Influence of the Current Field Non-stationarity and the Non-simultaneity of Hydrographic Measurements on ADCP-based Transport Estimates
Non-stationarity of the current’s field in combination with non-simultaneous measurements at stations of a hydrographic section leads to distortions in the ADCP-based assessments of total geostrophic barotropic transport over the section. These distortions over 49 particular sections from-shore-to-shore in different regions of the World Ocean are investigated on the basis of satellite altimetry data of Sea Level Anomaly, Absolute Dynamic Topography (ADT), and Formal Mapping Error (FME) available in the Internet (http://www.aviso.altimetry.fr). Distortions of barotropic transport have two components. The first is due to a change in the field of currents during measurements from station to station. It can be taken into account in the structure of the transport across the section from satellite altimetry data. The second is related to the displacement of streamlines of the geostrophic current at the ocean surface (ADT isolines) relative to the isobaths and represents the variability range of the instantaneous barotropic transport across the section track estimated on the basis of the same data during the time interval of measurements over this section. Assessments of these distortions are compared with the estimates of the errors of the barotropic transport over particular hydrographic sections. It is shown that the main component of these errors is the FME. Often, both components of the barotropic transport distortion are greater than the barotropic transport error, even for “rapid” sections, which are occupied in 6–12 days. Examples exist, in which significant transport distortions are accumulated during even shorter time periods of 3–5 days. Thus, investigation of the non-stationarity of the velocity field in combination with the non-simultaneity of hydrographic measurements is absolutely necessary for the analyses of the total transport and its structure across a hydrographic section.
The work was supported by the Russian Science Foundation, grant 16-17-10149. The author is grateful to E. G. Morozov, and K. V. Lebedev for useful comments when preparing the text of this contribution.
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