temperatures if you have droughts you have lower temperature this is again consistent with our understanding of clouds and rainfall and what they do to the temperature and at 200 HPA the same thing remains okay. So this is exactly what Simpson had said that that the surface temperature anomaly see sorry this is Kothawale and Rupkumar but what Simpson had said was that shows by statistics which is what I showed you generated by Kothawale and Rupkumar that the average temperature is much greater in years of bad rains which were the droughts then in years of good rains which is the excess monsoon rain that excess rainfall season that we looked at they begin to doubt whether they know the real cause of the monsoon. So what Simpson has shown is that the you cannot explain the observed space-time variation of rainfall by assuming that the rainfall is related to land ocean temperature contrast okay. So this is a very important statement he made back in 1921 and in fact this is consistent with our experience in the rainy season that days without rain are much hotter than rainy days right. So clearly rather than the land surface temperature determining the amount of rainfall why are they impact on the difference between land and ocean temperature the land temperatures determined by the rainfall are lack thereof. So if you do not have rain it is hotter and if you have rain it is colder. So this in fact shows thus the observation suggests that the land surface temperature varies in response to the variation of rainfall and it is not appropriate to consider land ocean temperature contrast as a cause of the monsoon rains okay beginning with reminding you of what Haley gave as a title to his paper what is the cause of the monsoon winds he said he was more concerned with winds but if we ask what is the cause of the monsoon rains we cannot attribute monsoon rains to land ocean surface temperature contrast okay. This cannot be considered as a cause of the monsoon rains in other words the observations are not consistent with the land ocean hypothesis land land breeze sea breeze hypothesis. So we now have to think of another hypothesis for the system which is responsible for monsoon rains the hypothesis that monsoon is a gigantic land sea breeze does not in fact give results which are consistent with observations of space time variation of monsoon rainfall and this is very simple space time variation variation of monsoon rainfall between droughts and good monsoon seasons variation of monsoon rainfall over space between the eastern part of the monsoon trough zone over Orissa and the eastern regions Visavi Rajasthan and the northwestern region. So land ocean temperature contrast cannot be considered as the primary cause of the monsoon over India and the Indian monsoon cannot be thought of as a gigantic land sea breeze. This is the conclusion we have come to now that we have shown that one hypothesis is wrong we have to show what is right before we can start interpreting what is monsoon variability. So in the next lecture we will look at what I believe is the correct hypothesis for the basic system responsible for the monsoon thank you.