 We are now discussing tropical convergence zones and the monsoon and we have already seen that the monsoon is a manifestation of the visit of a tropical convergence zone over the continent which we call the continental tropical convergence zone. And we have been looking at how it fluctuates. So let us now consider fluctuations of the CTCG in July-August which are the peak monsoon months. So during peak monsoon months there are fluctuations in both the location and the intensity of the maximum cloud zone over the monsoon zone with spells in which it is absent for several days over the monsoon zone. These are the cloud free spells and we have seen that during the peak monsoon months of July and August there are three types of MCG epochs which occur and those are type one is the one in which the MCG or the maximum cloud zone is generated over the equatorial region but which do not cross over onto the monsoon zone. That is to say they get generated there over the equatorial zone, fluctuate perhaps in intensity and then die. Now the mean lifespan of such episodes or such epochs MCG epochs which are born over the equatorial Indian Ocean and died over the same region the mean lifespan is only four days. Type two are the most interesting of the epochs. These are the ones which are generated over the equatorial region which cross over onto the monsoon zone. These are the northward propagating episodes or epochs. Now type three are those which are generated within the monsoon zone belt that is to say often over the head bay of Bengal and their mean lifespan is again six days only. So those that are generated over the equatorial region as well as those that are generated perhaps over the head bay or the monsoon zone have relatively small lifespan. The ones that are northward propagating modes have the largest lifespan. Now we talked about how the city CCG revised from cloud free spells. Now when we look at the observation there are two kinds two ways in which this cloud free spell can get terminated either with what Sikhan Gargill called in situ generation of cloud band within the monsoon zone that is to say within the belt of the monsoon zone latitudinal belt of the monsoon zone generally over the head bay. Alternately the one can also get revival from cloud free spells by northward movement via type 2 epochs. So this is revival by Genesis over the equatorial Indian Ocean subsequent northward movement. Now SG found that revival by type 3 occurred in 9 cases whereas type 2 occurred in 13 cases. So the two numbers are comparable given the small size. So frequency of revival by the two modes that is northward propagation of the equatorial CCG or westward propagation of synoptic scale systems from the bay is comparable. Hence the city CCG is maintained by systems generated on the equatorial Indian Ocean as well as over the Bay of Bengal. Hence the variability of the city CCG is naturally linked to the variability of convection over these oceanic regions. This is something that we have seen last time. Now there is a very special feature a very special sequence of events which one sees year after year in the evolution of this season. And this is the mid season disappearance and revival of the maximum cloud zone or MCG. Now this was noted by Sikha and Gadgil and what we see is that despite the variation from season to season some common elements of the variation of the MCG during the summer monsoon emerged from this study. And what are these common elements that Sikha Gadgil found that for all the seasons studied the following sequence of events occurs after the MCG is established over the monsoon zone at the end of the onset phase. So at the end of the onset phase after the MCG is established over the monsoon zone then every year the following sequence of events occurred. What happens? Now for the first 2 or 4 weeks 2 to 4 weeks after establishment the MCG fluctuates over the monsoon zone persisting almost day after day with only 1 or 2 short spells of 2 to 3 days in which it is absent and then disappears. So for the first 2 to 4 weeks the MCG is fluctuating over the monsoon zone and then disappears. Now a secondary band is generated over the equatorial region more or less simultaneously with this disappearance. So first of all at the end of the onset phase the MCG is established over the monsoon zone. Now in the first few weeks it fluctuates within the zone with few cloud free spells which are not very long and after which it revives and then disappears. Now when it disappears a secondary band is generated over the equatorial region more or less simultaneously with this disappearance. Now in 3 of the 5 seasons studied 4 to 8 days later a cloud band appeared in the region north of 25 north as a result of westward extension of a mid latitude band from China or Japan. Now all this is going to become very clear to you when I show you illustrate all this with an example from 1975. So but let us just try and list the events that happened. So first you have the establishment of the CTCG over the monsoon zone then for a few days 2 to 4 weeks it fluctuates within the monsoon zone more or less and at the end of that it disappears. Now after it disappears a few days after it disappears a band comes over the region again at 90 east but it does not necessarily penetrate to over the Indian region west of 90. So it is often not seen at 80. So another band appears and this is what we used to call the Chinese connection. This is a cloud band which goes from about it is generally north of 25 north and it goes from there right up