 So, we continue our discussion on seasonal transitions today, where in the last class we have already talked about the onset over Kerala. After the monsoon onset over Kerala which is short MOK, the monsoon advances across the Indian region and at the end of the onset phase the entire region comes under the sway of the monsoon. So, you may recall these are the dates from India Med Department of the onset dates over different regions. This is the date, this is when the monsoon onset occurs over Kerala. After that there is this, during the onset phase it propagates northward as well as westward and eventually by 1st of July most of the region is under the sway of the monsoon. This is 15th of July. So, these are the mean dates calculated by India Meteorological Department by looking at the onset date, looking at the rainfall, mean rainfall patterns and taking the centre of the penta during which onset occurred as the onset date. Now, the northward advance over the subcontinent from its commencement over Kerala and its culmination over the western part of the Indian monsoon zone takes about 40 to 45 days as we have seen this is 1st of June and this is 15th of July. So, the northward advance takes about 40 to 45 days and advance generally involves several northward propagations each pushing the limit of the monsoon farther northward. Now, we have already seen this when we looked at the satellite pictures of the cloud bands and how they propagate. As the monsoon advances over India major changes occur in the subtropical jet stream which in the winter and pre monsoon seasons has its core to the south of the Himalayas and is very strong and this subtropical jet stream shifts to the north. Now, this is something we have seen when we saw the difference between the summer monsoon and the previous season. See, in winter the subtropical jet stream comes over us and that is why Delhi is part of the mid latitude weather regime in winter. So, we have the subtropical jet stream coming over us and this is the air circulation at 3 kilometers even at 3 kilometers you see very strong western events this is the Tibetan plateau. So, the western jet the mid latitude western is which penetrate over the Indian region the jets actually splits part of it goes north of Tibetan plateau and part of it comes south. Now, why is there a split because this Tibetan plateau is 5 kilometers. So, we are looking at winds at 3 kilometers they cannot penetrate the plateau. So, they have to go around here and so you see these subtropical jet in winter comes like this. Now, if we look at the mean 200 millibar streamlines which is up in the upper troposphere for January then you will see that in January at 200 millibars also the winds are westerly. Now, there of course, a directly flowing over the Tibetan plateau which is lower than the 200 millibar level. So, this is the westerly wind what happens during the monsoon what happens during the monsoon is that the westerly flows entirely to the north of Tibetan region and to the south we have easterly flow this is the tropical easterly jet. In fact, what you have here is a subtropical ridge which we will talk about. So, you get the formation of a subtropical ridge in the middle and upper troposphere. So, this is high pressure now the lower pressure in the upper levels is here. So, naturally because of geography the winds will be easterly and this is the tropical easterly jet which was discovered by Koteshwaram in 1950. So, we have a transition from purely westerly flow here to easterly flow. So, the westerly flow in the upper levels get replaced by easterly flow during the summer monsoon. So, it has been shown that there is a shift of the subtropical ridge at 150 HPA from about 17 degrees latitude on day minus 20 in a composite of several years with MOK as 0 day to about 26 north on day plus 15. So, he is talking now of this ridge he is saying that about 20 days before the onset of the monsoon if you look at a composite made with saying that MOK is taking MOK as 0 day then on minus 20 it is around 17 and 5 days after the onset it goes to 26. So, you have see this is the latitude here this is about 26 or so. So, it comes to 26 from about 17. So, you see the shift of the subtropical jet before the subtropical ridge before the monsoon. Now, to remember that this high pressure region in fact marks the region where the pressure is low and you have a lot of convective clouds at the lower levels. Now rapid shift of the latitude of the ridge occurs only after MOK the easterly that 150 HPA spread northward with the shifting of the ridge resulting in the establishment of tropical easterly jet stream through peninsular India. So, we have the establishment of tropical easterly jet stream. Now, so this is an important feature and we will come to this when we talk of hiatus during the advance or temporary stalling of the advance of the monsoon which is observed in many many years. Now, let us go back to the dates of onset and see that the onset over Mumbai this is Bombay onset over Mumbai is around 10th June and this is 1st of June. So, the after the onset on Kerala the monsoon travels northward there is an advance of monsoon northward and by 10th of June the onset occurs in Mumbai. This is the mean picture now the average travel time of monsoon from Kerala to Mumbai is 8 days which is about 1 degree latitude per day. Remember that the average northward movement of the cloud bands that we saw also was about 1 degree latitude per day. So, this is a typical time of northward movement of