 So, we continue our discussion about the El Nino-Southern Oscillation today. You know the El Nino-Southern Oscillation cycle is an irregular oscillation with most of the variance concentrated in the 2 to 6 year frequency. But it also exhibits a pronounced biennial component that is to say year to year variation. Now the characteristics of the two phases in terms of the sea level pressure SST and OLR are brought out in the composites for July to November in the next 2 slides. Now we are just trying to see what are the important features of the 2 phases. So this is an El Nino composite for July to November and what you see here is sea level pressure for El Nino on top and this is of course the Pacific Ocean and then this is the sea surface temperature and this is the OLR. And what you see here is that there is a zonal band here of low OLR that is to say having deep convection right across the Pacific during El Nino and it is quite intense. This is where the outer curve is OLR of 240 and the shaded ones are 230 and below. So there is a reasonable amount of clouding in this band across which joins the West Pacific high intense clouding zone with a reasonably intense cloud zone of America here. Now actually the sea surface temperature is also very warm during El Nino and this is 29 degrees and 28 degrees is stretching right across and the low pressure center is around here. See this is the dateline right here, this is the dateline 180 and the lowest pressure is around the dateline in an El Nino composite and otherwise of course you have the generally low pressure of a West as compared to the East. Now the La Nina composite has obviously different characteristics. The low pressure has moved away remember it was near the dateline before and most important difference is here in the OLR. If one draws 240 then you still see a zonal band but you see that the convection is not at all intense here 230 does not occur here at all. So you have the West Pacific cloud zone, you have reasonably intense cloud zone here going up to the American coast but in between there is hardly any clouding here. So this is a major difference remember for El Nino it was cloudy with reasonable intensity right across. Now so if we look at sea level pressure those had all the features of a specific state. Now we compare sea level pressure of El Nino with La Nina and what you see is that La Nina the low pressure is much more over the West Pacific in El Nino it has now shifted to more to the Central Pacific here. So there is a major difference in the sea level pattern, sea level pressure pattern. Now you see clearly the warm event that is El Nino here see this entire region is about 29 degree centigrade, 28 degree centigrade contour is reaching right up to here and almost continuing here. That is to say this entire region here is about 28 for El Nino about the threshold for convection which we talked about. So this entire band becomes favorable for convection in El Nino look at La Nina on the other hand La Nina has 28 degrees here that is to say the warm water is here and there is also warm water here but in between it see this is 27 degrees. So in between it is less than 28 and actually here parts of it are here parts of it are less than 27 then it becomes 27 and 28 here. So this entire patch here is below the threshold for La Nina and naturally what you see is that for La Nina there is a gap in convection from here to here. This is where the sea surface temperature was below the threshold. What I show here is the OLR pattern for El Nino the mean pattern for July to November and La Nina pattern for La Nina OLR pattern. What you see is that there is a very major change which occurs between these longitudes here these longitudes which are 170 ways to 120 ways. You see there is hardly any convection in La Nina but you have intense convection here. So occurrence of convection in this band is a major distinguishing attribute of El Nino vis a vis the La Nina. See over this region any way there is convection and it becomes more intense in La Nina over this region also any way there is convection it becomes more intense here during El Nino. So but where there was hardly any convection in La Nina this is 170 ways to 120 ways now you are getting continuous band of reasonably intense convection. So this is a very critical region in which you see a major change in terms of occurrence of the TCG for the El Nino and non occurrence of the TCG for La Nina. Now note that the maximum variation in OLR as I mentioned is between these two phases is over 170 ways to 120 ways over this longitudinal belt the SST is about 28 degrees around 5 degrees north in the El Nino composite whereas it is below 27 degrees in the La Nina composite. Thus a coherent region of low OLR across the Pacific in the El Nino composite is consistent with a coherent zone of SST above the threshold. Now different phases now we have looked at all the different attributes of the two phases. So now we will look at what they look like in terms of both atmosphere and ocean these are normal conditions. During normal conditions you have convection primarily over the west Pacific in the atmosphere and you have this is the direct thermal circulation you have a descending zone here. Now this is the SST distribution SST is warm here and over the warm region there is rising in convection and then SST is very cold here along the coast of America and you have sinking here. Now this is the thermocline shown in blue and you see that the thermocline is much much deeper in the west than it is in the east this is the famous tilt of the thermocline