to China or Japan. Now the equatorial band continues to persist coexisting with this northern band throughout the latters life of 4 to 7 days with some fluctuations often. Now soon after the disappearance of the MCG or the band when it occurs see this band does not occur every year. So once the MCG disappears and this band disappears soon after that the secondary band begins to move northward. Secondary band is the band over the ocean which we also call the oceanic TTCG. Now this northward movement lasts for about a week. The MCG gets re-established over the monsoon zone by one or two such northward surges. Now this sequence of events Sikha Gargir noted occurred at least once in each of the 1973 to 77 seasons and twice in 73 and 75. So you have a sequence of events in which the continental TTCG disappears, a secondary band appears and the continental TTCG revised by northward movement of the secondary band. This is the sequence of events which we see year after year during the summer monsoon season and sometimes even twice. Now in fact what I will do is show all these events by an illustration of the typical sequence of events associated with the re-establishment of the CTCG by an northward moving epoch of the secondary or oceanic TTCG by looking at one example. And this is 14 to July 1975 to 8th August 1975. So let us look at this case and you will see all the features that I mentioned. So what you see here is a daily variation of the MCG, MCG axis which you see here as the solid line, the southern and northern limit of MCG also which you see here and in addition to that the 700 millibar trough which we have been talking about which is the axis of the 700 millibar trough which is the axis of the non-aurographic large scale rainfall. So this is the, this is what you get from the weather charts, this is what you get from the satellite imagery and both are shown here. So you see the daily variation of these things when they do not occur in the image, in the image when there is no maximum cloud zone you will get a gap like this. For example over this latitudinal belt there is a gap. So now let us follow the thing. See the CTCG has already been established over the monsoon zone. Now this is 14th of July, typically it gets established towards the beginning of July. This is 90 degrees east and this is 80 degrees east. So this is the central longitude of India and this is 90 degrees east which is the one that passes through Kolkata. Now so this band was there, this is the CTCG. Notice that while it was going strong another band appeared but soon disappeared within 3-4 days. You see that secondary band appeared but disappeared. Now at the end of this phase here around 21st of July this CTCG or the MCG has disappeared. And soon almost simultaneously you see that a secondary band has formed at 80 and 90. So the northern band has disappeared and a secondary band has formed. Now after a few days this Chinese connection has come up. A band which connects goes all the way to China or Japan starting at 90 degrees only here not seen at 80 degrees but you see this middle attitude connecting band, cloud band here. Throughout its existence more or less the secondary CTCG is around. It has not died for several days it continues to coexist it weakens a little bit but is there at the same place when the band dies. So this is when that band disappears finally. Now as soon as the band disappears this secondary CTCG which is here when it disappeared starts moving northward. You see that and at 90 degrees the movement is continuous and at 80 degrees it occurs in 3 spells that you can see. You can also see that during this movement the 700 millibar trough is more or less consistently moving with this. In fact let us note that when we began a clouds free spell here when the CTCG disappeared from the monsoon zone here we had a 700 millibar trough line around 10 north that was seen both at 80 and 90. You have to remember that this trough line is drawn from the weather charts. So one can primarily draw it on land and you know south of 8 degrees north is ocean. So this is why the southern most location is generally just 8 degrees or so. So a zone of cyclonic vorticity has appeared here even before the thing has started moving and then it starts moving and when it moves the 700 millibar trough also moves with it. So this is the northward propagation. So this is the sequence of events first the band being established then it hanging around then it dies once it dies it is all quiet on the western front for few days during that time the secondary band has appeared then this mid latitude band from china has come and when that dies then you get clean northward propagation at 90 and reasonable northward propagation at 80. So this is the sequence of events that we talked about and so at 80 and 90 the monsoon MCG or CTCG disappears on 20th July secondary band appears at 90 is the band north of 25 north is generated on the 24th of July and you have seen that already this is the generation of this band on 24th of July and persists till 29th of July this is the Chinese connection this band is not seen at 80. The 700 millibar trough occurs around 10 north during 21st to 28th July this is soon after the disappearance of the MCG or the CTCG from the monsoon zone. Now the secondary band disappears on the 26th but reappears on 28 so by and large it is present during the time the mid latitude band is also present and northward propagation then occurs up to 6th August which is clearly seen at 90 where it is continuous and at 80 where it occurs in 3 