Pacific Interannual to Interdecadal Variability
in a 1000 Year Simulation of the CSIRO
Coupled General Circulation Model
Daniel J. Vimont
2000
A thesis submitted in partial fulfillment
of the requirements for the degree of
Master of Science
Atmospheric Sciences
To determine the physical mechanisms responsible for modeled equatorial variability on interannual and decadal time scales, the heat budget of the modeled variability is examined. The heat budget analysis shows a positive feedback between tropical zonal wind stress and equatorial SST anomalies on interannual and decadal time scales. On interannual time scales, a delayed negative feedback is provided by westward propagating signals (caused by the same tropical wind stress anomalies) that reflect off the western boundary and counter the local positive feedback. The delayed negative feedback is not evident on decadal time scales. Differences between interannual and decadal tropical wind stress curl patterns are found to be consistent with the lack of a delayed negative feedback on decadal time scales.
Two proposed mechanisms that link mid-latitude and tropical variability are investigated. The first, the shallow thermohaline circulation mechanism, is not found in the model simulation. The second, the effect of internal mid-latitude atmospheric variability, is examined with the help of a mixed layer model. A mechanism is proposed in which tropical wind stress (or wind stress curl) anomalies associated with internal mid-latitude atmospheric variability force equatorial SST anomalies through equatorial wave propagation. Coupled dynamics amplify this signal, and create a hemispherically symmetric, ENSO-like signal. This mechanism appears to account for around 40% of the modeled decadal variability.