Tuesday 23 August 2005

P1
1000-1230 hours

157
Annual mean dynamics and nonlinear feedbacks in the central and eastern equatorial Pacific
Brown, Jaclyn¹, Godfrey, J. Stuart¹
1 CSIRO Marine Research, Hobart, Tasmania
Author email: jaci.brown@csiro.au
Understanding ENSO requires a solid knowledge of the dynamics and flow patterns in the equatorial Pacific. The linear features and strong zonal currents have been well documented and researched. Nonlinearity however has often been thought a second order correction to a flow dominated by windstress and pressure gradient forcings. Kessler et. al. (2003) developed an extended Sverdrup equation which highlighted the importance of nonlinearity in this region. We investigate this further using an eddy-permitting GCM for the annual mean and seasonal cycle, to which ENSO is phase locked. Our calculations are conducted on a full Lagrangian density layer framework, which is essential for physical understanding in this region (Hazeleger et. al. 2001) We show that nonlinearity is as important as windstress in forcing meridional flows and hence the strength of the zonal currents. We also show that in this particular model, our scale selective horizontal friction is insignificant for momentum dissipation (though it may be important for vorticity dissipation). The strongest nonlinear terms appear to be derived from flows on the time scales of Tropical Instability Waves. We hypothesize that the Tropical Instability Waves that are permitted in the model create their own friction to damp the system, unlike in previous, coarser studies. We also investigate the origin of the nonlinear terms, particularly in time, and attempt to explain the nonlinear feedbacks in this complex system.

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