A talk delivered as a tribute to J. Bjerknes on the 100th anniversary of his birth
October 25, 1997 by
S. V. Venkateswaran
1. THE TWO METHODOLOGICAL REVOLUTIONS OF THIS CENTURY
Jacob Bjerknes was a master builder of modern meteorology. At a very young age he became a member of the Bergen School of Meteorology founded and headed by his father, Vilhelm Bjerknes. Among other members of this school were several famous names in Meteorology: Bergeron, Rossby, Holmboe, Godske, Hoiland and Solberg, to name a few. Jacob Bjerknes knew them all. Seniors like Bergeron affectionately referred to him as "young Bjerknes." Bjerknes nourished these early Bergen contacts throughout his life. Several members of the Bergen school used to visit him at UCLA, giving the opportunity to his students to listen and learn meteorology from these giants.
Acting in concert and with almost missionary zeal, the Bergen school of meteorologists brought to fruition the grand dream of their mentor, Vilhelm Bjerknes. That dream was no less than to transform and develop meteorology as a scientific discipline in its own right, with strong ties to the already well-established sciences of fluid dynamics and thermodynamics. Thus was the subject of Physical Hydrodynamics, later renamed as Geophysical Fluid Dynamics, born. The Bergen school published an influential treatise under the former title.
I shall refer to this tremendous achievement of the Bergen school between the First and Second World Wars as the First Meteorological Revolution. The Second Meteorological Revolution was triggered by the advent of the digital computer in the 1940's and the happy decision by John von Neumann that the goal of weather prediction should have the highest priority in the use of his computer. We are riding on the high tide of the Second Revolution today. But let us not lose sight of the fact that the Second Revolution could not have happened without the first.
Jacob Bjerknes was, as I said, a leading participant of the First Revolution. He was a bystander and even a mild skeptic of the Second Revolution. One is tempted to ask why. I shall venture a guess. Bjerknes was deterred to an excessive degree by the failure of the great British meteorologist, Lewis F. Richardson in his heroic experiment on numerical weather prediction; also he did not correctly judge the capabilities of the computer or the cumulative talent of the galaxy of meteorologists and applied mathematicians who were challenged to make the von Neumann experiment a success.
There is yet another and perhaps more plausible explanation: Jacob Bjerknes, as was his habit, was far ahead of the game. After completing his studies on the general circulation of the atmosphere, he had turned his attention to the problem of climate and climate change which he correctly predicted was the wave of the future. He recognized at an early stage that climate variability on decadal time-scales were internally generated by the coupled ocean-atmosphere system. He probably concluded that the numerical simulation of this coupled system would encounter even more serious difficulties than the weather prediction problem, particularly because of the poorly defined state of the oceans at that time.
Whatever the true explanation is, the decision of Bjerknes to diverge from the popular path and follow his own trail enriched meteorology. Like Newton he "voyaged through strange seas of thought alone," to strike upon the El Nino problem, and place it at the center stage of attention of= both oceanographers and meteorologists. Three Questions on El Nino I shall now attempt to answer the following questions regarding El Nino.
1) What is El Nino?
2) What is the connection between J. Bjerknes and El Nino?
3) How is El Nino related to accompanying weather anomalies in other parts of the world?
Questions 1) and 3) are asked with curiosity and concern by the general public. Obviously, question 2 is relevant to this audience. Let me take up the first question:
What is El Nino?
The connotation of El Nino has evolved with time. Let me explain. The ocean current off the coast of Peru is normally a cold current. The coldness of the current comes about from coastal upwelling which brings cold water from below to the surface of the ocean. The upwelling water is rich in nutrients needed for the sustenance of rich marine life that the region supports.
Around December of each year a warm counter-current (a current close to the equator running from the Western to the Eastern Pacific) encroaches the Peruvian coast and over-rides the cool coastal current there. The name El Nino originally refers to this warm counter-current which brings with it beneficial rain for the arid coastal regions of Peru but also serious destruction of marine life through the suppression of coastal upwelling.
During some years, the counter-current and its effects are stronger than normal. The term El Nino is sometimes restricted to such abnormal years. It is now recognized that the El Nino phenomenon is not peculiar to the Peruvian coast; instead it is the large-scale oceanic response of the equatorial Pacific to the trade wind perturbation. The term El Nino is again used for this large-scale response.