spells we have seen this. Now this northward shift of the MCG occurs in association with successive formation and the northward movement of 3 disturbances. Now we have been emphasizing time and again that although we think of tropical convergence zone as a planetary scale cloud band which is east west actually generally embedded in this planetary scale band are synoptic scale disturbances and often in the northward propagation you see them as propagation of synoptic scale disturbances. Now that you see in the next few things this is 25th July remember this is a few days after the CTCG has disappeared from the monsoon zone and what you see here is a low pressure system this is at 700 millibar and these are streamlines and you can see a cyclonic circulation at 700 millibar just around 10 north or so and this is the 700 millibar trough that you can see here. So a trough has formed here on 25th July and you can see it is associated with this vortex. Now what happens on 31st July on 31st July there is a movement it has gone northward where it was around here on 25th and it has moved northward now and you see a very clear cyclonic vorticity pattern at 700 millibar on 31st July now it is centered closer to I believe 15 north or so and the next one is I am sorry this is again 31st July this is a repeat so this is 31st July and then you see 6th August it has now gone north of 20. So we have seen a northward propagation very beautifully in terms of the 700 millibar trough and the vorticity and it has gone all the way in one week from about 8 to 8 or 10 to about 22. So this is a example of the propagation and you can see that there are clearly two synoptic scale disturbances embedded in this trough here. So this is the story that occurs year after year now the period between the establishment of the MCG over the monsoon zone at the end of the onset phase and the appearance of the secondary band over the equatorial Indian ocean which was responsible for the re-establishment of the MCG over the monsoon zone varies from 25 to 33 days with a mean of 28 days and a standard deviation of 2.7 days. Now what are we talking about at the end of the onset phase the band gets or the MCG gets established over the CTCG that would be sometime in early July right then secondary bands may appear after that in fact it may be worthwhile to go back to this slide. Now this is 14th of July so for several days almost 2 weeks already the CTCG has been established over the monsoon zone. Now during this period a secondary band did appear here but this band did not lead to any propagations or re-establishment. This band appeared when the CTCG was active here and it just died nothing great happened with this band but then this band appeared and this is about 14 days or so about a few days I think 7 days it has appeared in this case. So it may be about 3 weeks or whatever since the establishment of the band and this is the band that actually led to the revival through northward propagation. So this is what we are talking about here and so there is a time scale emerging from here and that time scale is the period between the establishment of MCG over the monsoon zone at the end of the onset phase and the appearance of that secondary band over the equatorial Indian Ocean which is responsible for the re-establishment of MCG over monsoon zone varies from 25 to 33 days this is what they are saying with a mean of 28 days and standard deviation of 2.7 days. So a time scale is emerging here a time scale of about a month and this is very interesting because this has to be pursued further now. So what is this saying this is saying that the continental TCG or the CTCG cannot survive for longer than about a month after its establishment over the monsoon zone without re-establishment by northward moving epochs of the secondary or the oceanic TCG. So it needs re-establishment of the moisture from this oceanic TCG one month after its first establishment if it is not without that it cannot be maintained this is very clear because this is seen year after year. We have seen that the equatorial TCG plays a very important role in maintaining the CTCG over the monsoon zone and so we expect that you know in fact equatorial oceanic TCG is a friend of the monsoon it is helping it is helpful in maintaining the CTCG over the continent and therefore helpful in the last scale monsoon rainfall which is associated with the CTCG. But you know the relationship is not all positive the relationship between the CTCG and the oceanic or the secondary TCG over the equatorial Indian Ocean is complex and why is that because there can also be competition between the CTCG and the oceanic TCG. You may remember that in a TCG the air is ascending and TCG is a zonal belt from extending over some latitudinal range. Now north of the TCG as well as south of the TCG air that is pumped up by the TCG actually descends. So, this means that if there are two such animals which are at two latitudes over the same longitudinal belt then each will try and suppress the other through the descending limb. So, this we expect will lead to a competition between the two TCG's and in fact what Sikha Gadgir found in that study was that spells during which the monsoon MCG was almost continuously present for periods of two weeks or more and we have seen an example of this in the beginning from 14 to July onwards when a secondary band appeared. So, when it is continuously present for periods of two weeks or more the secondary MCG appeared intermittently with each epoch being rather of rather short life