A Meteorologist Looks at the Sea

Originally from Norway, Jacob Bjerknes had been studying the atmosphere for decades. During World War I he had worked with his father, Vilhelm Bjerknes, a pioneering meteorologist who coined the term "fronts" to describe the boundaries in the atmosphere where masses of warm and cold air meet and often spawn storms. The elder Bjerknes recognized that weather forecasting would require not only global data on atmospheric conditions but also much better knowledge of "the laws according to which one state of the atmosphere develops from another." Decades later in America his son would make an important contribution to that knowledge.

Central to Jacob Bjerknes's insight was his recognition that the interaction between the sea and air could have a major impact on the circulation of winds, rain, and weather. Bjerknes described a Pacific-wide air circulation pattern, which he called the Walker Circulation. This pattern of airflow, Bjerknes realized, hinged on the difference in sea surface temperatures in the western and eastern Pacific--a difference that creates differences in surface air pressure between the two regions.

Air above the cold waters of the eastern Pacific is too dense to rise high enough for water vapor to condense to form clouds and raindrops, leaving portions of Peru and Ecuador a desert. This desert effectively begins far offshore, where the cool dense air also creates a region of high air pressure. High pressure in the east and low pressure over warmer waters in the west (a large pressure difference in Gilbert Walker's scheme) moves air westward, generating and reinforcing the steady equatorial trade winds. The winds harvest moisture from the ocean as they blow toward the western Pacific; there the warm moist air rises, condenses, and then dumps heavy monsoon rains that nourish the jungles of New Guinea and Indonesia.

Jacob Bjerknes. (Photo courtesy of E. M. Rasmussen, University of Maryland)

Bjerknes recognized that during El Niño conditions, when the waters off northern Peru are warmer than normal and surface air pressure is lower as a consequence, the pressure difference between east and west weakens and so do the westward trade winds. As the winds falter, warm moist air rises over the central Pacific instead of farther west, effectively stealing the monsoon rains from India and Indonesia and spawning rainstorms that strike the west coasts of North and South America.

To determine whether Bjerknes's ideas had predictive power, atmospheric researchers now turned to computers. In the early 1950s mathematician John von Neumann, a key figure in the invention of the digital computer, led a group of scientists at the Institute for Advanced Study in Princeton, New Jersey, in some of the first efforts to use computer models to explore weather prediction. By the 1970s researchers were using computers to construct atmospheric general circulation models (AGCMs) to simulate the response of the atmosphere to a fixed sea surface temperature in the tropical Pacific. AGCMs divide an imaginary atmosphere into horizontal layers, subdivided into thousands of squares. Data on such variables as temperature, pressure, humidity, and wind are fed into a series of equations that produce new readings and outcomes for each of the grid points. The test is to see whether a model can reproduce observed real-world behavior given the same starting point, such as a certain sea surface temperature.

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