Impact of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) mission on ocean circulation estimates 2. Volume and heat transports across hydrographic sections of unequally spaced stations.

摘要

Accurate geoids are expected to improve our knowlegde of the dynamicsea surface height (SSH) as a mirror of the dynamic state of theoceans. The dedicated geoid mission GOCE is expected to be lauched in 2004.It will lead to a highly accurate geoid model with a resolution of degreeand order 200. We examine the impact of this missionon the assessment of large scale oceanic mass and heat transports via itsexpected error characteristics. We do so applying a linear box inversemodel and a non-linear section inverse model to hydrographic data andto (synthetic) sea surface height data. The results are compared tothose obtained when substituting the error estimates of the GRACEmission and the present day geoid EGM96.For the box inverse model, we find an average reduction in transportuncertainties in Experiment A (which includes model error at the levelof sea surface height variability) of about 9 % for GRACE geoid errorcovariances and about 17 % for GOCE over the ``hydrography only'' solution.In both GRACE and GOCE these average percentage improvements aresignificantly increased when the SSH variability signal is excluded(Experiment B) to 42 % for GRACE and 47 % for GOCE. We expect a greaterimprovement in the accuracy of ocean transports from GOCE when WOCEhydrographic data are used to enclose numerous, smaller box regions.The apriori assumptions of the non-linear model about the oceancirculation are much more conservative than for the box model. As aconsequence, the uncertainties of large scale transports are much biggerthan for the linear model. On the other hand, since this model buildson small scale balances, it can resolve small scale features of theflow field better. SSH data with GRACE geoid error covariances reducethe uncertainties on the average by 29 %, with GOCE geoid errorcovariances by 37 %. Exclusion of the SSH variability changes(Experiment B) these numbers by less than 5 % points.Summarizing our results and those of Part I, III and IV of this studywe conclude that the GRACE mission reduces the marine geoid uncertaintiessuch that altimetry becomes useful for the study of the steady stateocean circulation.The GOCE mission will improve the accuracy of the circulation estimates evenfurther on the large scales and introduce higher accuracy on shorterwavelenths as well.Furthermore, it will enable us to study individual ocean currents.