 So, last lecture we have seen that the system basic system responsible for the summer Indian summer monsoon is a tropical conversion zone and today we will talk about tropical conversion zone and the Indian monsoon. So, now summer monsoon the monsoon or the rainy season commences with the monsoon onset over Kerala MOK which is recognized as a rapid substantial and sustained increase in rainfall. So, we have already seen this this is the monsoon onset over Kerala composite for several years 80 years 0 is the onset date and what you see is a very sudden increase in rainfall which is afterwards sustained this is a very important facet of the onset. Now after the onset over Kerala what happens in the onset phase is that the monsoon actually the onset occurs more and more northern parts. So, it goes across the peninsula and then the onset over this part also is there by 10th of June. So, after this then there is an advance of the monsoon in this direction. So, there is an advance which is northward and advance that is westward and by 1st of July you can see most of India is under this way of the monsoon. So, these are the mean dates of course, of onset from India Med Department. Now, the retreat begins in September and slowly the monsoon retreats by October 15th it has retreated from most of India except for the southern peninsula and south eastern peninsula and to retreat from these regions it takes till December. So, October to December is the post monsoon season, but we will first see the summer monsoon season. So, the entire country comes under the sway of the monsoon at the end of the onset phase around the end of June. The monsoon begins its retreat from the northwestern parts around the beginning of September and so July and August are considered to be the peak monsoon months. By 1st of October the monsoon is restricted to the peninsula south of 15 degrees north. Now, for the country as a whole most of the rainfall occurs during the summer monsoon season June to September which we have seen already June, July, August, September. So, these are the four major rainy months and the focus of most of the studies has been the summer monsoon. However, we should remember that southern and south eastern peninsula receives substantial fraction of the rainfall during what we call the post monsoon season. So, in fact the mean monthly rainfall if you look at different places in this northern part which is the monsoon zone most of the rain in fact does occur from June to September as you can see here, but in the south eastern part you see Chennai for example, more rain in October November than in any other months and then Bangalore we have rain from the summer monsoon as well as rain in post monsoon, Palyam Kottayat the tip of the peninsula here we have rain primarily in October November December, but on the west coast we get rain in the most of the rain in the summer monsoon, but the rain continues in October November as well. So, this part whatever theory we have for the basic system responsible for the monsoon must not only explain the fact that over this part the monsoon zone the monsoon most of the rain occurs from June to September, but we should also understand from that basic system that a large amount of rain occurs here in October November and so on. So, now what is the mean rainfall during the summer monsoon and post monsoon seasons? So, this is the mean rainfall during the summer monsoon and this is the monsoon zone over which most of the rain occurs in addition and this is really the seat of the tropical conversion zone that we talked about last time. In addition to that there is the heavy rain on west coast which is partly accounted for by orography and heavy rain in the north east as well. The post monsoon on the other hand we get most of the rain south of 15 north over the southern part of peninsula to some extent over the east coast as well. Now, we have seen from the first study of satellite imagery that Sikha Gadger showed that the maximum cloud zone MCG associated with an active monsoon day resembles that associated with the canonical ITCG the intertropical conversion zone which was known to tropical meteorologists as the system that gives rain over tropical pacific for example. Now, the MCG over the Indian region is associated with cyclonic vorticity at 850 and 700 millibar we have already seen that furthermore Sikha Gadger showed that there is a high correlation between the axis of the MCG maximum cloud zone and that of the 700 millibar trough which is known to be associated with intense convergence in the lower troposphere and maximum non orographic rainfall. So, it was known to Indian meteorologists that the key element in the dynamics to look for is where the 700 millibar trough is because that is the axis of the non orographic rainfall. But Sikha Gadger showed that the what you saw from the satellite as maximum cloud zone axis of that zone and axis of the 700 millibar trough which is the axis of the non orographic precipitation which is to say axis of the large scale rainfall in the monsoon are in fact highly correlated this was shown by Sikha and Gadger. So, the intertropical conversion zone as considered by Chani is a prominent zonally oriented region of moist convection in the tropics associated with high cyclonic relative vorticity and convergence in the lower troposphere this is the these were the major dynamical attributes of the ITCZ. So, putting these two together putting the fact that the axis of the maximum cloud zone was highly correlated with the 700 millibar trough and had all the dynamical characteristics of the canonical ITCZ Sikha and Gadger concluded that it becomes clear that the organized moist convection associated with the monsoon may be attributed to a continental ITCZ over the region. So, they said that the monsoon MCG could be considered