span. So, it is like it just appears and within a few days it is gone. This is because the monsoon MCG or the CTCG is going strong. Now, the secondary MCG persists for longer periods only when the monsoon MCG disappears. So, there is a seesaw in the activity of the monsoon MCG and the secondary MCG or the CTCG and the oceanic TCG. There is a competition between the two and active spells of monsoon TCG will be associated with weak spells of the equatorial TCG and vice versa. This is what we expect due to competition and this was deduced from the kind of picture you have seen earlier that in several cases for example, here in 1975 you see that when the thing is going strong several times the secondary band has appeared, but of no consequence only when it disappears finally, then the secondary band goes strong and moves northward. Similarly, here now when the band is going strong oceanic TCG has appeared only for 2 to 3 days, but when it disappears then it starts moving northward again. So, just from these kind of observations they deduced and later on we will see correlations and so on which prove the point that in fact, active spells of the CTCG or monsoon MCG are associated with weak spells or short spells of the oceanic TCG. Only when the CTCG disappears the oceanic TCG has a longer life and furthermore moves northward. So this is what we have learnt now as the typical sequence of events that occurs and also the complexity of the interaction if you wish between the CTCG and the oceanic CTCG. So, since a lot of things were discovered in this very first study let us just recapitulate what are the major features that have emerged and many of them have been discovered for the first time in this study. The major features are the presence of 2 TCG's right. You remember we showed that if we look at frequency of occurrence of MCG's at different latitudinal belts then there is a mode over the continent if you look at July, August for example, there is a primary mode over the continent and a secondary mode over the equatorial ocean with kind of minimum frequency of occurrence around 7 degrees or so. So, one can talk of 2 TCG's occurring and we have also seen that sometimes they occur simultaneously sometimes only one of them occurs. So, this is a major new feature that the there are 2 TCG's here a CTCG in the north on majority of the days and sometimes simultaneously an oceanic one over the equatorial Indian Ocean and epochs of the oceanic TCG are characterized by longer lifespan only when the CTCG is absent. So, this is an important point to note now. So, the first feature is the fact that there are 2 TCG's over the same longitudinal belt. The second of course is something we have been talking about the important role played by northward moving epochs generated over the equatorial Indian Ocean in the transition phases as well as in the peak monsoon months. See the fact that the things move northward in the transition phase has been known for a very very long time because we have known that rain belts move northward and in fact Kali Dasa in his Megaduth talks about the cloud messenger is talking about northward movements. Yaksha is telling the cloud messenger that when he goes north he should take a message for his lover and so on and so forth. So, northward propagations during onset phase were very well known simply because they are manifested also as rainfall propagation. But what was not known before this study is the role played by northward propagation even during the season even during the peak monsoon months. So, this is the new thing and they also showed that the frequency of revival of the CTCG from a cloud free spell by northward propagation is comparable to the frequency of revival by propagation of systems generated over the bay. So, the so called in-situ revival propagation through in-situ revival by propagation of system generated over the bay or northward propagating systems giving revival are comparable and a time scale of about 4 weeks has emerged which is the maximum period for which the CTCG can survive without re-establishment by the oceanic TTCG. So, this is a very interesting result and which still I think has not been completely understood with our theoretical and modelling exercises. Now, within this period after a temporary disappearance the CTCG is regenerated by formation of disturbances within the latitudinal band of the monsoon so on. At the end of this period that is to say we are now talking about 4 weeks from when it was first established in the monsoon zone. So, at the end of this period a spell characterized by the absence of the CTCG occurs and revival from this situation is brought about by northward moving epochs of the oceanic TTCG. So, this is another important result and Sikha Gadgil actually even in that paper considered what are the possible mechanisms. Now, I shall in my later lectures go into the physics of the system in greater detail and elucidate the possible mechanisms and so on in greater detail, but I think it is worth mentioning right now the kind of feedbacks and so on that could lead to these observed fluctuations of the TTCG. Now, since the monsoon is associated with the continental tropical conversion zone monsoon variability can arise from basic feedbacks and processes which operate in an oceanic TTCG and also from processes special to the continental TTCG such as land surface atmosphere interaction. See this is the important fallout if you wish of our real recognition that the basic system responsible for the monsoon is a tropical conversion zone which in