The next pair of slides illustrates that we are learning a great deal about El Nino in terms of coupled systems. In this case it is the coupling between the atmosphere and the oceans. Now this is from a recent review paper. Shown on the right (fig.24) are two simple representations of the ocean and the atmosphere. That for the ocean, shown in red, is called SST, which stands for the average sea surface temperature in the eastern equatorial Pacific. And its anomaly (the mean variation has been removed) is shown versus time from 1950 to the present. In blue is plotted what is called the Southern Oscillation Index, which is a measure of the pressure gradient at the surface along the Equator in the Pacific. It's actually the difference in pressure between two land stations near sea level in the Pacific. And so it's a measure of the tendency for winds to occur along the Equator - the stronger the pressure difference, the higher the winds are likely to be. And what you see is a strong anti-correlation between those two signals. It's really quite remarkable.
What is plotted down here in the inset is the power in these two signals versus frequency in inverse years. So here's the power at one year, here's the power at two years, and the peak is at about four years. It is this coupled system between sea surface temperatures and winds represented by the Southern Oscillation Index that we sometimes call El Nino.
And an El Nino event like the one that occurred in 1997-98 begins with a strong warming of the eastern Pacific. The events in 1997-98 are shown in fig.25, in sequence from January '97, April '97, September '97, and January '98, on the basis of a recently deployed set of oceanographic buoys in the equatorial Pacific that have measured both the sea surface temperature, which you can also measure from Space, as the sub-surface temperature as well. You can see the development of the El Nino, and it can be correlated (although those data are not shown here) with the variation in the mean wind speeds along the Equator in this part of the world.
This is simply one example, a famous one because it affects the weather at one particular time scale - something like four years of how complicated is the ocean-atmosphere system. And of course the climate on our planet depends on both this system and many many other couplings at a variety of scales, and understanding which are the important mechanisms and which are the important forcing functions for our climate is a very important research area that will depend in part on our ability to unravel past events in the history of the Earth's climate.
I want to move now to the third part of the talk, and that is to illustrate, by example, three questions that are at this research frontier that I mentioned - namely the research frontier constituted by the part of the Earth beneath the ocean. And those questions that I have picked out simply because of my interests and the interests of my institution are these three (fig.26)：
* the formation and evolution of oceanic crust
* the dynamics of plate boundaries, and
* the nature of the sub-surface biosphere.
I will not talk at great length about any of these, but I simply want to use them to illustrate some of the outstanding questions. I do not want to imply that there are not other, equally important questions that are also at the research frontier that we will be talking about today. Other speakers will talk about these questions and some of the others that I do not have time to talk about.
The formation of oceanic crust - we know something about this topic, but there's much we don't know. We know that oceanic crust is formed at mid-ocean ridges. You see (fig.27) a map of a piece of the north-east Pacific on the left that includes the Juan de Fuca Ridge, and here (fig.28) are a couple of images of the sea floor at the ridge axis where you see a hydrothermal vent - what's called Black Smoker - very high temperature, nearly 400 degrees Centigrade, fluids exiting into the cold oceanic column and precipitating sulphides and other material. These vents are striking sources of energy that have been utilized by a diverse ecosystem of deep-sea animals of various sorts. Here an image also from the Juan de Fuca system. We know that this ridge setting is where the oceanic crust is formed.