cloud bands and its standard deviation is about 7 days the extreme values are 29 and minus 7 days the a negative value means that the monsoon has set in over Mumbai earlier than it does over Kerala and such a situation occurred in 3 years out of 84 year record for each of them it is minus 1 day. So, what we are saying is that instead of the monsoon first having an onset over Kerala and moving north in 3 years what has happened is the onset occurred first over Mumbai and almost simultaneously within 1 day after that it occurred over Kerala as well. But in general there is considerable variation of the time from when the monsoon onset occurs over Kerala to that over Mumbai. Now, Joseph has done an interesting study in which he shows that the correlation between the travel time of monsoon from Kerala to Mumbai which means onset date of over Mumbai minus onset date over Kerala what is the number of days. The travel time of the monsoon between Kerala to Mumbai and the date of monsoon onset over Kerala is minus 0.63 which is very large and highly significant. So, what it is saying is that later the monsoon onset occurs over Kerala the shorter the time it takes to further onset to occur in Mumbai after that. So, this is very interesting there is a negative correlation between the travel time of the monsoon from Kerala to Mumbai to the date of monsoon onset over Kerala and Joseph actually fitted a linear regression curve to this relationship and he found the following that when the monsoon sets in on 10th May in Kerala which is very early the estimated onset dates at Mumbai are 22. So, it takes more than the average period of 8 days it takes 12 days. However, when the monsoon onset occurs on 30th May then it takes only 10 days so less than half of the time to for the onset to occur in Mumbai and on a occasion when it occurred in 19th June I believe this was in 1979 actually the monsoon onset over Kerala occurred 2 days later which is on 21st June. So, this is the way it happens and it was intriguing at first as to how this should happen. So, Joseph has suggested the following explanation he says that in the North Indian Ocean around India the axis of the climatological monthly mean assistive maximum lies close to the latitude of Kerala in May, but shift to the latitude of Mumbai in June. So, there is a shift in the axis of maximum SST from about Kerala which is 8 degrees to Mumbai which is about 19 degrees North and so what happens is you have a shift in the SST from May to June SST axis of maximum SST. So, in a year when MOK is late and he does not say why it is, but he says in years when monsoon onset over Kerala is very much delayed say by mid June then the monsoon cloud band after its formation and bringing rains to Kerala quickly adjusts to the June position of the SST maximum axis which is close to Mumbai. So, what happens is since the TCC the tropical convergence zone is favored to form over the region of maximum SST when it is formed to the south of it it quickly goes to the place where the maximum SST is and that is why the travel time is much shorter when the onset occurs in the towards the middle of June. This is the explanation given to by Joseph of course, one has to test this with models which has not been done. Now, so far we have talked of the onset over Kerala over Peninsula and how it occurs under northward with by means of northward movement of cloud bands or rain belts. Now, over the monsoon zone the onset first occurs over the eastern part. Let us take here the example of June south west monsoon of 2007 and what you find is the lines in green correspond to the actual dates on which the monsoon onset occurred in 2007 and what you will see here is that this is the 8th June line then this is 9th June line which you can see. So, on 9th June it has occurred over the eastern part first notice of course, that after striking after the onset over here which was somewhat early by 28th of May then the onset occurred at more and more northern latitudes. So, by 15th June we have onset here over the west coast of the Peninsula and it is only by 18th June that the onset occurred over Mumbai. So, here it is a clear case of northward propagation, but when you come to the monsoon zone which is this part of the region then what you see is on 8th of June you see it is here on 9th June it is here these are northern limits of the limit of the monsoon 10th of June it is here 13th of June it is here 15th of June it is here. So, you see a very clear movement from east to west over the monsoon zone and then after that it of course, continues to move here. So, this is there by 30th of June from 15th to 30th it has come here and then later on by 4th of July it has reached its destination. So, the very advance of the monsoon occurs is by northward movement up to this point somewhere around here and westward movement from here onwards. This is the way the advance of the monsoon takes place and you see that by this example here. Now, there is considerable variation from year to year in the advance process and we have seen this example before that in some years you know it takes a very long time for the answer to occur over the north western parts or for the culmination of the advance phase. An example I will show you is the south west monsoon of 2009 and you will see what happened in 2009 the onset occurred very fast here this is on 22nd May it has not yet occurred by 23rd 24th May onset has occurred over Kerala here and then it progressed quite well up to 7th June by 7th June it had reached the