 So, in this lecture I continue from where I left off. In the last lecture continue my discussion on the El Nino Southern Oscillation. Remember we were looking at seasonal variation of the tropical atmosphere over the Pacific and we already looked at the seasonal patterns. Now we begin with looking at mean monthly patterns and what we will see an important feature of the mean monthly OLR patterns which we will look at now is the coherent low OLR region stretching from the Indian longitude across the Pacific up to the deadline. So, month after month you see this as a coherent low OLR region. So, it looks like what is happening over the Indian longitudes is part of a large system of which the other part is over the Pacific. So, it would be surprising if the variability of convection over Indian longitudes is not linked to that over the Pacific. In many of the ENSO studies people focus just on the Pacific, but we are interested in the monsoon. So, we have to focus on both and it is clear that understanding of the coupling over both the oceans is going to be important. Now during January, February as I am as we saw in the seasonal pattern there is no TCG over the central and east specific east of 150 west. So, this is January and you see there is no region of low OLR at all over the east specific here there is no low OLR region and you also see that sea surface temperature is below the threshold except for small patches here very very small patches here. So, it is not a surprise February also the same story only thing is by February this patch of warm sea SST higher than threshold has increased in extent as well as intensity. So, while in January the SST is below the threshold over the entire region in February there is no low OLR region over the east specific even though the SST is above the threshold we saw that over the west specific the SPCG is more intense than ITCG and the ITCG over west specific is a part of low OLR region which is stretching from the eastern Indian ocean. So, you see west specific ITCG is here and you see the ITCG is more intense than the SPCG already and there is of course no convection here and February despite the ocean being warm there is no convection over the east specific. Now, what happens in March now this region is becoming even warmer and still of course there is no low OLR region over central and east specific, but this remains a coherent zone right from the Indian ocean one goes to the ITCG and the other is of course the SPCG. In April the ITCG over the Pacific is seen eastward of 130 is right up to the South American coast. So, this is the first month in which we see a zonal band here see a zonal band of low OLR is stretching right across the Pacific here and this is the first month in which we see it remember it was not there in January February March where convection ended more or less over west and central Pacific this was the limit. And now you see that the warm SSTs have become even warmer and the region of warm SSTs here has extended very much more in fact this is the warmest this part gets. In May there is a coherent band of low OLR from 60 east over the equatorial Indian ocean stretching eastward you see this. Now what has happened is in April you did have a band which was coherent across this way, but it did not extend so far westward over the Indian ocean. Now we are seeing a band right from 60 degrees east which is going right across the Pacific. In April in between this was somewhat weaker, but now the Pacific ITCG here has strengthened and the ITCG is definitely stronger than the SPCG. So, in May already the situation has changed and you see a very coherent band across ITCG stronger and even here there is a region of low cloud you also see that it has become even warmer here of the coast of America's, but remember this is all north of the equator this is not on the equator on the equator the cold tongue persists. Now in June the West Pacific is much stronger than the ITCG over West Pacific is stronger than SPCG which was true in May as well and this entire thing has strengthened more you see this as a stronger one this part has strengthened more and you see now a very coherent band of warm accessories right across the Pacific in June. Now in July and August a secondary TCG is seen over the Indian ocean this is an interesting phenomena of course, in July now the ITCG over the West Pacific is very much dominating the SPCG and we have a coherent band here, but what July by July the onset of the monsoon has already occurred. So, you have most of the clouding and low OLR over the Indian land mass and the Bay of Bengal, but there is a secondary ITCG secondary TCG rather which is actually just along the warm SSTs here you see this is the Indian ocean tropical convergence zone which we have looked at before. Now August again the story is similar except this seems to have become more intense and we have a secondary band here and a secondary maximum in SST also over the equatorial Indian ocean you see that this one is still persisting, but not as warm and not as large an extent as in May you see in May this was much larger. May you also see another phenomena which was there also in April that a huge band of very very warm water here over the Indian ocean, but the clouding is restricted to a very small part of that. You notice that we mentioned that SST being above the threshold is a necessary condition, but not a sufficient one. This is a very nice demonstration of the lack of sufficiency obviously in April although the ocean is warm the dynamics is not favorable. So, now this is the situation in July and August the peak monsoon months and we have seen that the patterns for the peak monsoon