 Now, in the last lecture, we have seen the nature of monsoon variability as analyzed from rainfall data. Today, we will begin with looking at what monsoon variability looks like from measurements from satellites, the eye in the sky. And then we will look at the Indian monsoon, the seasonal variation of surface and so wind and so on and then develop the background required for understanding the various theories we have on mechanisms. Now, so let us first look at monsoon variability through the eye in the sky. In fact, satellites have contributed enormously not only to our understanding of the monsoon variability, but also to the elicitation of the basic system responsible for the monsoon. You know the physics of the mean monsoon. Now, even if one is interested only in the rainfall over the land region that is only over the rainfall over the Indian region, it is important to know about what is happening over the tropical oceans. Now, why is that? Because almost all the cloud systems which give rainfall over the Indian region are actually born over the warm parts of the tropical oceans, warm our Arabian Sea, Bay of Bengal, Equatorial Indian Ocean and Equatorial Pacific Ocean and warm parts of West Pacific are the breeding regions if you wish for the cloud systems which actually give us rainfall. Now, earlier without satellites, we did not really have good data on what was happening over the oceans. Now, with satellites it is possible to literally see the cloud systems over the entire tropical belt several times a day and in fact, some of us do that we see every day what is happening over the world. Now, this has had a major impact on our understanding of the monsoon and also on its variability and I believe in the ability to predict various facets of the monsoon and its variability. Now, at first when meteorological satellites came in the data we had actually were images. You know photographs taken by cameras from satellites and in these images clouds appear as white blobs because lot of sunlight gets reflected from the tops of the clouds. So, now if we look at a typical image that we got in that time which is what we will see in the next slide it is an image of an active monsoon day a day on which lot of rainfall occurred in the monsoon zone and it happens to be 8th of July 1973. So, what you see here is what the satellite actually generated it is a satellite image it is an image of the Indian region and what you see is an east west band of cloud over India it extends all the way from west of India over the Arabian Sea extends over India and you can see it continuously extends eastward as well. So, this is a big planetary scale cloud band sitting on us and over Indian region you can see that it is sitting on the monsoon zone. Please note the Indian longitudes what we can call Indian longitudes are 70 degrees east which is the longitude here 80 degrees east which is considered to be the central longitude of India that is this one and 90 degrees east which is the longitude that cuts across the bay and goes to the head bay. So, these 3 longitudes we consider are the Indian longitudes and we will focus our attention on the variation of the clouds over the Indian region. Now, in fact the very first study of you know how these clouds vary from day to day which was based on such satellite imagery actually revealed several new features which are particularly important in both the seasonal transitions such as the onset phase, the retreat phase as well as the fluctuations between active spells and break or the variations within the season. And this study was carried out by an eminent monsoon meteorologist Sikha and myself and the paper was published in 1980. Now, this very first study of the daily variation of the kind of imagery that you saw what we did was we looked at the extent of the band at those 3 longitudes I showed you 70, 80 and 90 the Indian longitudes. And looked at how the cloud extent over the Indian longitudes varies from day to day and it actually revealed many new features that were unanticipated. Firstly, we found that during the summer monsoon there are 2 favorable locations for the cloud bands. And you will see that in this picture, but if we go back one favorable location is over the Indian region which is a heated subcontinent in this time and another favorable location you see a kind of glimpse here is the equatorial region. So, cloud bands of the kind you are seeing have a high propensity to occur either here or here or we say there are. So, there are 2 favorable locations for the cloud bands one over the heated subcontinent and another over warm waters of the equatorial Indian ocean. Now, this is reflected the fact that it is a bimodal distribution that the cloud bands tend to occur at 2 places is reflected here. Now, this is again from the original paper that I talked about Sikha Gadgil paper. And what you see here is the monthly mean of the number of days on which the axis of the cloud band occurred at different latitudes along 80 degrees 90 degrees east. So, 80 degrees remember is the central longitude of India and we have to what this shows is the number of days on which the cloud band occurred here. So, let us look at June for example or begin with April. In April you can see that most of the time the cloud band is to the south of 10 degrees north remember the southern tip of India is 8 degrees north. So, in April you almost never get over Indian region any clouds very very few and over the bay you do get some clouding in April remember 90 degrees is the longitude that cuts across the bay. Now, this starts spreading northward in May and in June while still you have higher chance of clouding in the equatorial region you also start getting substantial chance of clouding in the northern parts right up to 20 north and beyond and that is to say right up to Bombay and north of that. Now, similar distribution here if