coast of Maharashtra west coast of Maharashtra. So, it has reached Konkan by 7th June now from here also we it had reached around here by 25th of May. But what happened after that is that between 25th May and 28th June there was no movement here at all you see. So, the monsoon was just static here and here also from 7th June to 28th June it remained here and 21st to 23rd it went there. So, what has happened is that there is a big hiatus is what they will refer to it later there is a within the advance there is a short spell and not very short in fact almost 2 weeks long from 7th to 24th in which no advance has occurred of the monsoon. So, such things also happen that there is a lot of inter annual variation and it is not as if there is a nice progression this way and progression this way to bring about the culmination of the advance phase. So, although the normal duration of the advance phase is about 38 days there is considerable variation in the duration of the advance phase from year to year with the duration being as long as 60 days or more in some year. Now this is the duration of the monsoon advance phase and this is the normal of 38 days you can see that there is considerable fluctuation in the duration of the advance phase and in 2002 which we will come to later. In fact, it took a very very long time more than 2 months from the monsoon onset on Kerala to actually achieve the onset over the north western parts of the country. So, in the monsoon of 2002 with an exceptionally long duration of the advance phase the monsoon advance stagnated or halted on 3 occasions 13 to 19 June 5th to 18 July and 20th July to 14 August. Such a stagnation is called a hiatus in the progression of the monsoon. So, such a hiatus occurred very often 13 to 19 June 5th to 18 July and 20th July to 14 August. So, 2002 was full of inactive spells in which the onset phase did not progress at all. So, these are called hiatus in the monsoon. Now there has been lot of studies of hiatus in the monsoon as well because during 1960 to 93 prolonged stagnation of 2 weeks or more was observed in 12 years with stagnation of the advance of both Arabian Sea and the Bay of Bengal branches in the years 76, 82 and 91. The variation in the number of days of stagnation or hiatus during the advance phase of the monsoon is plotted here and this is of course, 2002 which had a huge number and you have to remember here that this is not one hiatus, but it is some of all the all such spells in which the monsoon did not progress in the advance phase over that year. So, as you know in monsoon 2002 there were 3 such periods and combined they give this kind of a long more than 40 days of hiatus during the onset phase, but you see that even in other years there is a lot of variation. There are cases in which there is hardly any stagnation that occurred at all like this year of 63 and so on and so forth. Again here 96 there was hardly any stagnation, but typically you do have about 10 days of stagnation almost all the time and that is why the mean is of the order of 20 days or so. So, you know it is not as if advance of the monsoon is a steady progression from south to north and from east to west rather it occurs in burst of spells and in between these spells there are times when it does not seem to progress and those are the hiatuses and having a hiatus of some length is typical of the monsoon season. Now, why does this hiatus occur? It has been suggested the hiatus in the monsoon advance is linked with the abnormal circulation features with the penetration of the sub tropical ridge, the westerly trough activity in the mid latitude levels extended to southern latitude disrupting the monsoon flow when the synoptic forcing over north India is absent. As I mentioned before in winter we have only mid latitude flow over the region, but what happens is and we have also seen that soon after the onset of the monsoon the sub tropical ridge gets established, but if the sub tropical ridge comes to the south then you have a problem because then you have mid latitude influence in the monsoon and this is supposed to be one factor that lead to these hiatus events that we have observed. Another thing people have noticed is that intense convection over the Indian ocean just south of the equator near 90 degrees and comparatively warmer SSD over the same area also had been noticed during some hiatus events. So, these studies of hiatus events also suddenly threw up this region as being unfavorable. This is the region of the east equatorial Indian ocean which we refer to as EEO eastern equatorial Indian ocean and which plays a big role in major events such as Indian ocean dipole event and we will discuss this at length later, but this region convection over this region is found to be associated with hiatus events according to some of the researchers. Now, in fact, enhanced convection over this eastern part of the equatorial Indian ocean is a characteristic of break monsoons which we will discuss in a later lecture within the summer monsoon season. So, after the summer monsoon is established towards the end of June in July and August we get dry spells which are called breaks in the monsoon. Now, it is found that this kind of convection over the same region is a characteristic of the breaks in the monsoon. So, it is very interesting it is also shown that such convection is unfavorable on the inter annual scale also. So, for the seasonal total also enhance convection over that region is unfavorable for the summer monsoon over the Indian region. So, it