as a manifestation of a continental ITCZ and the secondary MCG and oceanic one. Now, the term intertropical conversion zone comes from the fact that convergence in the zone is between air from the two hemispheres. However, we have seen already that on Indian longitudes very often there are two cloud bands one over the equatorial Indian ocean and one over the monsoon zone. So, SG noted that since on occasion there are two MCG simultaneously present over the same longitudinal belt. It is clear that convergence in only one of them can be intertropical from the two hemispheres and so we have dropped the I from ITCZ and we only use the term tropical conversion zone to denote such a system and the monsoon is attributed to continental TCZ or CTCZ over the Indian region. So, this was the conclusion. So, now today we are going to talk about the variability of CTCZ. CTCZ is the basic system responsible for the large scale monsoon rainfall. So large scale rainfall over the Indian region during the summer monsoon is associated with a TCZ and this TCZ is called the continental TCZ as it occurs over land to distinguish it from the more common TCZ observed over tropical ocean. So, first let us consider the summer monsoon season and this is the monsoon zone that we have seen before over which the large scale rainfall occurs. Now, since we are going to look at the CTCZ in this lecture, we will look at first and foremost what the satellite shows us. So, what we now look at is the climatology or mean or the average outgoing long wave radiation map for June to September. So, average over 4 months and what you see here is the darker the shade means higher the clouds the lower the oiler. So, these are clouds with very very high tops here the dark ones and the entire region is marked so that only oiler below 240 watts per second is shaded. So, there are deep clouds almost everywhere here, but highest frequency of deep clouds occurs in this region here this dark region here. So, from June to September you can see this is what gives the rain over the monsoon zone here. You have this kind of a gradient that you saw also in rainfall, but you see a lot of rain occurs over the head bay during June to September. You notice that there is another tongue here over the equatorial Indian Ocean. This comes from the oceanic TCG that we have seen that occurs simultaneously sometimes with the continental TCG. So, during the summer monsoon low values of oiler occur the over the Indian landmass the Bay of Bengal and the equatorial Indian Ocean. Over the Indian region the CTCG occurs over the monsoon zone that is between 15 and 25 naught low values of oiler also occur along the west coast of peninsula and the north east that we have seen before ok. Now, the CTCG gets established over the monsoon zone at the end of the onset phase that is the spring to summer transition, the summer to autumn transition is the retreat phase of the CTCG. So, consider first the variation of the CTCG from spring to autumn on a monthly scale ok. So, April may are supposedly the spring months and first we see the picture in April and what you see is this of course, is the mid latitude rain that you see here this is nothing to do with the tropical systems. What you see is a tropical conversion zone if you wish over the eastern equatorial Indian Ocean. So, there is rain over eastern equatorial Indian Ocean on the average no rain at all over the western equatorial Indian Ocean and so, the rain is restricted to this region the deep clouds are restricted to this region in April. Now, in May you see that it has expanded northward and a little bit westward right. So, the region extends from 5 south to 10 north see it extends a lot over this region over 70 to 90 from 5 south to about 10 north it has see tip of India has also come under the low OLR region and it is much further northward over Myanmar and this part of the bay ok. Now, this is the June mean June rainfall remember the MOK or monsoon onset over Kerala typical date is 1st of June. So, entire June the monsoon should have been here so, the and furthermore you know that by 10th of June it covers this much of the country and then later on progresses. So, almost the entire peninsula is now under the sway of the monsoon when we consider the mean monthly OLR picture, but notice the high rainfall or low OLR region of the west coast and also head bay near Myanmar coast. So, now June is over now we come to the peak monsoon months of July and August and here you see a very typical pattern this is the continental TCG here now. This is the continental TCG, but remember that it does not end with the Indian region in fact as you go eastward it dips into the bay where it is even more intense ok. So, if you like you can say the CTCG here changes to an oceanic TCG here. So, we have an oceanic TCG and a CTCG here and notice we have a full flushed oceanic TCG here ok. So, this is the equatorial Indian ocean this is for the months of July and August and if you look at August sorry the last one was for the month of July this was for the month of July and you see already the CTCG is well established and this is the month of August and again you see the equatorial Indian ocean ITCG here the TCG here and this is the CTCG with convection over the bay. This is the month of September remember first of September the monsoon already starts retreating from here. So, September the rain is restricted more or less to the eastern part here of the monsoon zone and of course the peninsula gets a lot of rain in September. Now, the axis so to speak or where the deepest clouds are the lowest oiler is has already shifted towards the equator, but it is spread a little bit across the bay as well this is the September story. Now, during October and November the TCG is most intense over the east equatorial Indian ocean remember