Finally, it is known that the cycle of El Nino variations over the oceans is coupled to the cycle of atmospheric variations known as the Southern Oscillation. The acronym ENSO has come increasingly into use for this combined phenomenon.
1) Bjerknes and El Nino
Now I shall quickly summarize the insights provided by Bjerknes on the El Nino problem.
Bjerknes clearly recognized that El Nino is a manifestation of a large-scale air-sea interaction problem. Trade wind anomalies perturb the sea surface and produce anomalies in the sea surface temperatures (SST) and surface ocean currents; and the oceanic perturbations, in turn, produce trade wind anomalies.
Think of two coupled oscillators, one representing the ocean and the other the atmosphere. The oscillating characteristics of the two oscillators are different. When you couple them, the resulting oscillation is the ENSO oscillation which drives both the El Nino oscillation of the oceans and= the so-called Southern Oscillation of the atmosphere. The Southern Oscillation was discovered by Sir Gilbert Walker during his investigations of the variability of the Indian monsoon.
2) Telecommunications (Teleconnections?)
Bjerknes knew that El Nino had world-wide consequences for the weather. For example, the occurrence of El Nino is correlated with failure of the Indian monsoon, the occurrence of drought conditions in W. Australia and flooding conditions in S. West United States. The mechanism by which these global anomalies communicate with one another was what Bjerknes called "teleconnections." He sketched the details, for example, of the teleconnection between El Nino occurrence and California rainfall. This involved the strengthening of the tropical Hadley circulation and the shifting of the sub-tropical jetstream to lower latitudes over the United States.
Let me say a few words about what Bjerknes did not set out to do in his El Nino studies. He confined himself to studying the extreme phases of the ENSO oscillation cycle. He did not study the more difficult time-dependent problem which is crucial for forecasting the strength and duration of El Nino. The forecasting problem is a problem in non-linear dynamics to be solved in its full generality by numerical methods. Professors Ghil and Neelin at UCLA are, I believe, making progress on this important subject.
3) El Nino and California Rainfall
Now we come to the connection between El Nino and California rainfall= which is the third question we proposed to answer.
Los Angeles rainfall records reveal that over a period of 55 year, the average annual rainfall is about 17.6 inches. As compared to this, the rainfall during three major El Nino years were as follows:
El Nino Year Rainfall (inches)
There is no simple correlation between the intensity of El Nino and the amount of rainfall over Los Angeles. However, if El Nino for this year develops as strongly as predicted, we might expect a total rainfall amount between 40 and 50 inches over the Los Angeles area. That would be my prediction. Notice that the predicted amount is 2 to 3 times the normal amount. You have been warned! Arm yourself with sandbags and umbrellas and buy yourself a boat!
In conclusion, I wish to make a few remarks about Bjerknes as a scientist. Bjerknes was fascinated by the drama of weather. He watched its moods, its ups and downs, its exits and entrances, and most of all, its unfailing ability to surprise. Combining his unique insight with simple scientific principles, he developed his skills for predicting the weather to an exceptional degree. As a weather forecaster, his preeminent status was established at a very early stage of his career. Moreover, his intimate knowledge of weather gave him an enviable extra edge in his research.
Bjerknes chose his topics of research with supreme insight. Each subject that he studies and each paper that he wrote went straight to the core of a meteorological problem. Added to that, his papers, when studied in sequence, show an overarching unity of purpose and design. He appears to the reader as a polar explorer whose destination is pre-set and whose sense of direction is unerring.
Bjerknes's style of research is best exemplified in his work on the El Nino. He could marshal a mountain of atmospheric or oceanic data and see patterns in them and hidden order whereas other saw mostly randomness. He described the patterns that he saw with sufficient and simple theoretical underpinning. He seldom posed a problem or sought its solution in purely mathematical terms; that he left to his capable mathematical colleagues. But, again and again, he was successful in setting up the problem that others found fruitful to pursue.
Was Bjerknes a great scientist or was he a genius? He was both of these, of course, and more: he was a magician. It has been said of the great Newton that he was the last magician before the Renaissance. Was Jacob Bjerknes, the last magician of meteorological renaissance in this century?