appears that this region eastern equatorial Indian ocean is convection over this region is an unfavorable factor for each phase of the monsoon that is to say for the advance of the monsoon for the fluctuations in the monsoon during the peak monsoon months of July and August as well as for the seasonal rainfall as a whole. So, this is a very interesting thing we will have to follow it up later on when we discuss the physics of the monsoon. Now, the advance of the summer monsoon over the Gangetic valley and north west India is associated with the establishment of an active TCG across India that is the CTCG over the monsoon zone and we have seen this before and building up of a strong subtropical ridge to the north of 25 degrees north in the middle troposphere. So, once you have an active continental tropical convergence zone over the monsoon zone naturally overlying this low pressure region with intense convection will be a high pressure region and this is the subtropical ridge that they talk about. So, once the convection gets established once the CTCG gets established we have a very strong subtropical ridge to the north of 25 degrees north. It is believed that the warming up of the Tibetan plateau is one of the causes for the establishment of the subtropical ridge throughout the middle and upper troposphere. It is been suggested that the hiatus in the monsoon advance is linked to abnormal circulation features such as southward extension of the westerly trough activity which occur when the synoptic scale forcing over north India is absent. So, what we are saying is you know before the monsoon in winter months we have in fact mid latitude circulation over the northern part of India. With the monsoon during the peak summer monsoon season this has been this is totally pushed to the north of Tibet to the northern region, but what they are saying is when there is very little synoptic scale forcing over the monsoon region that is to say when there is very little convection associated with the tropical system often what happens is even after the onset of the monsoon these mid latitude systems tend to penetrate. And they it is suggested that in such situations the effect of the mechanical barrier of the Himalayan massive and elevated Tibetan plateau on mid tropospheric westerly troughs also becomes important in adversely affecting the advance of the monsoon. So, what they are saying is that one goes back partly to the winter like situation and because of that Tibetan plateau would act like a mechanical barrier as we have seen it does in winter and the fact that it is an elevated heat source also leads to an also has an important impact in adversely affecting the monsoon. So, it is suggest all the studies of Hayata suggest that the presence of the mid tropospheric subtropical ridge its location and the mid latitude westerly troughs during the onset phase of the monsoon appear to be important in determining the advance of the monsoon. So, we know that the advance of the monsoon comprises cloud bands moving from south to north cloud bands generated over the equatorial Indian ocean moving northward often in association with cloud systems over the Arabian sea and Bay of Bengal. And in addition to that we have westward movement of system generated over the Bay of Bengal across the monsoon zone. So, the advance process in fact comprises genesis of clouds and cloud systems over the surrounding seas and equatorial Indian ocean and their propagation on to the region northward propagation of cloud bands and westward propagation of synoptic scale disturbances from the Bay of Bengal. So, clearly genesis of these cloud systems and their propagation play a very critical role in the advance phase of the monsoon. But now we have seen that from studies of the Hayatas it appears that not only is it important to look at these genesis and propagation of cloud systems it is also important to consider the look at the establishment of a strong mid tropospheric subtropical ridge and adjoining Tibetan plateau because they also play an important role in the advance of the monsoon. Now, there has been a study an interesting study by Nithyanand Singh and others on the onset of the monsoon over the monsoon zone. Date of the onset of the monsoon over the monsoon zone has been derived as the date on which the cumulative rainfall over the zone exceeds 50 millimeters. So, he has used a threshold as in one day when the cumulative rainfall exceeds 50 millimeters he calls it the onset over the monsoon zone as a whole and he has derived these dates of the monsoon and remember that if one talks of onset on the monsoon zone as a whole this means this is the culmination of the advance phase of the monsoon right this is the culmination of the transition from spring to summer monsoon. And so when now we should look at the composites that he has derived of various important facets relative to the advance relative to the onset date of the monsoon zone and the features he has looked at is meridional winds over the Arabian sea low level jet of East Africa remember these were important circulation features of the monsoon. Tibetan anti cyclone which is the high which gets established over the low over the Indian monsoon region all of these undergo a rapid transition around the onset date of the monsoon zone and peak seasonal values are attained soon thereafter. Now, this is very interesting to see what you see here A is the rainfall over the core monsoon zone and you can see that this is the composite now for all the years and