that you see. These are normal conditions which are average over several years of what is seen over the Pacific and in the Pacific then comes the El Nino conditions what has happened is that the thermocline here has become deeper you see in the normal conditions the thermocline was almost at the surface here at 80 degrees west now it has become deeper as you can see. So the slope of the thermocline has decreased that means there is more warm water here you see here that the SST warm SST is like a river going across here entire Pacific and you get convection not only over west Pacific but over central Pacific as well. So in El Nino then because the convection has moved here what is the characteristic you have weak trade winds weaker slope of the thermocline which you have seen when the warm surface waters flow eastward and eastward shift of the tropical convergence zone whereas it was much more restricted to west Pacific now there is convection over central Pacific as well. Now let us go to El Nino it is the opposite of the cold phase and you see you have intense trade winds here intense trade winds. So intense upwelling here so the SST here is very cold cold water here relative to here and higher slope of the thermocline than normal. So from normal El Nino has lower slope of the thermocline it comes up to here and Larina has higher slope of the thermocline warm SSTs are largely confined to the west Pacific as you can see and so is the TCC the Tropical Convergence Zone. So these are the two extremes phases of this coupled ocean atmosphere system and you see how the two components atmosphere and ocean go together in these two phases. Now you have heard me use the word Nino 3 and Nino 3.4 before let us define what these regions are see these regions are supposed to be critical for El Nino and SST anomalies over these regions are used as indices for El Nino. So these regions are defined here first you remember the original definition of El Nino involved SST anomalies of the coast of South America. So Nino 1 plus 2 is the region over which the SST anomalies decrease and that corresponded to the traditional or old definition of El Nino. Now we have seen that their cold tongue comes in across the Pacific as well and across the equatorial Pacific then there are three regions Nino 3 which extends almost up to coast of America it goes all the way from 90 west up to about 150 west. So this is the eastern part of the Pacific going almost to South America this is Nino 3. Nino 4 is the adjoining region which covers the central Pacific. So Nino 4 is much more central Pacific and Nino 3 is East Pacific. Nino 4 as you can see goes from 150 west all the way to 150 east. So this is up to 160 east. So 160 east to 160 west actually is Nino 4. So this is just going across the central Pacific. So Nino 3 is East Pacific Nino 1 plus 2 is off the coast of South America and Nino 4 is central Pacific here 160 east to 160 west. Now Nino 3.4 is in between it has part of the East Pacific and it has part of the central Pacific as well but it starts east of the dateline. So it starts at 170 west and goes up to 120 west. This is Nino 3.4 all these three regions are from 5 degrees south to 5 degree north. So SST anomalies over these regions are used as indices for the occurrence of El Nino and La Nina. Now let us see how the sea surface temperature actually varies over these regions. So when we look at average SST for these different regions how does the sea surface temperature vary now we are talking of the mean of course. So we have first Nino 1 plus 2 you know that is a very cold part of the ocean. It attains its maximum which is actually around 26 or so in March but then decreases to a very low level here almost 20 degrees and then it starts increasing after September. So it has a high in March minimum in September and it is very cold right it is below 26 all the time. The next is Nino 3 which covers as you have seen Nino 3 is here it is the eastern most part of equatorial Pacific and that has a maximum in April and it is actually not as cold as Nino 1 plus 2 but it is somewhat cold and it goes above 27 degrees from March onwards. You can see March April May it is above 27 after that it decreases. So as you come towards the central Pacific then this is Nino 3.4 and Nino 4 both of them get maximum SST in May but you can see that Nino 3.4 is above 27 for a large part of the year all the way from March right up to July it is above 27. So it is somewhat close to the threshold but below the threshold of 28 this is Nino 3.4 but Nino 4 the mean is all the time above the threshold. So it is all the time above 28 this is a point to be born in mind. Now because I have included Nino 1 plus 2 in this graph these look like they are very gently varying. So in the next slide we will see only the regions over the equatorial Pacific. So as we have noted that while the mean SST of Nino 4 is 28 degrees centigrade or above for all the months the SST of Nino 1.2 which is off the coast of South America is well below 28 in all the months. The mean SST of Nino 3 and Nino 3.4 is also below 28 degrees in all the months but for Nino 3 it is above or close to 27 in March to April in March April and in Nino 3.4 it is above 27 in March to July. Thus positive anomalies of about 1 degree centigrade would imply SST above 28 degrees that is above the threshold for Nino 3 and Nino 3.4 in these periods where it is close to 27. Now here we see the same graph but without Nino 1 plus 2 and you can see better the variation of these other regions you can see a rather sharp peak for the Nino 3 region which is occurring