that the south eastern peninsula comes under the sway of the TCG in this month and consequently receives a large fraction of its annual rainfall. This is the picture we saw earlier that you know places like Chennai and Palemkotta and so on get a very large fraction of the annual rainfall in October November and what you see is that in October November the center of the CTCG is over the equatorial region ok, but and it is spread way up to about 15 north here you see. So, the center is here and we are looking at mean monthly picture and it is spread right up to here. So, associated with it we get rainfall over India as well, but you have to remember that the center is here this is different from the summer monsoon case that you saw earlier where see for example, July where there was a prominent band of cloud right over the land here which was connected to the band of ocean no doubt, but it was prominently here and this was where the deepest clouds were if you looked at the entire 80 degrees east the deepest clouds largest rainfall would be here. So, it is not as if the system has spread to that the system axis is here on the other hand when you look at the post monsoon it is a different story the axis is around here and this is the periphery of the system which is giving us rainfall in October November same story even more so in November you see the system has become more intense and shrunk a little more and only a small part of the peninsula is getting deep clouds in November. Now, an important question to address is why is there hardly any convection on the Arabian Sea in these months we can just note this if you look at October and let us look at the earlier season say July or August if you look at August there is quite a bit of low oil are here over the Arabian Sea that is east of 70 degrees east and so, but even then there is hardly any low oil are here, but in the equatorial region you see that this is extending right up to 60 east or so in August. Now, what happens in September also you see it up to 60 east, but October now it has begun to shrink, but the equatorial thing is fine in October, but November you see it has shrunk even further and there is absolutely nothing over the Arabian Sea in November whereas, in October in October also you did not have much over the Arabian Sea, but there was a little bit here, but in November it has totally disappeared therefore, an important question to address is why is there hardly any convection on the Arabian Sea in these months and in fact, I shall address this question at a later stage in this lecture series when we understand a little more about what determines where the TCG will occur over the oceans. We are going to look at that a little later and after we understand that we should be able to explain why there is hardly any convection over the Arabian Sea during October November, why is it restricted to the Bay. Now, in December which is the last month of the October to December season you can see that in the mean there are no deep clouds at all over India of course, there would be events in which they occur here, but other times there are no clouds at all. So, the average OLR does not reflect few days of deep clouds here. So, again you see here December is very much concentrated on over eastern equatorial Indian Ocean and this part of West Pacific with no deep clouds anywhere else. So, climatology of OLR for October to December is that the TCG if you wish the axis of the TCG is on the equator it extends basically from about 90 east which is here to about 110 east which is here, but on the periphery are you know south peninsula and so on which get rain during the post monsoon season. Now, question is what is the system which gives us rain in the post monsoon season. In the summer monsoon we have seen that it is a tropical conversion zone and we have called it a continental tropical conversion zone because it occurs on the continent. Now we should see what is the system that gives us rain in the post monsoon season. Now this is an example that we have seen before of what happens in the summer monsoon season this is 7th of August 2007 and what you see is a very bright cloud band stretching across here and you can see this is India I hope you can see that and that is Sri Lanka right there and this cloud band which we would call MCG or maximum cloud zone is over the monsoon zone here, but it does not stop here it stretches right across almost to the west Pacific. So, this is the tropical conversion zone which is responsible for our rainfall in the summer monsoon. This is another picture where we have 31st July where again you see this and interestingly at the same time we have an oceanic TCG here as well with a link between the two. So, there are two tropical conversion zone and this is what we meant you see we cannot have inter tropical conversion in both this and this it has to be only in one or the other that is why we call them tropical conversion zone and on 15th of October you see this is again a tropical conversion zone and beginning to look more like the classical canonical ITCG and what you see here is again very similar to what you saw earlier, but now to the south what you saw occurring here in our summer monsoon is now occurring here at the foot of the peninsula so to speak. The toe of India is dipping into the cloud band here and what you see is a cloud band very characteristic of a tropical conversion zone on 17th October. So, we are now in the post monsoon season and as far as the satellite is concerned the picture is very similar. Now this is in fact a global picture for the next day which is 18th October and what you see is the cloud band you saw here corresponding to our tropical conversion zone. What is interesting is across the globe you see this ITCG here see this is the Atlantic one and this is the Pacific