months are rather similar low OLR region stretches from the Indian monsoon zone across the Bay in a southeastward eastward direction to over Pacific and then as a zonal band. So, what is happening is see here because of the monsoon the TCG has moved much northward here it has remained south of 20 north. So, the band is going in this direction here and then zonal here the ITCG over the Pacific between 150 and 130 west as well as the ITCG over west Pacific is somewhat stronger in August. So, in August you have very well formed ITCG right across here and here also it is much stronger where it was somewhat weaker before. Now, September the system has become weaker the monsoon has started to withdraw from the country and of course, the warm pool region remains and a coherent band of warm water remains and so does the coherent band of low OLR stretching right across the Pacific and the same thing continues in October as well, but notice that now in October the monsoon has withdrawn from much of the Indian land mass and what you have here is a band more or less on the same latitude going all the way from the Indian region where the latitudinal extent is somewhat larger going right across the Pacific up to the Americas. So, this is around 5 north or so 5 to 10 north this is where the ITCG is and of course, it is consistent with the sea surface temperature. It is interesting that while there are major changes in the OLR patterns over the Indian and Asian region during May to October because of the changes in the locations of the TCCs with the onset establishment and retreat of the monsoon the ITCG east of the deadline persists in the same location throughout there is no seasonality at all. Well in a way it has no way to go it was not very prominent during December to February January February March, but since April it stays more or less in the same place thus from May to October the low OLR region over the Indian longitudes appears to be a part of a coherent region stretching from there eastward across the entire Pacific. Now this is a point to be emphasized that we have to remember that our Indian monsoon is a part of a system not only of the large scale Asian monsoon, but what you could call part of a system that is stretching right across the Indo-Pacific region. It is therefore, to be expected that variability of the Indian monsoon will be linked to the TCC over the Pacific. Now in November and December the warm SST region over the eastern most part has almost disappeared and you see that here you see the warm ocean region here has become much smaller than it was earlier and with that the low OLR region here has disappeared still we see an ITCG from Central Pacific, but it does not quite reach the coasts of America in November and in December the same story, but now actually the SST region has shrunk even further the warm SST region has shrunk even further and this also has shrunk to some extent and no region of low OLR of the coast of Americas. So, the zone of low OLR stretches from about 60 degrees east over the Indian ocean across the west Pacific only up to about 120 west and does not quite go up to South America, but still what is happening over the Indian ocean is a part of a large scale system, but this time the longitudinal extent is somewhat less than it was from May to October it does not quite reach South America. Now this is so much about the mean seasonal variation of the tropical convection. Now we have to consider the inter annual variation of tropical circulation and convection. Now this is where in a major contribution in elucidation of one of the most important facets of inter annual variation over the tropical Pacific Indian ocean regions namely the southern oscillation was made by Sir Gilbert Walker in the 1930s. So, this is a very very important facet of inter annual variation of the tropical circulation discovered by Sir Gilbert Walker and his colleagues this is the southern oscillation. So, in a set of papers from 1924 to 32 Sir Gilbert Walker and his collaborators reported the discovery for remarkable circulation parameter for low latitudes which he called the southern oscillation. In his words in general terms when the pressure is high relative to the mean in the Pacific oceans it tends to be low in the Indian ocean from Africa to Australia rainfall varies in opposite direction to the pressure. So, the fluctuations of surface pressure at Darwin in Australia tend to be out of phase with the fluctuations at Tahiti which is in the central Pacific and the correlation of sea level pressure at Darwin with the sea level pressure over the tropical Pacific is negative east of the date line and what you see here is a time series one of them is Darwin and one this dotted line is Darwin and the solid line is Tahiti and you can just see by the eye that by and large the variations tend to be out of phase. So, you had here Tahiti anomaly positive. So, Darwin was negative here Tahiti sorry Darwin was positive where Tahiti was negative here Tahiti is positive Darwin is negative. So, on the whole these are in opposite phase and that is why you get negative correlation. So, what you see here is a very famous diagram now this is the correlation into 10 of the surface pressure variation at Darwin which is here this is Australia. So, this is the correlation between the surface pressure variation at Darwin relative to that over the global tropics and you notice that over this part of the thing see the Indian longitudes vary along with Darwin same they have a positive correlation. But you see this side of the date line this is the date line here this side of the date line Tahiti has a large negative correlation minus 0.8. So, there