here in April and so the Nino 3 region of course has much more variation with season Nino 3.4 not too much variation and you can see that it is from March onwards for 4 months it is above 27 and even after that in August it is not that far down from 27. So 1 degree or more slightly more than 1 degree would take it to 28. Nino 4 on the other hand is always above 28. Now let us relook at what we had seen before OLR composites and mean for July to November which is the mature phase of Lino. What we see is we have already noted before that this was the region over which we saw maximum change between La Nina and OLR and we had marked it to be between 170 west and 120 west. Now it is so happens that this is precisely the longitudinal limits of Nino 3.4. So Nino 3.4 is on the equator between 5 is out to 5 naught and precisely between these longitudes here you see. So in some sense Nino 3.4 captures better than the other Nino regions the OLR variation between the two phases. So Nino 3.4 seems a very appropriate index. So as I mentioned in fact the longitudes of Nino 3.4 are characterized by major differences in OLR between El Nino and La Nina that is the last slide. Over these longitudes the SST changes between the cold phase when the SST is well below the threshold and the warm phase when it is above 28 degrees are also significant. Not surprisingly the NOAA official definitions of El Nino and La Nina are based on Nino 3.4 SST. Now I may mention NOAA is the official agency of United States of America National Oceanic and Atmospheric Administration NOAA. So how do they define El Nino and La Nina? They define El Nino and La Nina based on a threshold of plus or minus 0.5 degree centigrade for what they call an oceanic Nino index which is defined as the three month running mean of Reynolds SST anomalies. So based on a centered on 30 year base periods and so on and so forth. In the Nino 3.4 region which is 5 out to 5 naught, 120 to 170 west. So Nino 3.4 region SST anomalies of which they use three month running means that is what they think is based on and if this three month running mean ONI is greater in magnitude than 0.5 you get you say it is either an El Nino if it is positive anomaly or a La Nina if it is negative anomaly. So this is the ONI index now plotted for you right from 1950 to 19 to 2012, 2011 and what you see here is this red lines are the 0.5 which are the thresholds for the definition of El Nino and La Nina and you can see that when this is anomalies are larger than 0.5 these are all El Nino's all these are El Nino's when you have the ONI becoming positive and larger than 0.5 and these are La Nina's here you see this is when the index dips to below minus 0.5 and you have a whole set of La Nina's here and whole set of El Nino's here notice that the highest highest anomaly ever recorded is in 1997 this is the strongest El Nino. So now we have the definitions of El Nino and La Nina under our belt and we can now look at one more very important facet of ENSO. See ENSO phenomena is not only of great interest in itself but it is also of great importance because it has teleconnections with the rainfall and temperature over a large part of the tropics as well as mid latitudes. So it is a phenomena that seems to have an impact on many many regions in the tropics as well as mid latitudes. In fact in those seminal papers of birkenness that I talked about in the last lecture he also emphasized the likely impact of warm SSDs associated with El Nino on the prevailing westerlies in the mid latitudes and hence on the weather and climate of the mid latitudes. So birkenness also emphasized the connection of these El Nino events on with mid latitudes. So this is how the influence can be depicted first you see warm episode relationships between December and February warm episode is El Nino and June to August is of greater interest to us that is when our summer monsoon occurs and what you see is dry patches here. So El Nino has wet is associated with wet region over Central Pacific which we have seen and dry region over India. So you expect a dry monsoon during El Nino or a deficit monsoon during El Nino this is seen and if you see the cold relationship that is to say what happens when you have La Nina when you have La Nina actually it is drier here over Central Pacific and it becomes rather wet over the entire Indian region plus the adjoining part of the Asian region as well. So La Nina you get more rain and El Nino you get less rain. So during the boreal summer the Indian region has dry anomalies during El Nino and wet anomalies for La Nina. Thus it is clear that there is a link between Indian summer monsoon rainfall and Enso. Hence even if one was primarily interested in the monsoon it is important to try and understand Enso and consider its predictions which by the way have improved enormously over the last decade. I will talk about the monsoon Enso link in some detail in another lecture. Now irrespective of which Nino index is used Enso has strong links with OLR or rainfall of a large part of the tropics. This can be also clearly seen in the correlation of OLR over the global tropics with the different Nino indices for June to September. So now what you see here is correlation of Nino 3.4 SSD with OLR everywhere. Obviously if it is warm then you will get convection which means OLR will be negative. So the correlation is high and negative here. This is over the central pacific over this part. In fact the convection is high in an El Nino here corresponding to over a very large region here and you get a suppression over a very large region here. So it seems