this is the onset date. So, what you see here is this is when his cumulative rainfall has exceeded 5. So, this is the onset date, but you can see it has not only exceeded 5 in the composite it almost hits 10 centimeters. So, the composite actually means a very sharp increase from a few days before the onset about 10 days before the onset it is less than 1 millimeter cumulative rainfall and within this short time it attain something like 10 millimeters. So, it is the rainfall becomes 1 centimeter here and so, this is the rainfall then this is the precipitable water which also shows an increase this is the weafened field over the Arabian sea and this is the geopotential height. So, this is the 200 HPA level of the Tibetan plateau. So, what you see is basically that if you look at the monsoon zone itself this onset date of the monsoon zone is a very important date because before that one talks of the spring kind of a situation and after that is the establishment of the summer monsoon with all the characteristic features of the cross equatorial flow being established over the Arabian sea and of this being a precipitable water becoming high of course, along with rainfall and the Tibetan high also being established at 200 HPA. So, onset of the monsoon over the monsoon zone is the therefore, marks the culmination of the advance phase of the monsoon. Now, we have seen earlier that there is hardly any relationship between the total summer monsoon rainfall over India and the onset of the monsoon over Kerala. Earlier onset does not mean more rain or later onset does not mean less rain as one would have thought if in fact, it was a matter of stretching the season, but that did not happen. So, the onset did not have much of a relationship. However, there is some relationship of the duration of the advance phase with ISMR. This is the all India monsoon rainfall here on the y-axis and the number of days in the advance phase here and what you see is that if the advance is very rapid that is this part, this is the average period taken for advance. So, if it is faster than normal, then the chance of getting high rainfall is much higher than getting deficit. So, above normal rainfall the probability of above normal rainfall is rather high. If you have a very rapid advance say in 30 days or so instead of 38 days then most of the points are above normal and very few points are below normal. But if in fact, the advance takes normal or above normal then there is much less you can talk about whether you will get above normal or below normal rainfall. So, the relationship is there the correlation is not bad it is minus 0.34, but it is useful only partially one can say that if the advance is very fast then one is more or less guaranteed that the rainfall will not be below normal the chance of a below normal rainfall is very small. But once it is beyond this then there is too much of a scatter to make too much sense of it and notice that even when the advance took as long as 80 days you still had normal rainfall. So, the relationship for advance relationship with the duration of advance when the duration of advance is close to the average or above it is very poor with ISMR. So, the Indian summer monsoon rainfall does depend on how long the monsoon took to reach its destination in northwest India after starting from Kerala. Note that when the advance is faster than the average the probability of above normal ISMR is very high. However, when it is slower than average the chance of ISMR being below the average is only somewhat more than that of ISMR being above the average. So, with this now I come to the end of that transition discussion of the transition from spring to summer and if you look in the literature you find that most of the studies address the question of monsoon onset over Kerala which is the first time it makes it up its appearance over the Indian region. So, although the understanding and prediction of the entire onset phase is important this onset phase which commences with monsoon onset over Kerala and culminates with the establishment of the CTC over the monsoon zone is important for short range rainfall variability as well as the seasonal rainfall. The focus of most of the studies so far has been on the monsoon onset over Kerala. So, this is a lacuna that one should now try and address and it is very important to understand the variability of the advance phase and in fact if one is worried about all India rainfall it is more important to understand why the advance occurs faster in some years than in others if we want to say something about the all India summer monsoon rainfall. Now, I come to the retreat of the monsoon now I must mention that the retreat of the monsoon is also called the withdrawal of the monsoon both words mean the same thing. The normal dates of the retreat of the monsoon rains over different part of the country are derived as are given here and let me just tell you how they are derived. The norm again like onset the normal dates of the retreat of the monsoon rains over different parts of India is given by India met department are based on the long term average pentad rainfall graphs at for several observatory stations. So, what they do is to have mean 5 day rainfall graphs and the middle date of the pentad which shows an abrupt decrease in rainfall is taken as the date of retreat of the monsoon for each station and middle date of the one that shows an abrupt increase in rainfall is taken as the onset. So, this is how IMD calculates the onset remember this is all