|CGD's paleoclimate crew: Bette Otto-Bliesner, Caspar Ammann, Esther Brady, and Christine Shields. (Photo by Carlye Calvin.)|
This feat was accomplished using the PaleoCSM, a version of the Climate System Model (CSM). The recent success of Bette's group builds on the results of an intercomparison of ENSO in several versions of global coupled models within CGD. The intercomparison was organized by Jerry Meehl and included Peter Gent, Julie Arblaster, Bette, Esther Brady, and Tony Craig, with input from ocean modeler Bill Large. Bette and her group made several important changes to the PaleoCSM and then 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 Bette, although their average spacing has increased somewhat in recent decades.
Sea-surface temperatures from the Niño-3 region show that the El Niños in the model tend to be moderately strongcomparable 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 their real-world counterparts, peaking within a month or so of January.
Especially encouraging, says Bette, 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, Bette and Esther made two other key changes to the PaleoCSM:
According to Jerry, "The lower the diffusivity is, the sharper you can keep the thermocline [the boundary between the ocean's well-mixed surface layer and the cooler water below]. Otherwise it tends to diffuse away and become fuzzy." Bette and colleagues went on to show this relationship held not only across different models but within the same model. They expect that the lower diffusivities will be taken into account in future CSM runs at NCAR and elsewhere, and perhaps in other models as well.
Bette'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 statistic but in the whole geographic pattern of ENSO."
|This figure shows a realistic simulation of the El Niño/Southern Oscillation by the PaleoCSM (Climate System Model). 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. Positive anomalies are shaded dark gray and negative anomalies light gray. The bottom figure shows the time series of sea-surface temperature in the Niño-3 region, one of several areas in the Pacific Ocean used to diagnose ENSO. (Illustration courtesy Bette Otto-Bliesner.)|