you see in July and August which you remember were the peak monsoon months. In July and August you see what we consider a bimodal distribution over the Indian longitude that is to say there is a mode here and there is another mode here and between the two modes the chance of having clouds is very small. So, this mode is over the equatorial Indian ocean and this mode is over the heated continent around 20 north. Of course, the distribution is not very sharp it is in fact spread out and the chance of having clouds all over right from 18 north or so to 25 30 north is quite high. Same story for August same story for July here, but in August the Bay of Bengal the equatorial clouding has become less likely and so on. So, this is the bimodal distribution this is what we call the oceanic cloud band and this is the continental cloud band and they are separated if you wish by this no man's land you know by the these region by the region between these latitudes around 10 north when there is not likely to be the cloud band occurring. So, if we now make an average over if we now make an average over this part and call it a northern one and this part and call it a southern one when there are two distinct modes and otherwise look at the average of this distribution then what we get is how does the location of the cloud band vary with season vary from month to month. So, what you have here is both 80 degrees and 90 degrees and the sticks around it just indicate the spread or the standard deviation. So, what you see is in April and May the cloud band is located the mean cloud band is located south of India south of 8 degrees north of course, it has a tendency to appear sometimes towards the north, but the mean is located south of India. Now, it comes over India by June it is around 15 degrees north that is just north of Mangalore and Bangalore then by July it has reached this destiny it has reached over the monsoon zone. So, the center of the band is around 20 north August it remains there and September also it remains there. So, monsoon zone starts from around 15 north and you see that June July August September there is a the band is there. So, you see a very clear northward movement of the band this is the northward movement during the onset phase of the monsoon and this is the peak monsoon months this is the retreat which looks like a southward movement if you look at monthly picture, but note that when we look at the ocean band this is the average over south of about 7 degrees north or so, then the ocean band persists throughout June July August September. So, there is a band over the continent and there is a band over the ocean this is a very interesting feature and a somewhat unique feature of this part of the tropics. So, we have two locations where cloud bands can occur one is over the Indian region heated plains of India and the other is over the equatorial Indian ocean and throughout the main monsoon summer monsoon months of June to September the ocean band is also present if we look at the monthly picture, but if you now look at the daily picture you will see that in fact at both the locations the bands fluctuate I will come to that when we look at the next feature. So, in this very first study of the satellite imagery of our region a major feature of intracesional variation was discovered this is a new feature which was discovered in this study and this feature is shown here and I will go back to the description after you see the picture. What you see here is where the cloud band is at 90 degrees is. So, we are sitting on the longitude that cuts across the Bay of Bengal and we are asking the question on this particular day in 1973 where is the band. So, on this particular day in 1973 the band is located around 10 north. Similarly, on in 1974 also it is located around 10 north and even 1975 also. So, we see how does the location of the band vary from day to day when we are fixed at 90 degrees east which is the longitude that cuts across the Bay of Bengal which is the central longitude of our monsoonal region. So, what you see is there are these northward propagation you see band seems to get generated in the equatorial region and moves northward and these occur at different intervals for example, in 75 you got one northward movement and the band seem to die here having reached Indian region having reached the monsoon zone over the Indian region. Another band appeared and it moved northward and then it continued to move northward over a gap and after a few weeks another band appeared over the equatorial region and moved north and again moved north. So, these northward propagations are something that we observe year after year. So, what is the variation of the cloud band like location of the cloud band like year after year the cloud band gets generated in the equatorial region and moves northward how frequently at what intervals the intervals can vary from two weeks to several weeks six weeks or so like this one between the two propagation here. So, there are northward propagation of the cloud bands which occur with genesis in the equatorial region and northward movement towards the monsoon zone that is towards 90 20 degrees north. So, this kind of series of northward propagation is part of the repertoire of the monsoon year after year irrespective of whether it is a drought like 74 was a drought and whether it is a good monsoon like 75 was a good monsoon year. So, year after year what we see is a series of northward propagation. So, a major feature of the inter-seasonal variation that we discovered was the series of northward propagation of the cloud bands from the equatorial Indian Ocean on to the Indian region which occur year after year irrespective of whether it is a good monsoon or a drought. Now, the spacing between how the period between these two propagation to successive propagation varies and as we saw sometimes in the onset