its dynamical characteristics is the same as the tropical conversion zone over the ocean which is commonly called ITCG. And a great deal is known and great deal has been thought about why the ITCG fluctuates what are the possible feedbacks that give rise to these fluctuations and so on. Now, the point is that our system the monsoon system differs mainly in the fact that now the TTCG is over land and not over ocean. So, certainly the feedbacks that operate for an oceanic ITC or oceanic tropical conversion zone would also operate in this because it is also a tropical conversion zone, but in addition to that because it is on the continent there are other processes also such as land surface atmosphere interaction which will contribute to the variability of the TTCG. So, this is where the linking of the system to TTCG has made a difference in our understanding and in our proposing the mechanisms. Now, what are the mechanisms which have already been thought of for the tropical conversion zone the typical one seen over the ocean commonly called ITCG. There are cloud radiation feedbacks and there is another feedback which is impact of mid tropospheric warming by clouds on the vertical stability. Now, you remember we talked about how clouds were a result of an instability and that the atmosphere was conditionally unstable. Now, so clouds are basically a result of this manifestation of this instability. Now, what happens is when you have deep clouds they warm the middle atmosphere because that is where a lot of latent heat of condensation is being released. Now, because they warm the mid troposphere what will happen is that the instability will decrease because now the temperature increases. It has become warmer in the mid troposphere. So, as I said we will get into details of this at later stage, but let me just mention that the instability which is the source of these clouds that itself decreases because of the mid tropospheric warming associated with these clouds. Now, what will happen in that case that eventually the clouds will cease to be right because the what they feed on itself is becoming less and less intense and so the clouds will become less and less frequent and eventually they will be cloud free region. Now, when there is a cloud free region this instability will get re-established as it was earlier because now the mid tropospheric warming has gone and this is within a few days since the re-establishment. So, the thing gets established again and again you will have intensification of a tropical convergence zone there. So, these are fluctuations caused by feedbacks between the clouds and the vertical stability profile. So, consider so this is how we I just explained to recapitulate. If we consider the feedback between deep convection and vertical instability clouds lead to mid tropospheric warming and hence a decrease of the vertical instability. Such a decrease will lead to a weakening of the TCZ and eventually to a cloud free spell. The vertical instability will build up in the cloud free spell and the system can revive. So, in this way these feedbacks can lead to fluctuations of the TCZ. So, in addition as I said they will be impact of land also. Now, what Sikha Guide did was suggested possible mechanisms leading to the observed fluctuation of the CTZZ and in particular the observed sequence of events that they noticed is a part of the evolution of the monsoon season which occurs year after year. Now, what they suggested is the following that a few days after the establishment of the CTZZ conditions at the moist surface of land may not be very different from those at an oceanic surface. When cloud free spells occur within about two weeks of establishment the TCZ can revive just as the oceanic TCZ does. However, subsequently there may be a slow but steady depletion of moisture through absorption by soil runoff etc. So, that at the end of one month the conditions at the land surface become very different from those at the oceanic surface. So, what they are saying is see soon after the establishment of CTZZ it has rained everywhere and the land surface is extremely moist. So, there is plenty of water vapor available for evaporation and the system can revive just like a TCZ does on an ocean. But as the season progresses what is happening is a lot of water that is we come from the rain goes to enrich the soil moisture there is also a large amount of runoff and so on. So, eventually the soil surface is no longer as wet as it used to be. So, at the end of a month the conditions at the land surface become very different from those at the oceanic surface. When a cloud free spell occurs around a month after the establishment of the CTZ due to the negative feedbacks operating in a TCZ and with the disappearance of clouds the atmospheric conditions become favorable again example by build up of the vertical instability or whatever. Even when that happens the monsoon cannot revive without replenishment of the moisture because the conditions of land at the end of one month are very different from the condition at the end of 10 days after the monsoon has been established over the monsoon zone. So, in fact the conditions become similar to those prior to the onset and revival by a process which is strikingly similar to the onset which is the northward movement of the oceanic TCZ occurs. So, this is a very interesting hypothesis that has been proposed and it is been several years since that has been proposed, but as yet we have not been able to properly assess to what extent models based on physics are meaning