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Winter 2000

The PaleoCSM produces a realistic El Niño

by Bob Henson

Some scientists have considered it impossible. However, the paleoclimate group in NCAR's Climate and Global Dynamics Division (CGD) says they've done it: They've created a plausible depiction of the El Niño/Southern Oscillation (ENSO) within a coupled global model—the NCAR Climate System Model, or CSM.

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 Niño–like 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 1986–87 event and about half as strong as the 1982–83 or 1997–98 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:

  • A lower parameter of vertical diffusivity. This controls how quickly heat is transferred upward or downward inside the modeled ocean. An in-house group led by Gerald Meehl (CGD) recently studied how this variable affects ENSO in different versions of the CSM and NCAR's Parallel Climate Model, one of several models that have had some success in replicating certain aspects of ENSO. According to Meehl, "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." Otto-Bliesner and colleagues went on to show this relationship held not only across different models but within the same model.

  • A varying value of horizontal viscosity. The sides of narrow ocean currents, such as the undercurrent straddling the equator just below the Pacific's surface, can be difficult to model. According to Otto- Bliesner, it's now thought that "the ocean is actually more viscous across tight gradients than along the flow." The new scheme allows for this, and the resulting currents are about an order of magnitude stronger. "It intensifies and narrows them, much more in keeping with observations," says Otto-Bliesner. She's not sure how much of an impact this has had on the modeling of ENSO itself.

    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.)

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