ENSO dynamics and seasonal cycle in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general circulation model |
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Authors: | A Bacher J M Oberhuber E Roeckner |
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Institution: | Max-Planck-Institut für Meteorologie, Bundesstra?e 55, D-20146 Hamburg, Germany E-mail: bacher@dkrz.de, DE Deutsches Klimarechenzentrum GmbH, Bundesstra?e 55, D-20146 Hamburg, Germany, DE
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Abstract: | 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 midbasin, 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 observations, SST, windstress, and upper ocean heat content each exhibit a
single dominant large-scale amplitude and phase pattern, suggesting that the model captures the essential dynamics. The amplitude
of the essentially standing oscillation in SST in the NINO3 region attains the observed strength, but is weaker at the eastern
boundary. Anomalies of upper ocean heat content show off-equatorial westward and equatorial eastward propagation, the latter’s
arrival in the east of the basin coinciding with the SST anomalies. Equatorial wind stress anomalies near the date line provide
the appropriate forcing and clearly form a response to the anomalous SST.
Received: 14 June 1996 / Accepted: 11 November 1997 |
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