HOW WELL DOES A 1 4-DEGREES GLOBAL CIRCULATION MODEL SIMULATE LARGE-SCALE OCEANIC OBSERVATIONS/

Numerical high-resolution ocean general circulation models have experi enced a revolutionary development during the last decade. Today they a re run globally in realistic configuration with realistic surface boun dary forcing. To fully use the results of those models in understandin g various aspects of the ocean general circulation and to combine ocea n observations with models (state estimation) in a manner consistent w ith the data and model dynamics, stringent model-data comparisons are a necessary first step. In this paper a quantitative model-data compar ison is carried out for the global Parallel Ocean Climate Model (POCM) , known also as the Semtner and Chervin model, with nominal lateral re solution of 1/4 degrees. The focus is on various aspects of the simula ted large-scale circulation and their relation to the TOPEX/POSEIDON s ea surface height (SSH) observations and World Ocean Circulation Exper iment (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 c ycle in SSH, (4) two-dimensional frequency-wavenumber spectra of the l arge-scale fluctuations, as well as (5) the hydrography for WOCE secti ons. Recent improvements in external surface forcing fields including daily wind-stress fields and sea surface heat fluxes lead to a signifi cant improvement in the overall agreement of the simulated and observe d 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. B oth the causes and consequences of this low variability remain obscure .