is a seesaw between the pressure variations here the correlations are out of phase the variations are out of phase as far as the pressure is concerned. So, thus the variation of the surface pressure anomalies at Tahiti and Darwin and the Southern Oscillation index defined as a difference of Tahiti and Darwin sea level pressure anomalies is what we see here. So, these are now what you saw there were how the pressure varied out of phase now this is the sea level pressure anomaly this is Tahiti and this is Darwin. So, the difference between the two gives you an index which is the Southern Oscillation index. So, and this is the difference and these are these are the actual SLP anomalies and you can see that they are out of phase when one is positive the other tends to be negative. So, the difference is a very robust measure and it is a measure of what we call the Southern Oscillation index rather what Sir Gilbert Walker called the Southern Oscillation index. Now, clearly the series of surface pressure anomalies at the two stations are generally out of phase and the difference makes a robust index thus the difference of the normalized surface pressure between the east and the west generally taken as between Tahiti and Darwin this difference is conventionally called the Southern Oscillation index. In fact, the Southern Oscillation index is a measure of the strength of the Walker circulation when the Walker circulation is strong the pressure in the west is relatively low and the pressure in the east is relatively high right and when the Walker circulation is weak the pressure in the west is relatively high and the pressure in the so you have this is the case of a weak circulation where the pressure in the west is relatively high. Now, this is relative to mean or climatology and pressure in the east is relatively low this is when the actual pressure gradient which is from high pressure here to low pressure here weakens. So, this is a weaker Walker circulation and what you see is this is the stronger Walker circulation where you have very very strong the TCG located the rising limb located right over west specific and sinking limb here. Now, what has happened in the weak case is that the rising limb has shifted to the central Pacific. So, the Southern Oscillation which is an oscillation in the intensity of Walker circulation is associated with fluctuations in intensity and position of the ascending moisture that is Tropical Conversion Zone and Walker had already documented its relation to the winds and precipitation over the Pacific. Now, so far we have elucidated important fluctuations of the ocean state between El Nino and La Nina and an important oscillation of the Tropical atmosphere namely Southern Oscillation. Now, this is an oscillation of the east west pressure gradient over the Pacific Equatorial Pacific. So, this is the pressure gradient in the atmosphere oscillation of the pressure gradient in the atmosphere whereas El Nino and La Nina are warm and cold states of the Pacific which are seen as major oscillations in the sea surface temperature distribution over the Pacific. Now, the link between these fluctuations of the atmosphere and the oceans was only elucidated in the 60s with the important work of Birkenness. Now, the contribution of Birkenness is so important to our present understanding of ENSO that I thought it was worthwhile to actually discuss those two very critical papers seminal papers that Birkenness wrote in 66 and 69 and what the contribution is. So, in a set of seminal papers Birkenness in 66 and 69 pointed out the important role of a positive feedback between the atmosphere and ocean in transitions towards El Nino or La Nina which also gave an insight into how the mean state of the atmosphere over and of the Tropical Pacific is maintained. So, now I am going to consider in some detail Birkenness's elucidation of the physics of the fluctuations and their coupling. Now, as you recall originally El Nino was defined as an SST anomaly of the coast of Peru and Ecuador that is to say over East Pacific close to the South American coast. Now, during the international geophysical year 1957-58 and El Nino occurred and because it was international geophysical year there were a lot of observations and it was noted that the warm surface waters were not confined to the coast of South America, but extended far westward to the dateline. That event contributed to the present view of El Nino being associated with warm SST anomalies over the East Equatorial Pacific as well as of the coast of South America. So, Birkenness propounded a theory about the impact of warm anomalies of SST over the eastern and central Equatorial Pacific on the Hadley cell. See this is where the importance of coupling comes in. He actually propounded a theory about how these SST anomalies would impact on the convection in the atmosphere on the Hadley cell and tested it against observations of from the period of greatest known positive SST anomalies over the eastern half of the Equatorial Pacific in recent years in the winter of 57 and 58. So, he actually propounded the theory of how the SST would have an impact on the Hadley cell and compared it with tested it against these observations. Now, first of all he talks about the Equatorial cold tongue. South of the oceanic thermal equator a tongue of cold water flows westward aided by the wind stress exerted by the Equatorial easterlies. Now, the lowest surface temperatures of this tongue are centering right along the geographical equator which is what we have seen in so many pictures of SST that the center of the cold tongue is right along the geographical equator from where the Ekman drift diverges to