ENSO dynamics and seasonal cycle in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general circulation model

• Published in: Climate Dynamics
• Main Authors: Bacher, A., Oberhuber, J., Roeckner, E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1998
• Subjects: Pacific; Sea ice
• Online Access: http://hdl.handle.net/21.11116/0000-0003-2A35-3; http://hdl.handle.net/21.11116/0000-0003-2A37-1; http://hdl.handle.net/21.11116/0000-0003-2A38-0

The new version of the atmospheric general circulation model (AGCM), ECHAM4, at the Max Planck Institute for Meteorology, Hamburg, has been coupled to the OPYC3 isopycnic global ocean general circulation and sea ice model in a multi-century present-day climate simulation. Non-seasonal constant flux adjustment for heat and freshwater was employed to ensure a long-term annual mean state close to present-day climatology. This study examines the simulated upper ocean seasonal cycle and interannual variability in the tropical Pacific for the first 100 years. The coupled model's seasonal cycle of tropical Pacific SSTs is satisfactory with respect to both the warm pool variation and the Central and Eastern Pacific, with significant errors only in the cold tongue around April. The cold phase cold tongue extent and strength is as observed, and for this the heat flux adjustment does not play a decisive role. A well-established South Pacific convergence zone is characteristic for the new AGCM version. Apart from extending the southeast trades seasonal maximum to mid-basin, wind stress pattern and strength are captured. Overall the subsurface structure is consistent with the observed, with a pronounced thermocline at about 150 m depth in the west and rising to the surface from 160°W to 100°W. The current system is better resolved than in some previous global models and, on the whole, has the expected shape. The equatorial undercurrent is correctly positioned but the core is only half as strong as observed. The north equatorial current and counter-current also have reduced maximum speeds but the April minimum is captured. As with the companion publication from Roeckner et al. this study finds pronounced tropical Eastern and Central Pacific interannual variability. Simulated and observed NINO3 sea surface temperature (SST) variability is represented by a single, rather broadband, maximum of power spectral density, centered on about 28 months for the simulation and four years for the observations. For simulation and ...