How well does a 1/4° global circulation model simulate large‐scale oceanic observations?

Numerical high‐resolution ocean general circulation models have experienced a revolutionary development during the last decade. Today they are run globally in realistic configuration with realistic surface boundary forcing. To fully use the results of those models in understanding various aspects of the ocean general circulation and to combine ocean observations with models (state estimation) in a manner consistent with the data and model dynamics, stringent model‐data comparisons are a necessary first step. In this paper a quantitative model‐data comparison is carried out for the global Parallel Ocean Climate Model (POCM), known also as the Semtner and Chervin model, with nominal lateral resolution of 1/4°. The focus is on various aspects of the simulated large‐scale circulation and their relation to the TOPEX/POSEIDON sea surface height (SSH) observations and World Ocean Circulation Experiment (WOCE) hydrography. Comparisons are made for (1) the global mean sea surface circulation and absolute slopes, (2) rms SSH variability and eddy kinetic energy, (3) the simulation of the observed seasonal cycle in SSH, (4) two‐dimensional frequency‐wavenumber spectra of the large‐scale fluctuations, as well as (5) the hydrography for WOCE sections. Recent improvements in external surface forcing fields including daily wind‐stress fields and sea surface heat fluxes lead to a significant improvement in the overall agreement of the simulated and observed large‐scale mean circulation and its variability. However, simulated amplitudes of variability remain low by about a factor of 2 to 4 over a broad spectral range, including the long wavelengths and periods. Both the causes and consequences of this low variability remain obscure.