Tim Barnett, David Pierce, and Niklas Schneider, Scripps Institution of Oceanography
Lai-Yung (Ruby) Leung, Pacific Northwest National Laboratory

Sea level pressure (SLP) anomalies, in millibars, from the Parallel Coupled Model (PCM), for four cases: El Niño with a high North Pacific Oscillation (NPO), El Niño with a low NPO, La Niña/high NPO, and La Niña/low NPO. Values in the boxes show the difference between the SLP seen in the high NPO state and the low NPO state. Red boxes indicate a statistically significant difference at the 95% level. It can be seen that during El Niño, the state of the NPO is associated with significant differences in SLP over the Aleutian low region and the southeast U.S. During La Niña, there is a significant difference over the Aleutian low region. These model responses are similar to observations, and allow us to examine the reason for this link between El Niño and the NPO in the model.

Research Objectives
Our objectives are to examine the physical causes and potential predictability of two aspects of low-frequency climate variability: natural, unforced variability in the North Pacific/American sector, and forced variability in this region associated with changing levels of anthropogenic gases in the atmosphere.

Computational Approach
We use the Parallel Coupled Model (PCM), a sophisticated coupled general circulation model that includes the ocean, atmosphere, land surface, and sea ice. The individual ocean and atmosphere components from this model are also used separately. The resolution of the atmospheric component of the models is T42, while that of the ocean components varies from about 0.5º to 1º, depending on the latitude and longitude.

Accomplishments
The work of Gershunov and Barnett (1998) showed the possible role of North Pacific Ocean sea surface temperatures (SSTs) in modulating the effects of the El Niño/Southern Oscillation (ENSO) over North America. Using forced model runs with tropical SST patterns corresponding to El Niño or La Niña joined to extratropical SST patterns of either the positive or negative decadal phase, we tested the predictive ability of North Pacific SSTs. We found that the modulation effect is arising from internal atmospheric variability, and is not forced by North Pacific SSTs.

Our evaluation of climate response in the Pacific/North American sector to anthropogenic forcing (CO₂ and sulfates) is ongoing under the auspices of the DOE's Accelerated Climate Prediction Initiative (ACPI) pilot program.

Significance
Natural variability on the decadal time scale presents one of the biggest complications in the detection of anthropogenic climate signals, so determining the levels of natural decadal variability, and being able to understand the physical processes responsible for this natural noise, are a clear requirement of any attempt to make an early detection of human impact on climate. Thus our research of decadal climate research has a direct contribution to climate change research at time scales beyond a decade. Also, examining the large-scale, hemispheric links between the monsoon, ENSO, and North Pacific might increase the forecastability of any of this climate variability.

In the broader scientific sense, this exploration attempts to determine how forced changes in a complicated, nonlinear system (the climate) relate to intrinsic patterns of unforced variability in that same system. It is possible that all the forced changes lie outside of the realm of natural variability, or that all the forced changes are associated with changes in the probability density function of the natural climate modes. The real climate is likely somewhere between these two endpoints, and our investigations will help determine where. The answer will have payoff for prediction of future forced climate variability as well as for anthropogenic change detection strategies.

Publications
T. P. Barnett, D. W. Pierce, and R. Schnur, "Detection of anthropogenic climate change in the world's oceans," Science 292, 270 (2001).

D. W. Pierce, T. P. Barnett, N. Schneider, R. Saravanan, D. Dommenget, and M. Latif, "The role of ocean dynamics in producing decadal climate variability in the North Pacific," Climate Dynamics (in press).

N. Schneider and A. J. Miller, "Predicting western North Pacific ocean climate," J. Climate (in press).

http://cirrus.ucsd.edu/~pierce/acpi_pilot/index.html
http://www.pnl.gov/atmos_sciences/Lrl/ACPI.html