phase you get very quick propagations in very quick succession. So, sometimes 2 or 3 propagations occur in the onset phase in quick succession later on they occur at intervals of 2 to 6 weeks. So, in fact this is a basic feature then of the monsoon over the Indian region is this genesis of cloud bands over the equatorial region which move northward on to our monsoon zone at intervals of anywhere between 2 to 6 weeks which occurs year after year irrespective of whether it is a good monsoon or a poor monsoon. What you saw here was what happens in different years at 90 degrees east we sat on the central longitude of Bay of Bengal and saw. Now what you see here is what happens in the same year at different Indian longitudes. So, this is 70 degrees east this is 80 degrees east and this is our good old 90 degrees east which you saw earlier and what you see is that in fact the feature you saw at 90 is also seen at 80 and 70 and there is a great deal of coherence when you have northward band when you have northward movement of a band at 90 you see it also at 80 and you see it also at 70. So, in fact these northward propagations are not of cloud bands just around 90 rather they correspond to cloud bands which are stretching all the way from 70 to 90 which are stretching across the Indian longitudes and such a band is moving northward. In fact we saw the image of such a band in the first picture. So, this is the second important feature the first important feature was that in fact there are two favorable locations for cloud bands to occur one is over the heated continent in the north around 20 north and the other is an over the warm equatorial Indian ocean. Second was that every monsoon season we see a series of northward propagations of the cloud band which is generated over the equatorial Indian ocean and moves northward. Now the third feature that we found was that in fact there appears to be a competition between the band over land and the band over the ocean such that active phases of this one band occur during weak phases of the other and I believe you can see that here you see in this picture for example this is an active phase of the northern band in which equatorial band did not appear. It started appearing here and it actually was sustained for quite a while when there was no band in the north and then it moved northward. Once it was in a phase where it was active here it did not appear here as it became weaker it started appearing, but then disappeared because this one was continued and then again appeared and moved northward when there was no band here. So, there is a seesaw of activity between the this and this. So, there is a competition between the band on land and between the band over the ocean. So, this is the third feature then that we saw. Now all this was done by looking at satellite pictures you know the kind of imagery that you saw earlier and from 80s we got digital data. See otherwise it was a matter of assessment that this band is of bright cloud bands which stretch over a large longitudinal range and so on, but in the 80s we started getting digital data and this makes a big difference because now the you can do analysis in a more objective manner. Now what was this data on? This data was on outgoing long wave radiation. When we when I talk about the background I will talk about this, but let me just mention at this point that the source of energy for atmospheric circulation is the sun. So, it is the solar energy that drives atmospheric circulation. So, we get radiation from the sun and this radiation is short wave radiation because the sun is very hot. In turn the earth atmosphere system emits radiation back to space that is how the balance is maintained. If it was not emitting back then we would earth atmosphere system would get hotter and hotter, but that does not happen. So, the balance is maintained and so we have the earth atmosphere system radiating back to space. Now the earth atmosphere system is much colder than the sun. So, the radiation that the earth atmosphere system emits is in the long wave and what satellites can measure very efficiently is the outgoing long wave radiation. So, this is the outgoing long wave radiation from the earth atmosphere system. So, now there are measurements daily measurements grid point measurements of what we call OLR or outgoing long wave radiation from the earth atmosphere system. Now with these digital measurements of OLR we can deduce the regions where there are clouds. How is that? Because outgoing long wave radiation depends on the temperature of the radiating surface right and so under cloud free conditions what the surface that emits the radiation is the land or the sea which is rather hot, but when there are clouds the emitting surface is the cloud top. So, when the cloud top is at a very high level in the atmosphere then the OLR or the outgoing long wave radiation will be much less because the temperature is much lower. So, when OLR is high we do not see we deduce that there are not any clouds, but when OLR is low we can certainly say that there is a cloud top at a very high height at a very high level in the atmosphere which is emitting the radiation. So, the nature of variation of low OLR bands that is where is the outgoing long wave radiation low that is where the cloud tops are very high remember. So, if we now trace using this digital OLR data digitalized OLR data we can actually look at regions of low OLR and that corresponds to what we saw with the eye or in the images as cloud bands. So, regions of OLR are the regions with clouds which have tops at a very high level in the atmosphere and if we analyze that data the kind of picture we get is rather similar to what we got from analysis of satellite images. So, this is not a surprise because after all a feature that we saw in the imagery has to be reflected also in the digital data and what you see here