one you see ours is more intense and somewhat larger in latitudinal extent, but you can see that is the same animal which is sort of girdling the earth in the equatorial regions which is responsible for our post monsoon rain and this is a case of a tropical conversion zone from which a cyclone was born and now the cyclone is taking off you can see this very beautiful outflow region of the cyclone and the cyclone is taking off from this region and the same picture here now you see because the cyclone has become so strong the ITCG horizontal extent has decreased, but the ITCG persists over the Pacific. So, the system responsible for organized rainfall in the summer monsoon as well as the post monsoon is the same system it is a tropical conversion zone. However, the summer monsoon June to September season is called the south west monsoon season because the low level winds are from the south west and the post monsoon season October to December is called the north east monsoon season because the low level winds are from the north east. So, you can see here this is a slide you have seen before that in July the winds are from the south west and in November they are from the north east you can see that very clearly here this is the south west monsoon because it is winds are from the south west and this is the north east monsoon because the winds are from the north east, but I believe these are misnomers why are they misnomers because they give the impression that the system responsible for the rainfall in the two seasons is different which is clearly not the case as we have seen that the same system namely the TCG is associated with the rainfall in both the seasons. Also the term south west north east give the wrong impression to lay people that rain comes from the south west in the summer monsoon whereas, it comes from the north east in the post monsoon season. See simply because the wind is in that direction does not at all mean the rain comes from that direction, but by calling a rainy season by name which conveys the direction of the wind people who do not know the system which includes a vast majority of the people in the monsoonal region assume that in south west monsoon rain must be coming from the south west and in north east monsoon it must be coming from the north east. I therefore suggest that the term summer monsoon and post monsoon and not south west and north east monsoon be used for the two monsoon seasons that we experience in India. Now, note that the CTCG. So, the lesson so far is that right from the onset of the monsoon over Kerala in 1st June to the retreat around mid December from the entire Indian region the system responsible for the large scale monsoon rainfall of our country is the tropical convergence zone. Now, note that the CTCG is established over the monsoon zone at the end of the onset phase towards the beginning of July and the CTCG primarily fluctuates over the monsoon zone during the peak monsoon months of July and August. The retreat of TCCG from the western part of the monsoon zone commences in September. Consider first the spring to summer transition. Now, let us see how the transition occurs northward shift is accomplished and here I would like to go to the picture first and we will come back to the description. What you see here is a for a particular year these are bands. Now, this is latitude going from 10 south to 25 north and we are sitting over 90 degrees east which is the latitude of around Calcutta. So, sitting at that longitude we are seeing where are the deep clouds. So, on this specific day this is March on this day in March there is a very deep cloud here and shallower clouds here. Now, you see below 180 watts per second is the darkest color lighter than that is 180 to 160 I think below 160 is the darkest 160 to 180 is the middle one and 180 to 200 is the lighter one. So, everything colored in green corresponds to deep clouds because they involve on a daily scale OLR or outgoing long wave radiation of the order of 200 watts per second. So, what you see now this is time and this is latitude. So, what you are seeing here is a northward surge which went from 10 south to about 5 north then it hung around for a while another northward surge here which went further northward and then finally, another surge here which took it to its final destination around 20 north. So, this is the spring to summer transition it is made up of several northward surges one after the other and that is what you saw here northward shift of the TCG from its mean location near 5 degrees north in April may to 20 degrees north in July, August is accomplished by successive generation of northward moving epochs over the equatorial region. HG found that the average rate of northward progression is about 1 degree latitude per day and this is the rate of progression that we saw in the next slide. So, what is the average rate of this northward progression that you see they found that it was about 1 degree latitude per day, but remember this process is not linear it is not as if it will cover 10 degrees in 10 days rather it is a very jerky process because the basis is all non-linear instabilities and so it is only an average we are talking of order of magnitude of the rate sometimes you know it will go up to some point and hang around there and then go northward as you see here this happens very very often. So, even here you see it started and then it went here very fast and then it kind of slowed down and then continued. So, it is not a uniform progression, but typically it is about 1 degree a day that is the progression. The maximum latitude attained by each such band is generally further northward than that of the preceding one and after one or more surges of this type the CTCG gets established over the monsoon zone towards the end of June or beginning of July. So, you