Nutzung von Argo-Driftern und Satellitenaltimetriedaten zur Ableitung der Zirkulation im Nordatlantik

Abstract

In recent years, two major observational systems have become operational. The first is the satellite altimetry which maps the ocean surface quasi synoptically. By referencing sea surface height to geoid models, such as derived from GRACE, ocean surface currents can be diagnosed. The second observing system is the Global Array of Profiling Floats (Argo) which provides in situ subsurface temperature and salinity data. Both types of measurements are consistent with each other to such an extent that altimetry can be used to check the quality and performance of individual temperature and salinity profiles measured by Argo. It is shown that both types of measurements complement each other, and a joint analysis is required in order to fully exploit the information content. The model used in this study to combine these two datasets solves for temperature and salinity which are close to observations and simultaneously respects stationary dynamical balances. The solution produces the estimates of the ocean circulation and transports. The momentum part of the model diagnoses consistent stationary flow fields, given prescribed fields of temperature, salinity, wind stress and open boundary fluxes. Simultaniously the residuals, appearing in the advection-diffusion equations for heat and salt are minimized. These residuals together with data misfits enter a cost function (weak constraints) which is reduced under the dynamical part being exactly fulfilled (strong constrait). We analyse the North Atlantic circulation by combining data from Argo profiling buoys and satellite altimetry measurements into an inverse finite-element ocean circulation model. The ocean circulation is analysed separately for each of the years 1999 to 2008. Numerical experiments carried out with and without use of the altimetric data are discussed. Including altimetry is shown to improve the model solutions, in particular, producing more realistic heat transports. The use of altimetry changes the model topography on large scales, this difference is imprinted in the corresponding change in circulation. The resulting temperature and salinity fields that minimize the model/data misfit are discussed. It is found that both types of observations are to a large extent complementary to each other. The changes due to Argo-data are similar to those due to altimetry mainly in regions of dense Argo-data coverage. By analysing inter-annual variabilities of sea surface height, heat transport and heat content it is found that the Argo-data coverage sufficient for reconstructing the inter-annual changes is reached only from the year 2002 onwards. The work shows the possibility of extracting the North Atlantic circulation combining both in situ and altimetry data. The derived trends in sea surface height, heat transport and heat content are discussed. The results compare well with other estimates based on measurements.