based on the mean rainfall patterns. So, the retreat of the monsoon is here it commences around 1st September from the northern northwestern parts and keeps retreating and finally, phase of the over the southern south peninsula is completed by December. So, now we have these are the mean dates of retreat you can see that by end of September this is 1st of October. So, by end of September it has retreated from a very large part of the monsoon zone and by 15th of October it has retreated from about north of 15 north or so. So, totally from this part Gangetic plain monsoon zone and it has come well to the south and then it keeps on the rainy season continues towards this end with the final retreat of the monsoon rains only occurring in December from the Indian region. So, the climatological retreat of the monsoon from northwest India begins in the 1st fortnight of September its timing has considerable inter annual variability. The retreat of withdrawal of the monsoon and its gradual equator word movement and the deceleration of the low level westerly flow is heralded by the seasonal cooling of the Asian continent. See by after October in November December this part begins to start cooling. So, with the this is accompanied by the retreat of the monsoon. The withdrawal of the summer monsoon has been associated with the southward displacement of surface pressure trough, the establishment of dry continental air and development of anti cyclonic flow over central India. So, now they are talking of withdrawal of the summer monsoon and not the entire monsoon. Remember what IMD has drawn is if you like the withdrawal of the rain giving system and that takes a long time that takes till December, but we had divided into rainy season. So, summer monsoon withdrawal corresponds to that occurs over this region by about early October and so, when one talks of central India we you know the surface low pressure trough will move southward from there and you get an establishment of dry continental air and instead of having cyclonic flow you get anti cyclonic flow over the central part of India. At the time of onset and advance of the monsoon the rain belt associated with the tropical conversion zone moves north, but monsoon withdrawal is associated with the inability of the TCG and its associated rain ban to advance to higher latitudes. We have seen this before. At the time of onset each successive northward moving ban moves further and further northward. At the time of retreat or withdrawal it is not as if things start moving southward not at all the genesis is still around the equatorial region. It is just that the culmination occurs more and more southward or the ban cannot move as far north as it used to before. So, the culmination of the propagations becomes occurs at a much more southerly latitude during the retreat. So, this is an interesting feature again which we had pointed out earlier that the before the satellite era people used to think that just like you have a northward movement of the rain belt you would have a southward movement of the rain belt during the retreat of the monsoon. But actually if you look at cloud bands it is northward movement which culminates at a lower and lower latitude as with successive northward propagation. Now, we have seen that the basic system responsible for the monsoon rainfall is the TCG which is the same as that which is responsible for the large scale rainfall over other tropical regions such as the Pacific. And we have also seen that it is the same system that gives us rain irrespective of whether we are in the south west monsoon or in the northeast monsoon the so called south west or northeast or the summer monsoon or post monsoon. Now, the dates of retreat or withdrawal of the monsoon determined from the rainfall at different stations actually which is the this is again from IMD we have looked at these onset and withdrawal. So, this corresponds actually to the onset and retreat of the TCG from the Indian region. So, we have dates of retreat or withdrawal of the monsoon determined from the rainfall of the different stations correspond to retreat of the TCG. So, the TCG retreats from this region which means the TCG which gets generated to the south no longer reaches these higher latitudes but actually reaches only up to here and then only up to here and so on and so forth. So, the retreat corresponds to retreat of the tropical conversion zone from our midst. At any place the period between the onset and the retreat of the monsoon can be considered as the rainy season and the mean rainy season can be derived from the dates given by IMD for the onset and retreat. So, given that we have a onset of the monsoon and retreat here one can actually derive what is the mean rainy season over any part. However, the rainy season over the Indian region is divided into summer monsoon and post monsoon season and as we mentioned in the first summer monsoon most of the flow is from the southwest most of the winds are from the southwest. So, it is also called the southwest monsoon. So, the onset of the monsoon over Kerala corresponds to the onset of the summer monsoon as well as the onset of the monsoon as a whole because that is the first season and during the onset phase the TCG gets established over the monsoon zone north of about 15 north. The summer monsoon is characterized by winds from the southwest at the surface and has been traditionally and even now called the southwest monsoon. For some reason which is not clear to me at present IMD declares the withdrawal