is for another year May to September of 2007 and what we have plotted as color is regions of low OLR. Again the plot is very similar we have latitude here and day here. So, if this tells you that on this day the band extends from about 3 degrees north to about 10 degrees north. Of course, now with this satellite data we are able to also go to the southern hemisphere and look at the entire equatorial belt with the imagery that we analyzed it used to be either northern hemispheric or southern hemispheric. So, we were restricted to the northern hemispheric imagery as soon as we got OLR data we could in fact include the entire equatorial region 10 north to 10 south and that is what you see here and again what you see is very marked feature is the propagation the northward movement of the cloud band. Of course, it is a complicated feature there are other things happening, but again you see here this is the onset phase you saw one propagation here and another here and then the third propagation here this is the phase when it got established then again the equatorial oceanic band got flared up again this flared up and then another northward movement. And you see again like we saw before that these northward movements are coherent across the Indian longitudes that is to say what we are seeing is a band which stretches right across the Indian longitudes and moves northward. Now, since in the tropics high rainfall occurs from deep cloud tops with high deep clouds with high tops OLR is used also as a proxy for rainfall in the tropics and low values of OLR being associated with high rainfall. Now, with the new satellites there are also direct measurements of rainfall possible these are based on microwave measurements from satellites and this is actually a figure derived this is the rainfall as seen from satellites what you see instead of shading regions of low OLR now we are shading regions of high rain. So, rain above say 14 millimeters a day or something like that or maybe 1 centimeter a day is what is appearing as bright red there and what you see here from data of this microwave sounder MSU data is very beautiful northward propagation and again the presence of two you know oceanic band and a band to the north and very nice northward propagation are seen with rainfall data as well. So, we learnt a new thing by looking at satellites that throughout the season there are these northward propagation of the cloud band occurring right from April to October. Now, you may recall that we had looked in the last lecture at what are what are the dates of onset of the monsoon and we had seen that the monsoon onset over Kerala occurs around 1st June and afterwards there is a northward progression of the monsoon or the advance of the monsoon in the onset phase there is also an eastward progression, but the this northward progression eventually leads to the establishment of the monsoon in the monsoon zone. So, this is the onset phase in the retreat you remember was the steps were reversed and the retreat appear to be southward movement that is to say the rain gets more and more restricted to the southern parts the edge of the monsoon comes southward this is the retreat of the monsoon. Now, so, one would think just by looking at these maps that the system that the rain system moves northward in this case and moves southward in this case, but what satellites told us was something different see looking at the mean dates of onset and retreat one gets the impression that the rain belt moves northward across the Indian region in the onset phase and southward during the retreat phase. In fact, before people started looking at satellite pictures people thought that retreat was the reverse of onset in a sense and there is a book by a great tropical meteorologist by Herbert called Herbert Reel on climate and weather in the top tropics this was in 1969 I believe all the references will be provided to you. In fact, what Reel says is that the retreat period is in a sense a mirror image of the onset. So, they expected that the reverse of retreat will occur during onset, but when we see what happens with satellite imagery in fact, April to June at 90 degrees east again this is a plot very similar to what you have seen before shaded are the regions of low oiler or the regions of clouds and this goes all the way from March to end of November and because you know large period is covered these northward propagation look very fast, but what you see and this is in fact, the red line is the envelope of these propagation. So, that is where they tend to go. So, if you look at the envelope then it goes northward and it comes southward. But if you look at the actual cloud bands you there is a surprise certainly in the onset phase as we expected we get northward propagation and what is happening is that you have successive genesis of clouds in the equatorial region which move northward, but they move further and further northward in the onset phase. So, initially it moved only up to 5 north then it moved up to 10 north or 12 north up to Bangalore, Bangalore latitude then the next northward propagation moved much further up to 20 north north of Mumbai and so on and so forth. So, onset phase then comprises northward propagation each successive northward propagation taking the cloud band further and further north. Now, if you thought that retreat was the mirror image of onset phase you would think that in during the retreat phase you would get southward propagation, but that is not at all what you see in the retreat phase. What you see is when the envelope is going southward the bands are not going southward again the bands are cloud bands are being created over the equatorial Indian ocean and they are moving northward, but this time they are moving to a lower and lower latitude that is to say the destination of the northward movement becomes more and more southward, but retreat