saw here as you can see very clearly in the envelope we have drawn that the maximum latitude attained in each surge is further and further northward till it comes to a month of July and then it is going here. Now this is the retreat of the monsoon where if you look at the envelope is going southward, but are the bands moving southward no not at all. In fact what is happening is the bands are still generated over the equatorial Indian Ocean and they are moving northward, but the latitude they reach is becoming more and more southward. So, this thing has gone right up to beyond 20 north, but the next one here has gone only up to about 7 or 8 north. So, this is what happens. So, actually it is interesting that this more about the spring to summer transition we had surges that went more and more northward as with every succeeding surge. It is interesting that this mode of northward shift of the TCCG is similar to northward progression of monsoon range over India which is also accomplished in such surges. So, monsoon range onset also occurs in surges and sometimes one surge will take it up to a certain point and the next surge will take it further northward and so on and so forth. Very similar thing we see in cloud bands. Now when we see the summer to autumn transition again the transition begins with the final disappearance of the TCCG from the region north of 20 degrees north in September. Note that this transition also comprises northward moving epochs just like the spring to summer transition and north southward moving ones as would be expected from the monthly location of the TCCG. During this transition the maximum latitude attained in each northward moving epoch decreases progressively we have seen this already in this picture. So, we have a situation in which we have northward movement throughout, but the final destiny of the northward epochs actually becomes further and further northward in the spring to summer transition and further and further southward in the summer to autumn transition that is how it occurs. Thus in the summer monsoon as well as the post monsoon season an important feature of the inter-seasonal variation is the northward propagation of the oceanic TCCG on to the subcontinent. So, this is a very very important feature of the variation of the cloud bands or the inter-seasonal variation or variation within the season. Perhaps the most dominant feature if we start looking at clouds is the northward progression and this is a picture that you have seen earlier from Sikha and Godgate and what you see here is that these northward progressions occur from April right up to October and they occur year after year we have already seen that and this is from 70, 80, 90 degrees east in 75 and you see that the northward progression is coherent across the Indian longitudes you see it particularly here for example 80, 70 and 90 which means that a band stretching right across from the Arabian sea to the Bay of Bengal is going all the way from the equatorial region to 20 north or so as a band that is why these northward movements are coherent that is why these are movements of an MCG. So, they are coherent across now the question is what is the contribution of these propagations to the maintenance of the CTCG. See we call it a continental tropical conversion zone because it is on the continent but we also notice that all the time we are getting bands from the equatorial Indian ocean which come and end up in the CTCG. So, obviously CTCG is maintained partly by these northward movement TCCG from oceanic side. So, question is to what extent does CTCG existence depend on that what is the contribution of these propagations to the maintenance of the CTCG and in fact in that paper Sikha and Gargil also address this. Now, how do they address it to that to do that we have to again do a define a few things properly. First of all we have already noted that there are two favourable locations for MCG one over the heated subcontinent and another over the equatorial Indian ocean and remember they assume that the boundary between the two is a kind of no man's land where the frequency of occurrence was minimum around 7 degree north or so to the south of that is what they call a secondary band to the north of that is what they call a primary band or a monsoon band which we can call the CTCG. So, variation of the mean location of the axis of the primary MCG and at this point this is the primary MCG. So, we should not call it the CTCG at this juncture becomes the CTCG only here when it is overland. So, this is the primary band and this is the mean location of the axis of the primary band moving northward and moving southward, but throughout the season June to September intermittently we saw secondary band appear over the equatorial Indian ocean and that is what you see here and that axis remains more or less fixed standard deviation is higher here when it is to the north and the standard deviation is roughly same all around here. So, these are the two bands we are talking of what is the contribution then of the oceanic TCG to the maintenance of the CTCG. Now, they also derive the standard deviation of the axis and this is the standard deviation of the axis and what you see is generally it is below 7 degrees. The spread is a little large in July, August, but it is all less than 6 degrees or so where a standard deviation here is about 4 degrees for the secondary band. So, because the standard deviation of that band that is how much the axis varies just due to fluctuations in situ is less than 6 degrees, SG considered northward movement greater than 7 degrees latitude within the lifespan of an epoch is significant since the standard deviation of its axis is less than 7 degrees for every month. So, remember epoch is the starting of an MCG epoch or an event it starts at a certain latitude