the north and south as a result of the change in the sign of the Coriolis parameter from one hemisphere to another. We have actually seen how Equatorial upwelling occurs because of this Ekman flow diverging there. We have pointed out that particular meteorological interest is associated with the case of extreme weakness of the Equatorial easterlies and resulting elimination of the upwelling. So, he has this is again the link which was known before his time, but pointed out by him that if the trade winds become weaker these easterlies become weaker then one can have much weaker upwelling including elimination of upwelling. And he talks about how the upwelling has got eliminated, how the thermocline has become deeper he says the 80 degrees Fahrenheit isotherm actually has its greatest depth of 500 to 600 feet at the equator he says. The great equatorial warming down to such a depth from October to December 57 must have come about by sinking motion possibly added somewhat by anomalous water advection from the west. So, he is actually now elucidating the physics of the process how did the thermocline become deeper when the upwelling was cut off and what else besides the cutting off of the upwelling could have contributed to the deepening of the thermocline. Now, this is where he talks of anomalous sinking at the equator I mention all this because this is this still holds true several years 50 years after Berkness's paper almost 50 years Berkness attributed the anomalous sinking to the geostrophic flow of the ocean below the Ekman layer associated with the east west pressure gradient. Remember we had pointed out that the wind drives the ocean from east to west because of this there is a piling of water on the west relative to the east that is to say the sea level on the west pacific is higher than that in the east. So, this means that within the ocean there is a pressure gradient which is pressure which is high on the west side where there is a larger column of water above any point relative to the east. So, you have a pressure gradient which is high on the west and low in the east in the ocean and therefore, this to this pressure this pressure gradient will lead to geostrophic flow. Now, along the equatorial troughs he points out the ocean level is rising from South America to New Guinea by about 0.7 dynamic meters due to the wind stress of the prevailing equatorial easterlies. So, because the winds are piling up water against the west we have a pressure gradient in the ocean where there is high pressure in the west and lower pressure in the east. Now, let us see how this geostrophic flow comes about. So, we have in the ocean in the northern hemisphere a high pressure in to the west low pressure to the east and this means that the pressure gradient force is this way and a geostrophic wind in which there is a balance between the pressure gradient and the Coriolis force has to be towards the south or towards the equator because that is the only way only when it is to the south can the Coriolis force balance the pressure gradient. If it were you know that geostrophic wind is always along lines of equal pressure. So, it has to be along this line, but if it were in the opposite direction say it was going northward then pressure gradient and Coriolis force would be in the same direction. So, there is no question of one balancing the other. So, the geostrophic current in the ocean has to be towards the equator. Now, in the southern hemisphere also it has to be towards the equator remember the pressure gradient is the same in the southern hemisphere, but in the southern hemisphere since the Coriolis force acts to the left of the wind when the when the current is going towards the equator you have a Coriolis force this way balancing the pressure gradient. So, both in the northern and the southern hemisphere the geostrophic current which is generated by the east to west pressure gradient is towards the equator. So, this is very interesting. Now, in the Ekman layer the situation is different geostrophic current is below the Ekman layer. So, below the Ekman layer then you have convergence in the Ekman layer you actually have divergence at the equator and that is why equatorial upwelling, but below the Ekman layer you actually have convergence from both north and south of currents at the equator. Now, such geostrophic convergence of surface water must prevail at the equator at all times, because the zonal slope of the equatorial ocean although somewhat variable with time never ceases to be directed downhill from west to east. So, this pressure easterlies are always maintained irrespective of what happens and so this gradient pressure gradient is always maintained because the sea level is higher over the west specific than over the east specific. Now, we can combine geostrophic and the windriff motion into a composite schematic flow model applicable to the vicinity of the equator and this is the schematic model that Berkness proposed. What you have is east wind here and this east wind will create along the equator current which goes in the same way from east to west this is what is leading to piling up of the water, but the geostrophic current which is below the Ekman layer is in fact going in this direction this is the so the inclination first we should look at the inclination of the thermocline. Now, we see that there is piling up of water here and so there is deeper thermocline here then there is there. So, this is the thermocline which is inclined here and actually the inclination of the thermocline is going to be much larger than the inclination of the surface here. See surface is higher in the west specific than in the lower than in the east