also comprises northward movement just like onset does. So, this was a surprising result from study of satellite imagery which we would not have anticipated. So, we say note that the southward movement of the envelope in September, October is also characterized by northward and north southward propagation. So, the cloud bands have this tendency to go on propagating northward. Now, if it is in the onset phase they will go further and further northward if it is in the retreat phase they will start going less and less northward with every successive propagation. So, northward propagation play an important role in both the transitions in the onset phase as well as the retreat of the monsoon. Now, let us see what else we learnt from satellites. We have already been introduced to active spells and brakes which were defined on the basis of conventional observations rainfall. Now, let us see what is the signature of these active and weak spells, what does the structure of active and weak spells look like the rainfall pattern is the structure we saw with conventional data. Now, what does the structure look like from the satellite data and what we see here is when we define brakes or active spells on the basis of rainfall over monsoon zone we know the dates on which they occur then what we do is take the average OLR for those days and this is the composite of brake spells, but note what we have plotted is not the actual OLR, but what we call the OLR anomaly that is to say how OLR differs from the average. So, here for example, the blues represent negative anomaly this means OLR is less than the average this means there is clouding there or rain there. Negative OLR anomalies mean more rain positive OLR anomalies mean less rain. So, during the break obviously you will have less rain over the Indian region that is very clear, but now this satellite data allows us to see the signature of these brake spells over the other parts particularly now over the equatorial Indian ocean we saw that the equatorial Indian ocean played a very important role in onset and retreat phases because that is where the cloud bands were generated. You saw cloud bands being generated over equatorial Indian ocean moving northward. Now what is the equatorial Indian ocean doing during a break spell? Well it turns out that the eastern part of the equatorial Indian ocean is very active lots of clouds in our brake spells and exactly the opposite occurs in our active spells. So, this is the average picture OLR anomaly for brake spells and this is the average pictures for our active spells and what you see here is that naturally in the brake spells convection that is to say clouding is suppressed over the Indian region at that time convection flares up over the eastern equatorial Indian ocean and that means there is more rainfall than average over the eastern equatorial Indian ocean during the break phases. You also see a signature here of the break phase and this is also been popularly called the quadrupole we will come to that when we discuss brakes in detail, but you see that this part of the west specific also has a break that is to say suppression of convection or rainfall and to the north is a region where there is more rainfall and more clouding. So, this is the signature of brakes over the Indian region you also see some signature over each specific. Now, you see exactly the opposite happening in active spells, but in active spells you see that the active spell is rather coherent stretching across right from the Indian region to the west specific. So, you have a active spell which is stretching across here and in a way you can say brake spell is stretching across here too. So, the signature of active and brake spells from satellite data tells us that perhaps this eastern equatorial Indian ocean is playing some role in the variation of rainfall we observe over the Indian region during the monsoon between active and weak spells. So, with the eye in the sky what have we found? We have found that the cloud systems over the equatorial Indian ocean play a very important role in seasonal transition as well as the intracesional variation that is the variation between active spells and weak spells and also a new feature of intracesional variation has been found by looking at the satellite imagery the series of northward propagations that occur throughout the monsoon season. Now, can we get any hint of what is the signature of droughts and excess monsoon seasons on this from the satellite data? Can we get any hint of what is the signature over the rest of the tropics of a drought monsoon drought or a monsoon excess rainfall season? So, there is a so far we have been looking at composites we looked at the OLR anomaly corresponding to a brake this was average over all brake days. Similarly, we looked at OLR anomaly corresponding to an active spell this is the OLR anomaly average over all the active days. There is another way to look at the signature of a relationship and that is to compute the correlation of the quantity you are interested in with the OLR over the entire region. Let me explain you remember we looked at all India monsoon rainfall all India summer monsoon rainfall we called it ISMR that is the one from which we detected droughts of the Indian monsoon and excess monsoon rainfall season. Now, this ISMR is a time series right we have the values right from 1876 to the present and in the earliest in the first lecture I had shown you how this Indian monsoon rainfall varies from year to year. So, ISMR varies from year to year and you may remember that the standard deviation is only 10 percent of the mean and when the magnitude of the ISMR anomaly is larger than the standard deviation we have the extremes which are if it is a deficit then it is a drought and if it is a positive anomaly then it is an excess monsoon season. So, we have a whole time series of ISMR and we want