PREDICTABILITY EXPERIMENTS IN THE NORTH-ATLANTIC OCEAN - OUTCOME OF AQUASI-GEOSTROPHIC MODEL WITH ASSIMILATION OF TOPEX POSEIDON ALTIMETERDATA/

In a recent paper, Blayo et al, [1994] described the simulation of the North Atlantic general circulation during the period October 1992 to September 1993, as obtained from the assimilation of TOPEX/POSEIDON (T IP) altimeter data in a high-resolution, quasigeostrophic model of the Atlantic basin between 20 degrees N and 60 degrees. in line with this paper, the purpose of the present study is to investigate the usefuln ess of the quasi-geostrophic model for predicting the time evolution o f the mesoscale flow field from the perspective of operational oceanog raphy. Forecast experiments have been conducted to address the followi ng three questions: (1) how does the model with assimilation differ fr om a so-called ''pure forecast'' trajectory, as produced in the absenc e of altimeter data; (2) can the model evolution controlled with T/P a ltimetric residuals be considered as a decent description of the actua l ocean evolution; (3) is the assimilation useful to predict the futur e of the system at short and medium ranges? The simulations consist of twin experiments obtained from the assimilation of subsets of the ori ginal T/P data set (covering, for instance, only part of the total. du ration of the reference experiment) or from the assimilation of synthe tic altimeter data extracted from the reference simulation. In additio n, predictability experiments have been carried out to determine the d oubling timescale of small errors in the specification of the model in itial conditions. The statistics derived from the simulations indicate that the assimilation of altimetry is efficient in reconstructing the past and the present state of the ocean on an operational basis. The procedure is shown to be able to recover both the surface and the deep flow during the assimilation period. The transport of the Gulf Stream is kept consistent with observational evidences as long as altimeter data are taken into account. However, forecasting mesoscale eddies is realistic only for a couple of weeks in advance. It is found that the averaged doubling time of initial errors is about 20 days, and credit of forecasts over more than 2 months in advance is questionable. As ex pected, predictions of the future are bounded by the predictability ti mescales inherent to the physical nature of the system.