specific because the winds have pushed the water up, but thermocline slope is in the opposite direction and if you go very much below the thermocline then the two have to balance each other and there should not be any pressure gradients or currents because in the deep ocean there are hardly any currents. This is why the thermocline is tilting the other way and he mentions that the inclination of the thermocline between is between 2 to 3 orders of magnitude steeper than that of the sea surface and is adjusted such that the isobaric surfaces are quasi horizontal in the deep water. So, this is the way the pressure field is with that kind of pressure field the zonal flow component of the water must be negligible below the thermocline because there are no pressure gradients below the thermocline while being eastward directed and rather strong just above the thermocline. So, so with this kind of pressure gradient just above the thermocline it is going to be directed to the east. This is the equatorial undercurrent which actually generated a lot of interest when it was first discovered and this was in the 50s. It does not normally extend to the surface because there is an overcompensated it is over compensated by westward wind rift because there is a water is being pushed by the wind towards the west, but the geostrophic flow is towards the east. So, that becomes an undercurrent in the extreme case of a cessation of easterly wind the wind rift would immediately cease to while the eastward gravity flow would continue as long as the ocean surface is tilting. So, what happens when you have calm is that because the ocean surface is tilting you will get this kind of a gravity flow going towards the east. So, this gravity flow which is occurring because the level here is higher than level here will be the only current that will be seen if there are no winds to push the water up. So, all the water above the thermocline would then flow to the east this is the so called surfacing of the equatorial undercurrent which does occur temporarily when westward wind stress becomes too weak to balance the eastward downhill component of gravity. Now, this is a picture from George Philander which shows how complicated things are here this is the equatorial region. So, you have at the surface you have a flow like this going towards the west and this is the equatorial undercurrent this is the section if you wish and you have divergence here because of Echman drift in Echman layer, but you actually have convergence here because of geostrophic this is what Bairkness showed and this is of course the upwelling of the Peru coast. So, life is very complicated, but it it was very elegantly explained this complicated flow pattern of the ocean by Bairkness. Now, he says the cessation of the easterly winds permits the geostrophic convergence to act unopposed also at the ocean surface. If remaining sufficiently long such flow would make the ocean surface bulge upward at the equator instead of downward and the water sinking at the equator would depress the thermocline. Remember we had shown that if you have an eastward flow you would have water bulging at the equator because of the convergence and that would that can be only compensated by water sinking at the equator and depressing the thermocline. So, the latter process was probably responsible for most of the descent of the thermocline in 57. So, he explains how the warming occurred because sinking took place around the equator instead of upwelling sinking took place because the winds was so weak. So, Bairkness attributed the deepening of the thermocline in the eastern equatorial pacific associated with El Nino with weak easterly winds that is weak phase of the walker circulation also of the southern oscillation. Should be noted the deeper thermocline in the western pacific is caused by the westward winds at the surface of the ocean. Thus the stronger the westward surface winds say due to a strong walker circulation the deeper the thermocline in the west and the shallower the thermocline in the east. Hence the cooling of the east will be enhanced because of the shallow thermocline thus strong phase of southern oscillation is linked to cold events that is La Nina. Now, impact of warm SST anomalies Bairkness pointed out that there were large positive SST anomalies in Galapagos waters and west of that up to Canton Island we talked about Canton Island how it became warm and the rainfall increased enormously. And the SST of the central pacific was about 28 degrees centigrade near Canton Island which we saw as well as Christmas Island during the El Nino 57. So, Bairkness suggested that the equatorial heat source for atmospheric circulation over the eastern equatorial pacific must have been operating at considerably greater than normal frequency in 57 58. So, he is saying because of the warm SSTs the heat source is operating at a much higher frequency and Bairkness was the first to point out the most important facet of ocean atmosphere coupling namely the relationship of precipitation or strength of the Hadley cell or what we call the tropical convergence zone to the SST. Now, this actually Bairkness showed that the great positive water temperature anomaly observed along the equator in central and eastern pacific from November 57 to February 58 was accompanied by an anomalously great heat supply from the equatorial ocean to the ascending branch of the atmospheric Hadley circulation and intensification of that circulation. We saw a manifestation of that in the increase of rainfall over Canton Island. Thus, he elucidated the impact of SST variations on the inter annual variation of the tropical convergence zone which is