to ask the question how is the variation of ISMR or all India monsoon rainfall related to events over different regions events such as the mean rainfall over the equatorial Indian ocean so on and so forth. So, the way to do it is that you have one time series which is the rainfall ISMR with that you correlate the time series of each and every grid for which you have from satellite now we have OLR data which is daily from which we can get June to September average OLR for each year. So, what do we have we have a time series for every grid point of variation of June to September average OLR we also have a time series of June to September all India rainfall which is ISMR. Now, given those two time series we can correlate them and derive the correlation coefficient. Now, when the correlation coefficient is positive that means the series tend to go together when ISMR anomaly is positive this anomaly will also tend to be positive and so on. When it is negative it means the series are in opposite phase and we have to make sure every time that the correlation coefficient that you get is not really a figment of your imagination, but is statistically significant that also we will look at. So, another way to look at signatures of events we are interested in or links between variation of quantities such as ISMR and the convection and rainfall over the tropics is by computing the correlation and what you see now is a map of the correlations. Blue regions are regions where correlation of ISMR with OLR is negative why negative because when OLR is negative that is when you get rain. So, if we were calculating rainfall you would see the blues as positive in other words rainfall over the Indian region seems to be positively correlated to rainfall over the western equatorial Indian ocean. See when this is blue this is blue it is negatively correlated to the rainfall over the eastern Indian ocean. So, you have correlation between ISMR saying rainfall over India is positively correlated with rainfall over western equatorial ocean that is negatively correlated with the OLR over this region because OLR and rainfall themselves negatively correlated. Negative OLR anomaly corresponds to positive rainfall anomaly. So, we have a link between the western equatorial Indian ocean rainfall and the Indian rainfall ISMR such that when this is above normal this is this will also tend to be above normal when this is below normal this will tend to be below normal. So, this is what a same sign of correlation with OLR reflex. Now, if you go to this side you see the signature is opposite that is to say if you get rainfall here it is not it is heavy rainfall here is associated with below normal rainfall here and heavy rainfall here is associated with heavy rainfall here. So, you get a positive correlation between rainfall over western equatorial Indian ocean and over the Indian region and negative correlation between rainfall over central Pacific and Indian region and this is actually a region which has been studied extensively. This is a site of one of the most energetic phenomena in the tropics the El Nino and Southern Oscillation. This is something about which we will talk in fact there will be at least two lectures on this event. This is the most active phenomena in the tropics and it also happens to be the best understood phenomena in the tropics. The physics of which now has been elicited and which models are able to also predict and simulate. So, this is the El Nino Southern Oscillation and what you see here is the link between El Nino Southern Oscillation and the Monsoon. These are opposite in phase rain here and rain there are opposite in phase whereas rain here and rain here are in phase. So, this is the link with what is happening over equatorial Indian ocean. Now, this mode this seesaw between the east and west when it rains more here it rains less here when it rains more over the east it rains less over the west when it rains more over the west it rains less over the east this seesaw between rain over the equatorial Indian ocean is called equino and these two modes Enso and equino actually play a very important role in the inter annual variation or year to year variation of Indian summer monsoon rainfall. So, with satellites then we are also able to see what we call teleconnections that is to say connection with far off events connection of the monsoon with Enso connection of the monsoon with event over the equatorial Indian ocean which is nearer of course and these teleconnections become evident when we look at satellite data. So, the links of the inter annual variation of the summer monsoon to Enso over the Pacific and equino over the Indian ocean are clearly seen. Now, in fact as I said these phenomena will be discussed in detail in later lectures because they are very important events very important phenomena in the tropics. Now, so far I have focused only on rainfall and cloud bands right and why did I do that because I said to people like us who are living in the monsoonal regions of the world what really concerns us is the seasonality of rainfall and not so much seasonality of winds which was the basis for the Arabs definition of monsoon or monsoonal regions of the world. So, that is why I first focused on rainfall and we looked at what is the mean rainfall like and how does the rainfall vary with season and we also looked at different patterns of variability how does the how do the transitions occur the onset phase the retreat phase and fluctuations within the season. Now, all this we looked at on the basis of rainfall because that is where the emphasis has to be in understanding and prediction, but you know rainfall is one facet of the atmosphere. The other important climate elements are temperature pressure winds and they are all interlinked. So, one cannot say that I am interested only in rainfall therefore I will only look at rainfall if you