like an east-west dipole with the entire pacific in the northern hemisphere primarily convecting more except for parts of the west pacific which seem to be convecting less. And from here onwards we have suppression of convection here associated with the El Nino and opposite would be associated with the La Nina. This general pattern is there for all the indices. But notice that Nino 3.4 seems to have a higher correlation with the Indian region, higher correlation in magnitude than the other indices. And here we have Nino 3 again very similar pattern to Nino 3.4. Nino 4 is also somewhat similar notice that all of them have negative correlation with the rainfall over this region just off the coast of South America. This is around 20 south or so. But the more important one is this India pacific east-west dipole if you wish. Now Nino 1 plus 2 has the minimum correlation with Indian region and in magnitude of course it also has a huge negative correlation all along here which is in the northern hemisphere. You know this is where the band was the low L R region was in the mean. So somehow it tends to suppress Nino 1 plus 2 warming tends to suppress this entire Indo pacific region here uniformly. Where Nino 1 plus 2 whereas the other regions the ITCG over the pacific which occurs always in the northern hemisphere you know 5 to 10 north actually gets enhanced during Nino and this is seen with the other indices. But this index is somewhat different and this is corresponds to looking at the way Nino was originally defined by seeing SST anomalies of the coast of Peru and Ecuador. Now just as a point to make we have one should not think of El Nino and La Nina as always having mirror images of effects of entirely opposite effects. See we have noted that in terms of anomalies the distinguishing attribute of the warm and cold phases is the reversal in sign of anomalies. So for El Nino you see throughout here actually the SST anomalies are positive for La Nina throughout here the SST anomalies are actually negative. But it must be kept in mind that in many ways cold and warm phases are fundamentally different because the quantities that affect the remote atmosphere are not the SST anomalies. SST anomalies are our own creation it is saying how different the SST is from the average but what the atmosphere sees is the actual absolute SST. So it is not anomalies that affect the remote atmosphere but rather the mean location of the regions of persistent precipitation. So directly the teleconnections are with regions of persistent precipitation and we know what does that depend on? These regions of persistent precipitation depend on the SST particularly if SST is below the threshold you would not get any precipitation at all. So it depends on regions of persistent precipitation and also on spatial gradients of SST because as we have seen if SST is maintained above the threshold whether you have convection or not depends on whether you get convergence of low level air or not. And that itself depends on spatial gradients of SST amongst other things. So if you have a very sharp maximum SST then that would lead to convergence and if the SST is above the threshold that would lead to crowding intense deep convection and precipitation. So what really matters for the atmosphere is the absolute values of SST and as we have seen in our El Nino line in our composite the regions over which SST is above the threshold vary considerably between the two phases and that is what is going to matter it is not anomalies although they can be inverse of the things. The precipitation regions how they vary cannot be thought of being inverse of one another. The SST anomalies can be inverse of each other but the mean location of the heat source which drives the response of the low and the mid latitudes is very different. In the warm phase of Enso persistent precipitation extends into the central pacific while during the cold phases of Enso it reaches to the far western pacific we have seen this. Now because the rest of the world is forced by these regions of persistent precipitation and because these regions are in different locations for warm and cold phases of Enso there is no expectation that the global effects will be the negative of each other. See what is involved is a shift in the location of precipitation and also in its intensity and these shifts will not lead to effects which are positive and negative obviously. Now after the groundbreaking work of Birkness in the 60s there has been a phenomenal progress in understanding the physics and modeling of Enso since the 80s. Models of the coupled atmosphere ocean system are now capable of generating predictions of Enso with reasonable skill. Here I discussed what is understood about the basic facets of Enso and how this understanding was achieved with the hope that some lessons can be learned for studies of the monsoons which is really the focus of this lecture series. Now this discussion is primarily based on the excellent books by Philander and Saracic and Cain which I have referred to earlier and these people Josh Philander, Ed Saracic and Mark Cain have been major players in the elucidation of the physics of Enso and modeling of the phenomena. Now so what is the understanding that has been gained about Enso? We have mentioned this in several contexts before but let me just quickly recapitulate the mean state of the tropical Pacific and the atmosphere above can be understood as follows. The equatorial sea specific is 4 to 10 degrees colder than equatorial west specific. The east is cold because of equatorial upwelling, the raising of