on a certain date and then after a lifespan of a few days it dies within its lifespan it may propagate it may not propagate. So, consider first the latitude and date of origin of an MCG epoch that is latitude and date at which it first appears at the given longitude. Now, we have to remember that we are looking at the band and as such we are not worried about what happens east-west progression and so on at this point we are focusing on north-south. So, the appearance of a band at a given longitude may be the result of a generation of a new cloud band at the location or an advection of a band generated from east or west. If we are sitting at say 90 degrees east and we say MCG epoch started at 90 degrees east on a certain date it could have started because something moved there from 100 degrees east or it could have started because a cloud system got generated there at 90 degrees east this is something that we have to bear in mind. Now, look at the variation of the number of MCG epochs with latitude of origin. So, how many epochs are generated at different latitudes going from 0 and all the way here till 30 north and solid lines indicate epochs with significant northward movement. These are epochs with significant northward movement and these you can see up to here and what you see from this picture is that vast majority of the epochs are generated over the equatorial ocean relatively few are generated over the continent. This is something very clear that most of the MCG epochs are generated over the equatorial region 0 to 10 north from April to October. Now, we have to remember that in the Sikha Gadgil study the data they had available was hemispheric imagery or hemispheric cloud mosaics. So, their data was restricted to 0 to whatever you wanted in the northern hemisphere restricted to the northern hemisphere. So, they could not go south of the equator, but later studies have found that it is equator is not a Laxman Resha at all. In fact, equatorial Indian Ocean means 10 south to 10 north also. So, actually it is 10 south to 10 north, but Sikha Gadgil's data is only from 0 to 10 north. So, they found that most of the epochs are generated 0 to 10 north also MCG epochs without significant northward movement dominate over equatorial region. So, what we saw was over the equatorial region that the dashed lines are much much longer than the solid lines. So, MCG epochs which do not dominate which do not move northward are dominant here and they are dominant almost everywhere and north of 18 there are no northward moving epochs at all. Because for it to be northward moving it has to move at least 7 degrees and there is not that kind of a span left for them to move. So, without significant northward movement dominate over equatorial region, well north of 18 degrees only such epochs are generated which is natural because a minimum displacement of 7 degrees is considered necessary for significant movement. Now, this is a table showing percentage of epochs starting in different latitudinal belts 1, 2, 3, 4, 2, 6 and so on in different months and what you can see here is that in April vast majority are generated right near the equator in May the mode shifts a little bit here, but in June it is more flat right up to here and July it is still equatorial, but you are getting some more being generated in 10 to 12 and so on. Now, you see longer and longer tails develop here. So, when you come to June to September you do get quite a few epochs being generated even north of 20 degrees. These are generally the systems generated on the head bay and so on and so forth which we will come to later, but note that the percentage of epoch generated north of 15 degrees north in the peak monsoon months of July and August. So, if you want to look at how many epochs are generated north of 15 north that is over the monsoon zone in the peak monsoon months of July and August when CTCC fluctuates primarily over the monsoon zone there only 30 percent in July and 10 percent in August. So, even in the peak monsoon months when the CTCC fluctuates primarily in the over the monsoon zone most of the epochs are still generated south of the monsoon zone. This is an important point to remember thus even when the CTCC fluctuates primarily over the monsoon zone the contribution of the northward moving epochs is very large. Now, frequency of epochs with different lifespans. So, if we look at frequency of epochs with different lifespans then it is very interesting in April they are very very short lived epochs. Typically 3 to 4 is the maximum number I have lifespans between 3 to 4 days several have between 5 to 7 days none last longer than 13 days. Now, long living epochs longer lifespans epochs characterize the major peak monsoon months here July and August they are the longest ones long duration epochs occur in July and August primarily, but rest of the months they are typically shorter with worse majority being less than 5 days or so. And you can see that here now this is of course combined for all the months and here you see separately the northward moving epochs from the non northward moving epochs. And you see that the longest epochs are those with northward movement see solid bars here which indicate the epochs which move north actually dominate. In fact, they are only epochs that survive up to about 15 days and beyond are solid or the northward moving ones. And later on actually here the ones that have shortest lifespans are the ones that do not move northward at all. So, you can see all epochs with lifespans beyond 18 days that is here onwards and almost all with lifespans beyond 10 days that is beyond here almost all of them are northward moving epochs. This is an interesting thing to note. So, already we see the dominance of northward moving epochs in July and August. Now what Sikha Gargill did was to systematically