want to understand the system you have to look at the entire atmospheric system and only then you can understand the physics and model it. So, we have to also look at winds temperature pressure how do they vary and what are the linkages between rainfall and winds rainfall and temperature pressure and so on. So, this is what we will consider next and so, let us now look at the Indian monsoon which is the seasonal variation of surface winds and pressure because see the basis for the original definition of the monsoon is the seasonal variation in the direction of surface winds that is how monsoonal regions of the world were defined initially. Now, the mean surface winds over the Indian region and the adjoining season for July and November we have seen here. Now, this is a typical weather map as they say, but this is not a weather map this is a mean picture, but this is the way meteorologists produce maps and it is a good idea to know what this map is representing. In fact, these arrows indicate the direction in which the wind is blowing. So, here you can see the wind is blowing from the south west to the northeast. So, the arrows indicate the direction of the wind and the number of teeth on the arrows. I must also mention that imagine if you wish when you see an arrow like this that you are holding an umbrella with the handle here and this is the tip of the umbrella. So, the wind blows from the handle to the tip and how strong the wind is is determined by how many teeth are drawn on the handle. So, for example, these are very very weak winds and we will see pictures later on of much stronger winds as well. So, what you see here basically is that the direction of the wind reverses. Now, here it is blowing from south west here it is blowing from northeast and this you see this is the seasonal reversal of winds that our Arab sailors were concerned about. This seasonal reversal of winds is really the basis of the definition of monsoon the original traditional definition of monsoon and it is because these winds are from the south west during our summer monsoon which is July. July represents our summer monsoon because the winds are from the southwest in our summer monsoon this is called the southwest monsoon season. Notice that in the north in the November month of November October November December is what we call the post monsoon season November is representative of that. You see that the winds have completely reverse direction it is now blowing from northeast to southwest initially it was southwest to northeast now it is blowing from northeast to southwest. So, this is a seasonal reversal in the direction of wind which was considered to be a very important feature of the monsoon. Now, as I said we have been taught in school and these are words still unfortunately this terminology is still used even by meteorologists. They call this summer monsoon due to September as southwest monsoon because the winds are from the southwest they call October November December as northeast monsoon because the winds are from the northeast. Now as we so you know while the mean summer wind in July in the summer monsoon season is from the southwest the mean surface wind in November which is representative post monsoon season is from the northeast and the name southwest monsoon for the summer monsoon season and northeast monsoon for the what we call the post monsoon season are derived from the direction of the wind prevalent in these seasons. Now, when I talk to you about the basic system which is responsible for the monsoon then you will realize that these are in fact misnomers and they are also they are highly misleading because to a lay person when you call this the southwest monsoon and you see that the wind is coming from the southwest they imagine that this means that the clouds that give us rain all come from the southwest in this season and they all come from the northeast in this season. This is not something that is explicitly taught but this is an impression people have because of the names we use for the season southwest and northeast okay but it turns out that these are highly misleading and are in fact misnomers and therefore we should actually formally abandon them but right now I do not think I will expound on why and therefore that explanation will come only when I explain what is the basic system see somehow when you say the system comes from here and the system comes from here the idea may be that actually the basic system responsible may also be different and it may here it may be originate in the north here it may originate in the south and so on. Now it turns out that that is another misunderstanding that is generated only because we insist on names which were originally generated by sailors because sailors what they care about is actually the direction of the winds. So this is why I think we should stop using these words and actually now consider what is the monsoon system like we have seen already what the monsoon system is like in terms of rainfall. Now we have also seen that monsoon system another important characteristic is seasonal reversal of the direction of surface winds. Now we have to see that for the atmosphere over India as a whole what are the characteristics of the monsoon system so we have to look at the temperature pressure winds and so on only then we will be able to unravel the dynamics of the system that gives us our monsoon rainfall. So in the next lecture then what I will do is actually build up the background necessary for us to understand you know how the winds temperature pressure and so on are interrelated and thereby then I can talk about what is the basic system that gives us rainfall in the tropics as a whole and what is the basic system that gives us rainfall during our monsoon season what is the system that visits us which we call the monsoon. So that is what I will start with in my next lecture. Thank you.