the thermocline exposing colder waters and the transport of cold water from the south specific. All of these are dynamical features driven by the easterly tradements. So the atmospheric circulation is driven by the SST gradients which lead to a low pressure over warm SSTs in the west and a high pressure over cold SSTs in the east. Now over the equatorial regions the surface wind is partly driven by the surface pressure gradients and they has an easterly component obviously because the pressure is low over the west and high over the east. The wind will go from east to west that is to say it will have an easterly component. Now large scale circulation in the tropics on time scales of weeks or longer corresponds to direct thermal circulation. This is a very important thing to remember. Thus in the zonal walker circulation air rises over the warm western tropical Pacific and sinks over cold eastern tropical Pacific. The moisture laden air carried westward by the trade winds converges over the warm west Pacific. Now ENSO so that was the mean state we were talking about how does ENSO operate. The relaxation of the westward surface winds during El Nino produces less upwelling and therefore less cooling. Consistent with weaker westward winds the thermocline is not as tilted and any upwelling in the eastern Pacific would bring warmer water to the surface because the thermocline is now deeper. Consider next the nature of ENSO and mechanisms leading to this oscillation. As I mentioned there has been a phenomenal increase in the understanding of the physics of ENSO since the 60s. So elucidation of the nature of ENSO until the 1960s there was relatively little data on ENSO and the El Nino was regarded as a departure from normal conditions. It was considered as an event that suddenly occurs. It was believed that the phenomena starts to develop at a certain time because of triggers that lead to its growth. So one talked of an event which was being triggered by certain factors. The subsequent decay of anomalous atmospheric and oceanic conditions restores the normal state. So one thought of El Nino as an event that is triggered that develops that grows that dies and with the death one gets back to the what we may call the normal state. This was the perception based on the earlier data. Now a lot more information was available about El Nino by the early 80s because of the recovery of earlier measurements made by commercial vessels over many decades as well as new data collected since the 60s. So a huge database became available in the 80s because of this and with an in-depth analysis of all data including data from ship tracks over critical regions for 1950 to 73 Rasmussen and Carpenter in a paper which is very famous now elicited the nature of El Nino and how it evolves. So there were seven El Ninos between 50 and 76 and spectrum analysis of the SST along the coast of South America now showed the following. What you see here is that there is a very clear peak now this is the normalized spectral density on the y-axis and this is the frequency on the x-axis and actually here the period is also shown in months and what you see here this is 3 years or 36 months and this is 48 months or 4 years. So the major peak is between 3 to 4 years this is the major peak in the spectra. So they showed that this phenomena had a periodicity of about 3 to 4 years. Now they also elicitated the nature and evolution of a typical El Nino. They showed that although the amplitudes of the different El Ninos varied considerably their phases are very somewhat similar. So you see these are all the El Ninos drawn and this is the SST anomaly here of the coast of South America and this is the month. So what you see is by and large they all start developing around here around January of the El Nino year then they reach a peak around here somewhere somewhere around June or so and then start decaying and so it is about an 18 month kind of period here this is January and this is June or if you want to stop it here this you can call as the El Nino year when the anomalies are all positive that is from early from around the beginning of the year to around the end of the year. So all the profiles look somewhat similar and therefore one can make composites because if they were totally out of phase then the average makes no sense at all composite makes no sense at all. But because by and large they are in phase this is what you see is the composite SST anomaly along the ship track and this is for a specific station. So by and large one can now think of a composite El Nino or an average El Nino if you wish which is whose evolution we can look at by adding together all the information of the 7 El Ninos that occurred and that is what Rasmussen and Carpenter did. Now what they did was to show how this SST anomaly patterns evolve. So first they looked at the antecedent anomaly composite August to October for the year preceding El Nino and what you see is the anomalies are negative almost everywhere except here there is a small part here which is positive. Now what happens then during November to January prior to maximum SST anomaly this is the beginning of the El Nino event if you wish what you see is rather large positive SST anomaly is here but notice interestingly there is also a region of positive anomalies around the equatorial regions of the dateline central Pacific. So this is how things change from August to October to November to January. Now comes what Rasmussen and Carpenter called peak phase composite because they insist on still defining El Nino by SST anomalies of the coast of South America. And