divide the epochs into 3 types type 1 is generated in the equatorial region which do not cross over onto the monsoon zone and this had a mean lifespan of about 4 days. See these are cloud bands that appear over the equatorial region and disappear in about few days. Type 2 was generated in the equatorial region which cross over onto the monsoon zone which had a mean lifespan of 22 days. And type 3 are generated within the monsoon zone as I said these are typically systems generated over the Bay of Bengal which then move westward onto the monsoon zone. The type 3 epochs generally involves synaptic scale systems generated over the Bay or appear over the Bay by westward propagation of synaptic scale systems generated over west specific moving on to the monsoon zone. Now SG found that the northward moving epochs that is type 2 epochs contributed 82 percent of the days on which MCG was present over the monsoon zone and 78 percent of the days on which MCG was present over the equatorial region. So, whether we consider the equatorial region or over the monsoon zone the maximum contribution seems to be coming from epochs which are northward moving epochs which are generated over the equatorial Indian Ocean and which move northward onto the monsoon zone. So, this is a very convincing proof of how important this is these epochs are. Thus the convective days associated with the CTCG over the monsoon zone in July, August are dominated by epochs generated over the equatorial Indian Ocean which move northward. Hence the contribution of northward moving epochs of the oceanic CTCG to the maintenance of the CTCG is substantive. So, we expect the variability of the CTCG on different scales to be linked to the variability of convection over the equatorial Indian Ocean that is very clear because convection over the equatorial Indian Ocean now has turned out to be a lifeline of the monsoon. A lifeline of the continental tropical conversion zone which is the basic system responsible for the monsoon. So, naturally variability of the monsoon will be linked with variability of convection over the equatorial Indian Ocean which is the source of this lifeline. Now, synoptic system generated over the Bay of Bengal which move westward onto the Indian region also contributes significantly to the maintenance of the CTCG. An example is the systems of the summer monsoon that we see here and see 99 summer monsoon for example, so many monsoon depressions were born here and a low pressure system was also born and you can see they are all generated here and move along the monsoon zone this way. So, these are tracks of lows and depressions. So, we do get contribution from such systems also and these will not be type 2 because type 2 means the systems come from here. These are type 3 which means the what we call in situ which is to say within the latitude of the monsoon zone the system is getting generated and giving green. So, this is the other system the tracks of all cyclonic storms and severe cyclonic storms from July onwards. This is from IMD Atlas track of all cyclonic storms and this is the mean July rainfall and you can see that there is an association of the July rainfall with the tracks of these storms and part of the large scale rainfall certainly can be attributed to genesis and propagation of synoptic scale systems. For the season as a whole the maximum number of low pressure systems are generated over the head bay of Bengal. See this we have seen before this is the frequency of genesis of low pressure systems in different places and the maximum occurs here this is 340 and they all move across and give us this kind of June to September rainfall over this is the monsoon zone. So, these systems do contribute a great deal these are the what Sikha Gargill called the type 3 events. So, I think now we have to also see how often does the propagation from the within the system occur type 3 and how often type 2 occur and in fact Sikha Gargill have also looked at how typically in a season the CTCG gets established how it fluctuates and when it fluctuates between active spells and weak spells how does it revive after a weak spell and that is what we are going to look at next time. So, what we have seen this time is that in fact the tropical convergence zone is the basic system responsible for the summer monsoon rainfall as well as the post monsoon rainfall and the rainfall that we experience the variation of the monthly rainfall that we saw across India is consistent with this as we saw from the OLR data how the TCG moves from month to month. Secondly, we saw that northward propagation which is the most important feature of inter seasonal variation of the maximum cloud zone in fact dominate the entire picture. They are a key element of the seasonal transition with the spring to summer seasonal transition occurring with northward propagations which take the monsoon further and further northward and the summer to autumn transition which is the retreat again inverse northward propagation not southward which take the band the culmination of the northward propagation is at more and more southern latitude that is how that has occurred and we have also seen that if we look at the contribution of these northward propagations to CTCG over the monsoon zone and to the oceanic TCG then the maximum contribution occurs from northward propagating MCG is more than 80% for the CTCG and very close to 80% for the equatorial oceanic ITC. So, northward propagating epochs of MCG are a very critical element of the monsoon of the seasonal evolution of the monsoon as well as the inter seasonal fluctuation of the monsoon. This is what we have seen by the analysis done so far. Now, we will see how the monsoon evolves and how the different revival by different processes takes place and so on and so forth in the next lecture. Thank you.