This feat was accomplished using the PaleoCSM, a version designed for the study of past climates. The problem may seem outside of the paleoclimate group's purview, but group head Bette Otto-Bliesner says, "The one criticism we'd gotten at meetings was, You're really not getting [ENSO] right in your present-day simulations, so how can you possibly know if you're getting it right for past climates?" In their quest to get ENSO right, Otto-Bliesner's group made several important tweaks to the PaleoCSM, following up on the results of a divisional experiment and input from CGD ocean modeler William Large. They ran the new version for a 150-year period under preindustrial conditions.
The output shows a series of El Niñolike warmings in the central and eastern tropical Pacific occurring about every two to four years. This is very close to the frequency of actual El Niños during much of the 20th century, notes Otto-Bliesner, although their frequency has increased in recent decades.
Sea-surface temperatures from the eastern tropical Pacific, which modelers call the Niño-3 region, show that the El Niños in the model tend to be moderately strong: comparable to the 198687 event and about half as strong as the 198283 or 199798 events. The modeled El Niños tend to unfold in a sequence very much like that of their real-world counterparts, peaking within a month or so of January.
Especially encouraging, says Otto-Bliesner, is that ENSO is well portrayed not only at the ocean surface but in three dimensions. Cold underwater anomalies pushed down by El Niño move west across the Pacific, then bounce back eastward to help cause the event's demise a few months later. The new findings are consistent with two of the leading theories behind El Niño formation, the "delayed oscillator" concept and the "buildup" hypothesis.
Along with increasing the north-south resolution across the Pacific, Otto-Bliesner and oceanographer Esther Brady (CGD) made two other key changes to the PaleoCSM:
Otto-Bliesner's group is now going to try to simulate ENSO during the last glacial maximum and the period 80 million years ago. "The CSM is doing a good job now," she says, "not just in the Niño-3 [region] but in the whole geographic pattern of ENSO."
|This figure shows the realistic simulation of the El Niño/Southern Oscillation by the PaleoCSM. Top figure shows December-January-February anomalies of precipitation (millimeters/day), sea-level pressure (millibars) and surface wind, sea-surface temperature (°C), and equatorial ocean temperature (°C) for a composite of 13 El Niño events in the CSM. (Illustration courtesy Bette Otto- Bliesner.)|
Edited by Carol Rasmussen,
Prepared for the Web by Jacque Marshall
Last revised: Wed Feb 7 15:34:33 MST 2001