this is where the peak anomalies of the coast of South America occur this is from March to May of the El Nino year. So this is what they call the peak phase composite and you notice that these SST anomalies have now actually spread to almost this part here. So if we went to the previous picture then the SST anomalies were just here and here in the next picture now you have peak SST anomalies which are occurring here but also positive anomalies have spread up to here. And in the next picture which is August to October of the El Nino year you have a very huge band of warm SST anomalies. So you can see that it appears that anomalies have spread westward you see originally they were constrained to this part of the equatorial Pacific. Now they have spread westward to occupy a very large region of the Pacific. So in the peak phase composite March to May of the El Nino year strongest positive SST anomalies occur of the coast of South America. The magnitude of the positive SST anomalies is high over a belt extending westward up to 120 west. In August to October the SST anomalies are higher than 1 degree centigrade over a belt from 180 east right up to the South American coast. So there is a clear westward propagation of anomalies. Now comes the mature phase and here we still have very high anomalies positive anomalies of SST. This is December to February following El Nino and now after that this is SST anomaly from May to July following El Nino now it has all become negative. So this is the end of the episode if you wish. So in the mature phase the largest SST anomalies are from 180 degrees to 100 degrees with anomalies larger than 1.2 degree centigrade over that critical region that we had seen 170 west to 120 west. So here the anomalies are very large over that critical region from 120 west to 170 west see this is that belt here. This was the critical region which flared up during El Nino and which was suppressed during La Nina you remember this is that Nino 3.4 longitude region and that is where the anomalies in the mature phase are very very high. May July the following and El Nino here negative SST anomalies over the equatorial belt east of 140 degrees and positive anomalies over the central Pacific weekend. So Rasmussen and Carpenters suggested that the westward shift of positive SST anomalies to the central Pacific coincides with the strengthening of western event anomalies along the equator and a southward shift of the ITCG over the west Pacific. This is also accompanied by a northeastward shift of the SPCG. SST anomalies of the magnitude characteristic of the composite El Nino can have a major impact on the atmospheric convection and precipitation for regions over which they imply that the SST becomes higher than the threshold. Positive SST anomalies over the eastern equatorial Pacific in March May can also lead to some enhancement of convection over the region. What you see here is the mean for March May this is over 27.5 and you see that there are large SST anomalies here exceeding 0.5 and so on. So this can lead to intensification of this and can lead to part of this region going above the threshold in March to May. In August to October over 150 ways to 120 ways the mean SST is just above the threshold in a narrow band and OLR is not very low over this region. So now we are looking for mean of August to October and what you see here is a thin band here of OLR not very intense and you can see that SST is just above 28 in this thin band here this is 27.5 is just above 28 and if you look at the anomalies corresponding to El Nino then you find very large anomalies occur where actually there was relatively thin band which is above the threshold and so the positive SST anomalies associated with an El Nino would lead to higher SSTs over this region and imply enhanced convection which is what we saw in the composites. That these kind of anomaly pattern makes this entire region highly warm and would therefore lead to enhanced convection over that region. So the most important result of the Rasmussen Carpenter study was considered to be the westward propagation of the SST anomalies during the establishment of the composite El Nino. It suggested that monitoring the SST of the eastern most part could lead to anticipation of anomalous conditions over central Pacific. See we are always very happy to see propagations of this kind which occur over a period of several months because this means that if we see the event where the propagations begin then we could anticipate what will happen in the region to which the event is propagating. However, the year the Rasmussen and Carpenter paper was published which was in 1982 another El Nino occurred this was the El Nino of 82 and 83 which did not behave like the composite El Nino in some aspects. This clearly showed that the phenomena is more complex than the composite picture had indicated. Just when we thought that we know how a typical El Nino evolves came an El Nino which did not quite evolve in the way Rasmussen and Carpenter composite had evolved. In what way was it exceptional? The El Nino of 82, 83 was exceptional firstly because it was a very high amplitude very large magnitude of SST OLR anomalies and so on. Also it evolved in an unusual way. In fact the warm SST is propagated eastward from the central Pacific instead of westward propagation seen in the composite and OLR anomalies also propagated eastward in the central Pacific. So, what this again is a paper from a paper by Gil and Rasmussen. So, Rasmussen also documented the El Nino which departed from his classical picture of how El Nino evolved and