which are governed by Newton's laws are able to simulate these basic features and whether the mechanisms leading to these fluctuations have been unraveled. So, I will discuss all this in later lectures, but I thought it is a good idea at this point just to point out that we do have some ideas about why the fluctuations occur in the TCZ and why there are fluctuations in the monsoon rainfall and in the only intracesional scale. Now one thing that came out extremely clearly from this study of the satellite imagery was that the lifeline of the monsoon is the clouds over the ocean. So, oceanic TCZ we have seen plays a very very important role in maintaining the CTCZ by northward propagation both during the onset phase, during the retreat phase as well as during the peak monsoon month. So, northward propagation of the oceanic TCZ is very very important, but we have also seen that the relationship between the CTCZ and oceanic TCZ is very complex and there is also a competition between the CTCZ and the oceanic TCZ on the intracesional scale that is the sub seasonal scale we know that there is a very clear competition between the two. Now we have if we look at anomaly OLR anomaly composites of active spells active spells are one in which there is sustained rainfall sustained high rainfall over the monsoon zone and the so called breaks of the monsoon in which you have a huge rainfall deficit over the entire monsoon zone. So, when we look at we have actually defined all these objectively and by objective definitions based on rainfall criteria criteria based on rainfall over the monsoon zone we can actually define active spells and break spells. Once we define them we know the dates on which they occurred we can actually make a composite of this to see what is the convection pattern like associated with active and break spells and what you see is shown here these are OLR composites of break spells on top these are break spells and remember here pink and red is positive OLR anomaly right positive OLR anomaly means negative rainfall anomaly. So, here the convection and rainfall are suppressed and this is a break monsoon condition and here is the active condition all blues this is when convection and rainfall is enhanced. So, these anomalies mean convection is enhanced here and notice that there is a seesaw between here and here during our breaks there is excessive convection over the eastern and central equatorial Indian Ocean and during our active spells there is suppression of convection over this region. So, the competition we were talking about between the oceanic TCG and the CTCG seems to be manifested as this dipole of anomalies that you see here positive anomalies here negative here or negative here and positive here. So, this dipole is a reflection or a manifestation of the competition that you have seen between these two and interestingly this dipole is not extending this side of 80 degrees at all in other words the western equatorial Indian Ocean is not competing with the CTCG only the eastern one seems to be. See our idea of saying there will be competition because suppression from one system will lead to weakening of the other and so on did not really specify how much the weakening would extend in the east-west direction and where its center would be, but from this data it is clear that most of the weakening or suppression of convection occurs over the eastern equatorial Indian Ocean eastern and central and almost nothing over the western equatorial Indian Ocean. Now, this is interesting because we knew that the relationship is complex the oceanic TCG is actually very much required for maintaining the CTCG by not forward propagation. So, it appears that the positive relationship is with the western part and the negative relationship with the eastern part. So, there is a division of labor if you wish that this part takes care of eastern part takes care of the competition between the two and western part takes care of the positive interaction between the two which is again an interesting thing which we had not anticipated when the Sikha Gadget paper was written. Now, this competition is also clearly brought out in the correlation of the monthly OLR average over monsoon zone for July-August with monthly OLR over the Indo-Pacific region. So, what this is is this is the monsoon zone. So, we take the average OLR over this region and correlate it with OLR at every grid point in this region and naturally the correlation is highest here over the monsoon zone because we are correlating with the average OLR of monsoon zone and it is positive here. Notice negative correlation is over the eastern equatorial Indian Ocean and positive correlation is over the western equatorial Indian Ocean. So, this is very interesting as I said before what we saw from the active break thing it appears that the eastern equatorial Indian Ocean has taken charge of fighting with CTCG whereas the friendly oceanic TCCG is on this side and the same story you see for August also. So, while the correlation with convection over eastern equatorial Indian Ocean is negative suggesting competition between the CTCG and the TCCG over this region that with convection over the western equatorial Indian Ocean is positive suggesting the contribution of this region in maintaining the CTCG. In fact, this relationship of ISMR or the Indian summer monsoon rainfall with correlation over equatorial Indian Ocean is very similar. This is now the correlation of ISMR all India summer monsoon rainfall is correlated with OLR at every grid points and you see that