Altimeter assimilation in the OCCAM global model - Part II: TOPEX/POSEIDONand ERS-1 assimilation

Maps of combined TOPEX/POSEIDON (T/P) and ERS-1 altimeter sea level anomali es for 1993 are assimilated every 10 days into a high resolution global oce an model, OCCAM. The assimilated mean sea level is based on the model simul ation, but modified in the: Kuroshio and Gulf Stream based on a dynamic sea level from climatological hydrographic data. Sea level updates are accompa nied by Vertical thermocline displacement and geostrophic current adjustmen ts away from the equator. Sea level errors were used to weight the sea leve l updates, with the model forecast errors calculated at low spatial resolut ion using an adaptive method based on the work of Dee [Dee, D.P., 1995. On- Line estimation of error covariance parameters for atmospheric data assimil ation, Mon. Wea. Rev. 123, pp. 1128-1145]. The modified mean sea levels are very successful at deflecting the western boundary currents to more realistic separation points which results in bett er surface temperature fronts and diagnosed air-sea heat fluxes, based on t emperature relaxation. The altimeter anomalies are successfully assimilated and the model is able to propagate these between assimilation times, altho ugh with some degradation and damping of the smaller scale features. Predic tions of sea level changes over 10-, 20-and 40-day periods are better than the corresponding persistence forecasts except for the Antarctic Circumpola r Current area where there is little difference. AVHRR and subsurface hydro graphic and ADCP current data from WOCE cruises were used for validation, a nd the model run with altimeter assimilation shows significantly better agr eement with these independent data sets than the model simulation, for all areas away from the equator. (C) 2000 Elsevier Science B.V. All rights rese rved.