what you see here is see this is time and this is July 82 and time is increasing downwards. So, this is July 83. So, what you see is this is warm, warm water is actually moving westward with time. You see the longitudes are here moving eastward. So, it all began near the dateline this is the dateline here and this is 100 west. So, this is the South American coast is here. So, it begins here and moves eastward throughout. So, you see warm water moving eastward and OLR anomaly moving the same way and this is the mean sea surface temperature which actually has is simply decreasing as you go towards the east and this is the Westerly wind anomaly. So, the evolution of the SST and OLR over the equatorial belt and OLR over 5 to 10 degree north where SST where you know the ITCG occurs in the mean pattern for three El Nino events we look at in the next sign. Because this is a very interesting phenomena they showed that in 80 to 83 things began in the central Pacific not in east Pacific and then moves to towards that. So, what you see here is what meteorologists call a kind of homular diagram and this is daily OLR now most of the time we have been looking at monthly OLR again the shades the darker the shade the lower the OLR the deeper the clouds. And what you see here now time is going upwards in these graphs not downward like in the earlier one. So, this is January 82 and this goes all the way this is January 83 and January 84. So, this is for 3 years and here we are centered at the equator itself. So, this is from 5 degree south to 5 degree north. So, this is OLR and this is weakly SST from Reynolds. So, what you see here is initially you see that the SST for example is warm right from the this is actually 40 degrees east onwards. So, this is the Indian Ocean and now this is the West Pacific here and what you see here is that West Pacific is warm. But as you go towards July 82 this warm SST region is moving towards the east and this is near the South American coast. So, you see a tongue of warm water moving westward and just at the same time you see this is the low OLR region. Low OLR region was more or less confined to the west of 160 degrees east that is to say to the West Pacific before July 82. But from July 82 it started moving eastward. So, this is another way of looking at what we saw this eastward propagation of both the warm water and the convection in the atmosphere was a characteristic of 82, 83 and you know. But remember the actual cloud band is between 5 and 10 north in the mean picture. So, what you see here is this is the equatorial belt and this is the belt just to the north of it 5 to 10 north again for the same period and what you see is this 5 to 10 north belt is actually active throughout here up to about January. It is active throughout this period very much so up to July 82. Now in this patch from about October 82 to April 83 there is hardly any clouding over West Pacific in the northern one. But you see it seems to have come to the equatorial region. So, here in this part of the thing you had clouding going all the way from 5 south 5 to 10 north over the entire region and over the equatorial region up to about 160 that is to say West Pacific was convecting over equatorial region as well as to up to 10 degrees north and the rest of the region was mainly 5 to 10 north as we have seen. Now you see the movement here see the movement eastward movement has occurred only in the equatorial band when there was no convection at all to the north that is to say when the tropical conversion zone moved to the equatorial region that is when the propagation has occurred. This is for 82 83 and similar situation you see for 97 El Nino as well. Again you see things moving eastward here this is the equatorial region see the clouding has moved just when the sea surface temperature warm water also moved or spread if you wish. And again you see that actually the clouds disappeared from the northern part and were restricted to the equatorial belt and that is when the propagation occurred. This is very interesting because you remember in Birkenness's argument he had argued lucidly as to how because of the higher pressure in the west Pacific in the ocean relative to the east Pacific you have a current along the equator which goes from west to east this is the equatorial undercurrent. So perhaps what we are seeing is warm water being advected by the equatorial undercurrent here and because equatorial undercurrent is restricted to the equator you do not see any advection of warm water or any convection propagation in the northern part. Now there are events like 87 in which there was no propagation of eastward at all it may have been more classical Rasmus and Carpenter kind of evolution. So there is considerable variation in the way in which El Nino evolves from one event to another and I will consider the anomaly patterns of El Nino events in the satellite era in the later lecture. So in this lecture now we have learnt what are the important attributes of El Nino La Nina we have learnt what the analysis of the data that became available by it is has shown us the periodicity of El Nino La Nina and how it seems to evolve in a large number of cases. But we have also seen how complex the phenomena is and as soon as one starts generalizing say saying that an El Nino evolves with propagation of anomalies from the east immediately an exception occurred which told us that not all El Nino's behave in that way. So we will continue to look at El Nino and the understanding gained on this very fascinating phenomena in the last two decades in the next lecture. Thank you.