the correlation of course is negative with OLR because it is tall clouds which have lower values of OLR. So, lower values correspond to high values of rain. So, negative correlation is what you expect between rain and OLR and you can see the same sign here whereas it is opposite here over central and eastern equatorial Indian Ocean. So, this is very interesting. So, convection over western equatorial Indian Ocean is favorable for the monsoon while that over the eastern equatorial Indian Ocean unfavorable. I shall discuss the implications of these links for variability of the monsoon on inter seasonal and inter annual scales at a later stage. Now, consider the next link to convection over the Bay of Bengal. We have said both are important equatorial Indian Ocean as well as Bay of Bengal. Now, genesis of synoptic systems over the bay which we have seen and subsequent propagation on to the Indian land mass is associated with rainfall over monsoon zone. This is an example of actually synoptic scale systems. One synoptic scale system over the bay another has already travelled to the other end of the monsoon zone and you can see in fact that this is a strong synoptic scale system a depression whereas this is a low here. So, this is the weather map and the relationship of the spatial variation of the rainfall over the Indian region with the convection anomalies over the bay is again a little bit complicated and we illustrate that with July 98. What happened in July 98 was very interesting. Most of the systems were generated here in this region rather than in the central and northern bay. So, there was a shift in the genesis region and because of that we got negative oil or anomalies here you see this is where most of the systems got generated and they moved on to land. In fact they came here and gave us a lot of land a rain, but since the systems were generated here and they moved to land you saw nothing was generated here and this whole belt got deficit rain. So, you can see the link is very nice between the region of genesis and where you get rain, but it is a complicated kind of link. So, this anomaly pattern is a manifestation of the genesis of most of the synoptic scale systems over the southern rather than the northern bay. These systems moved westward on to the Indian region and the rainfall was in excess over the peninsula and deficit over the monsoon zone. So, now it is very clear that the variability of the CTCG is linked to the variability of convection over the surrounding seas and the equatorial Indian Ocean. So, the million dollar question is what determines the variability of convection over these oceans. Now, it is not quite a straight forward problem because we know that the convection on land also has an impact on convection over ocean and vice versa. These are interacting systems, but we still would like to know whether there are factors that determine the convection, the variation of convection over tropical oceans because those will be then critical for understanding variability of the monsoon as well. So, major question to address therefore is what determines the variation of convection over tropical oceans such as these. Now, when we talk of variation of convection we are talking of organized convection. Organized convection over several hundred kilometers, convection organized over synoptic and larger scale which is hundreds of kilometers and we are interested in the variability of such organized convection over tropical oceans. But to go back to the basics see what does what is the basic source material for clouds it is water vapor. Clouds are condensed water vapor, clouds contain liquid water because in clouds water vapor actually gets transformed to liquid water. So, the most important input to a cloud is obviously the water vapor. Now, so naturally it will depend on the availability of water vapor. Now, where is the water vapor available? It is available at the surface of the ocean and we are talking of convection over the ocean. So, the water vapor is available at the surface of the ocean and obviously the more water vapor in a air parcel touching the surface of the ocean the more chance there is of getting clouds is not it. So, convection will depend on how much water vapor is in air and this is what we will discuss next time, but basically we all know how much water vapor can air can hold depends very much on its temperature. This is why when we put a glass full of cold water in a room water droplets start condensing at the outer surface of the glass why is that because at the outer surface of the glass air hugging the glass becomes colder and as it becomes colder its capacity to hold water vapor decreases. So, some of the water vapor condenses on the glass to form liquid water. So, the most critical element then being water vapor leads to the fact that the air temperature near the sea surface is very critical and air temperature near the sea surface is strongly related to the sea surface temperature itself right the two go together. So, in fact the convict the first candidate if you wish for addressing the question what determines the convection organized convection over tropical oceans and its variability is the sea surface temperature or SST and in the next lecture we will actually try and understand what is the relationship between organized convection and sea surface temperature organized convection over the ocean and sea surface temperature because that will certainly give us a clue in understanding what leads to the variability of convection over the ocean. Thank you.