the basis for seasonal predictions in the tropics. Now, you must remember that while atmosphere changes very fast the ocean changes much slowly much more slowly it is a sluggish fluid and so this is of great importance because the atmosphere is responding very fast to the sea surface temperature as you saw the Canton Island case that Bairkness showed and because the atmosphere responds fast, but the SST is evolving more slowly because it is a feature of the ocean. So, it became possible to think of predictions because of the atmospheric phenomena because of the slowly evolving boundary condition. We will come to this when we talk of seasonal predictions this was this is what Charney pointed out that in the tropics we should be able to have predictions on the seasonal scale because in the tropics a major the most important part of the circulation the convection and precipitation is driven by sea surface temperature that is boundary forcing which is evolving slowly. So, we should be able to generate predictions of rainfall in the tropics this was suggested by Charney and the basis of that is this relationship between SST and rainfall which Bairkness first elucidated. Now, Bairkness also suggested the feedbacks that operate in the coupled atmosphere and tropical ocean system as follows. These are the Bairkness feedbacks of particular dynamic significance are the processes connected with the changes in slope of the pressure profile along the bottom of the walker circulation by bottom of the walker circulation he means the pressure atmospheric pressure at sea level. A change towards a steeper pressure slope at the base of the walker circulation is associated with an increase in the equatorial easterly winds and hence also with an increase in the upwelling and a sharpening of the contrast of surface temperature between the eastern and the western equatorial specific. So, what is he saying? He is saying that if the pressure slope becomes steeper this means that the winds will strengthen and this means strengthening of the winds will give more upwelling this means the temperature gradient will also strengthen, but this temperature gradient is what drove the winds in the first place. So, you have a feedback or a chain reaction which shows that an intensifying walker circulation also provides for an increase of the east to west temperature contrast that is the cause of the walker circulation in the first place. Trends of increase in the walker circulation and corresponding trends in the southern oscillation probably operate in that way this is what he pointed out. On the other hand a case can also be made for a trend of decreasing speed of walker circulation as follows. See we looked at what happens when there is an increase of the slope now if we had a decrease of the equatorial easterlies that would weaken the equatorial upwelling and thereby the eastern equatorial specific becomes warmer and supplies heat also to the atmosphere above it. So, this lessens the east west temperature contrast within the walker circulation and that will have an impact directly on the walker circulation which will slow down. So, here again you have a positive feedback and so there is ample reason to for a never ending succession of alternating trends by air sea interaction in the equatorial belt, but just how the turn about between trends takes place is not yet clear this is from Berkness paper. So, we note that Berkness had realized that the ocean atmosphere interactions could amplify an initial modest weakening of the trades into an El Nino, but he could not explain why those conditions once established did not persist indefinitely why is it that once El Nino is established it does not continue forever. In other words Berkness's explanation of the warm El Nino state and cold La Nina state is as follows. So, I repeat again what are the basic feedbacks that Berkness proposed suppose the east starts to warm for example, because the thermocline is depressed then the east west specific contrast is reduced. So, the pressure gradient and the winds weaken the weaker winds bring weaker upwelling a sinking thermocline and slower transports of water cold water such a positive feedback between the ocean and atmosphere can lead to an El Nino. On the other hand if the east starts to cool the winds will strengthen and tilt the thermocline further and also lead to enhanced upwelling this can lead to a La Nina. Thus the eastern Pacific SST and the pressure gradient the southern oscillation are considered in this framework to be components of a single coupled mode namely El Nino southern oscillation or what we call Enso. Now, it is important to note some caveats it is important to note that ocean atmosphere interactions proposed by Berkness are confined to the neighborhood of the equator at the equator there is no Coriolis force. So, along the equator a westward wind drives the surface water westward and creates an east west SST gradient that reinforces the wind. So, this is only at the equator because there is no Coriolis force an easterly wind will directly drive surface water westward a wind coming from east to west drives the surface water westward only at the equator. See such interactions are impossible far from the equator because the Coriolis force deflects the motion of the ocean from the direction in which the wind is blowing we have seen that we have seen that in the treatment of the Ekman layer. In fact the because of the Coriolis force the transport of the is actually at an angle to the wind. So, it is very important to remember that all these atmospheric ocean interactions actually work only at the equator. Now, Berkness's analysis of the system in the classical