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December 2000

The PaleoCSM produces a realistic El Niño

CGD's paleoclimate crew: Bette Otto-Bliesner, Caspar Ammann, Esther Brady, and Christine Shields. (Photo by Carlye Calvin.)

In a landmark achievement for paleoclimatic modeling, the Climate and Global Dynamic Division's paleoclimate group, led by Bette Otto- Bliesner, has created a plausible depiction (see graphic below) of the El Niño/Southern Oscillation (ENSO).

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 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 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 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 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:

  • Lowering the vertical diffusivity. This parameter controls how quickly heat is transferred upward or downward inside the modeled ocean. The CGD intercomparison studied how this variable affects ENSO in different versions of the CSM and the Parallel Climate Model, or PCM. The results are now in press at Climate Dynamics. The PCM (now being merged with the CSM—see sidebar) is one of several models that have had some success in replicating certain aspects of ENSO.

    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.

  • Varying the horizontal viscosity. This allows smaller, more realistic values away from the western boundaries of the oceans, where an unrealistically high value is needed for numerical reasons. With the new scheme, developed by Bill Large and coworkers, the resulting equatorial undercurrent in the Pacific Ocean is about an order of magnitude stronger. "It intensifies and narrows the undercurrent, much more in keeping with observations," says Bette. She's not sure how much of an impact this has had on the depiction of ENSO itself.

    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."

    • Bob Henson

    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.)

    CSM, PCM, CCSM: Who's who and what's what

    The recent appearance of the acronym CCSM (Community Climate System Model) has engendered some confusion among those more familiar with the two NCAR global coupled models, the CSM (Climate System Model) and the PCM (Parallel Climate Model). According to CGD director Maurice Blackmon, here's what the various names refer to:

    Climate System Model
    CSM-1.0: 300-year present-day run
    CSM-1.3: 1870 control run, 20th -- and 21st-century simulations

    Parallel Climate Model
    PCM-1 for present-day control run, 1%-per-year increasing-CO2 run, and 20th -- and 21st-century simulations

    Community Climate System Model
    This is the project combining the CSM and PCM. The merged model is still being formulated, so the acronym CCSM should only be used for model versions to come.

    PCM/CSM Transition Model (PTCM)
    The Climate Change Research Group is using this to evaluate various aspects of the two models prior to their merger. The code will not be released on the Web.

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    Edited by Bob Henson, bhenson@ucar.edu
    Prepared for the Web by Jacque Marshall
    Last revised: Wed Dec 13 17:30:40 MST 2000