of the summer monsoon or the southwest monsoon for the country. So, while the mean dates of IMD for withdrawal of the monsoon correspond to the end of the rainy season for the as a whole for that part of the country IMD also declares the end of the summer monsoon which is part of the rainy season. And it is not clear to me whether that is a useful thing to do or not. But let me talk about what is actually done. So, the retreat of the summer or southwest monsoon corresponds to the disappearance of the TCG from the monsoon zone and hence the disappearance of southwestern is from north and central India. So, India meteorological department at present uses the following guidelines for declaring withdrawal of the southwest monsoon. So, withdrawal from extreme northwestern parts of the country should not be before 1st of September. So, this is an instruction to for operational declaration of withdrawal of the southwest monsoon. So, they say even if you feel that the conditions look like the monsoon has withdrawn you should not do it before 1st of September. After 1st September for the first withdrawal from the western parts of northwest India there are many criteria they look at cessation of rainfall activity, establishment of anti-cyclone over the in the lower troposphere this is over the monsoon zone. And considerable reduction in moisture content as inferred from satellite data. Now, for further withdrawal of the country they say may be declared keeping the spatial continuity reduction in moisture seen in water vapor imagery and prevalence of dry weather for 5 days. And this is southwest monsoon should be withdrawn from the south southern peninsula and hence from the entire country only after 1st October when the circulation pattern indicates a change over from southwestern event. So, only when the winds become northeasterly should you declare this. Now, I must mention here that this is a very interesting terminology that IMD uses they say they feel that they are the players who actually bring on the monsoon or withdraw it which is an amusing thing. So, they say southwest monsoon should be withdrawn what they mean is of course, it could be declared that the southwest monsoon has withdrawn. But it is a very amusing way of putting things, but which IMD uses has been using for decades. Now, since people were interested in summer monsoon some work has been done on how does one define you know the duration of the summer monsoon. And there are two Japanese who have defined an index for monsoon withdrawal based on the characteristic low level circulation associated with the summer monsoon. And this is a daily circulation index which is defined as the difference in average 850 HPA zonal winds between a southern box and a northern box. Southern box centered around 10 north and a northern box centered from 20 to 30. So, what this is capturing is we have seen earlier that during the southwest monsoon there is shear between the west always to the south and east always to the north. And what this index is capturing is the shear vorticity above the boundary layer. And we have to note that during active phases of the monsoon this shear vorticity is cyclonic. For the mean monsoon as well this shear vorticity is cyclonic. And remember that vorticity is closely linked to organized rainfall. So, if one has to use a circulation index it makes sense to use 850 millibar vorticity as the index and that is what they do. And what they say is that the date of monsoon withdrawal is taken as the first of 7 consecutive days for which the smooth DCI is negative. So, this vorticity has to change from cyclonic to anti cyclonic with the withdrawal of the summer monsoon. And so they look at the vorticity from about 10 to 20 north 20 to 30, 5 to 15 we surveyed 20 to 30. So, over that band they are looking at the shear vorticity which is the dominant component of vorticity. And they look at the data to determine when it becomes negative and it should not be just a day or two of fluctuation of vorticity, but it should be a permanent transition to a withdrawal state. And so they wait for 7 days and only the first of 7 consecutive days for which the smooth DCI is negative is taken as the withdrawal date. And the withdrawal dates that they derived are shown here and you can see the withdrawal of the south west monsoon or the summer monsoon dates are here. And the latest is sometime in November, but the mean date is about 20th October. So, 20th October is the date of the withdrawal of the south west monsoon from the country by this definition. Now I will start a discussion on the rainy season over the Asian monsoon region and variation of rainfall profiles. Remember here we have been talking of mean rainfall. And we have also said that the patterns of mean rainfall vary across the country from places like Chennai or on southeast peninsula where maximum rain occurs in October, November to places over the Indian monsoon zone like Chai Bazaar and so on, where the maximum occurs only in July, August and the rain is restricted to June to September. So, we have already seen that there is a great deal of variation in the patterns. Now there have been some studies of monsoon rainy season defining what a monsoon rainy season is and looking at its variation over the Asian region as a whole and circulation. There have also been studies of determining regions over which the rainfall variation patterns are similar. The monthly mean rainfall profiles are similar. I will talk about both of those in the next set of lectures. Thank you.