papers of 66 and 69 showed clearly the strong link between El Nino and Walker Suther oscillation. Thus El Nino is now considered to be the ocean component and the southern oscillation the atmospheric component of Enso which is the irregular oscillation of a coupled atmosphere ocean system. So, now we talk of this oscillation as an oscillation of the coupled system. We do not talk of it as an oscillation only of the ocean component or the atmospheric component because the two are intimately linked the two oscillations are intimately linked. So, El Nino and La Nina are associated with different amplitudes of the southern oscillation and have distinct characteristics in terms of the ocean structure, the atmospheric circulation, convection etc. Now, the coherent variation of the atmosphere and the ocean is clearly seen in the variation of SSTs along the equatorially specific the sea level pressure at Darwin and high rainfall over central Pacific. So, this is now you know found even in popular literature because El Nino has indeed become a very popular topic and what we see here is the SST along the equator in eastern Pacific and you can see that the SST along the equator has becomes high and low and coherent variations with of the SST along the equator as the SST varies along the equator you see the sea level pressure at Darwin is varying coherently. You see here peaks coincide with peaks and troughs coincide with troughs and rainfall at Christmas Island in central Pacific is again showing very very similar things. So, as the ocean is warming these are El Nino conditions here at the same time we see signal in the sea level pressure at Darwin which has become higher and central Pacific there is very clear large amount of rainfall occurring like we saw in Bergenes case of Canton Island, but this time it is shown for Christmas Island. So, just to show you the location Darwin is here Christmas Island is here and we are looking at SST here. So, what we see here is how coherent the variations are of SST along the equator in the eastern Pacific. So, SST is along the equator in the eastern Pacific here then the rain at Christmas Island and the pressure right across the Pacific at Darwin and what you see is the coherent variations here. The tight linkage between the southern oscillation in the atmosphere and fluctuations between El Nino and La Nina in the ocean is clearly seen in the close correspondence in the variation of the anomalies of Nino 3 SST I will define the regions later, but Nino 3 is equatorial Pacific going right up to the east Pacific and Darwin sea level pressure. So, the southern oscillation here we have. So, this is the SST anomaly of the east Pacific and this is the Darwin sea level pressure and this is for many many years and you can see even by eye how closely the two are being the two are oscillating. So, it is a very very tightly linked system anomalies of SST anomalies in the Nino 3 area measured in degrees centigrade and again you see the same thing and sea level pressure at Darwin and the former is a good measure of El Nino and the two are obviously interrelated. So, what we have now learnt is that what began as a what was thought as a very exceptional event that occurs along the coast of South America and has major implications for Ecuador and Peru namely the El Nino and its opposite phase the La Nina or the cold phase actually happened to be first of all it is not at all restricted to the coast of South America. It is much more a global phenomena a phenomena that extends right across the Pacific with the SST anomalies extending over eastern equatorial Indian Pacific ocean as well. So, the first recognition was that it is a much larger scale phenomena then being restricted to the Pacific, but still the El Nino and La Nina were identified by sea surface temperature anomalies, but now then this side by side with this there was a recognition that the atmosphere actually there is a southern oscillation with pressure variations at Darwin or on the towards West Pacific being negatively correlated with pressure variations over East Pacific that is to say there is a sea saw when the pressure tends to be high over one region it tends to be low over another over the other region. So, there was a sea saw between East West and we are talking now atmospheric pressure which meant that there is a variation in the easterlies across the Pacific the trade the easterly component of north north east and south east trade. So, that would vary along the equator and that would vary from time to time and this was called the southern oscillation and what Birkenes did was to elicited very nicely the link between the warming and cooling of the coast of Peru which also went coherently with warming and cooling on the equatorial East Pacific link that with the southern oscillation as well as very importantly the rainfall or the Hadley cell. So, strengthening of the Hadley cell during El Nino is what he showed. So, what has now been established very clearly is that El Nino La Nina oscillation seen in the ocean and southern oscillation for seen in the atmosphere are all components of one interactive system the El Nino southern oscillation coupled atmosphere ocean system. And these are manifestation of oscillation of this coupled system and now in fact this is how the thing is treated as a coupled system and so the phenomena is a manifestation of the coupled system and to understand the physics we have to look at the coupled system and the coupling plays a very very important role. So, we will continue in the next lecture with how what was the progress in our understanding beyond Birkenes very pioneering contribution to this field. Thank you.