Quality assessment of a satellite altimetry data product DT18 in the Norwegian Sea: A comparison to tide gauge records and drifters data

Abstract

• The altimetric product DT18 reproduces the sea level variability in the Norwegian Sea with an average error 3–4 cm. • Statistical tests conducted for the DT18 reveal some problems. • Errors of the DT18 have a shape of “diamonds” in the open sea. • Ageostrophic factors influence strongly the real current velocities in the Norwegian Sea. The objective of this paper is to assess the quality of the gridded altimetric sea level product DT18 as well as altimetry data from the Sea Level Climate Change Initiative in the Norwegian Sea. We compare the satellite altimetry data with two types of instrumental data. We use tide gauge records to assess the quality in the coastal zone and data from surface drifting buoys to assess the quality in the open ocean. Thus, we aim to complement previous studies by providing the objective assessment of the quality and performance of satellite altimetry in this important region. We demonstrate that the product DT18 reproduces the sea level variability in the coastal zone at an acceptable level (an average error is 3–4 cm), and it reproduces the structure of the time series quite well (almost with a 96% correlation). We found out that altimetry substantially smoothes the pattern of the sea level. This pattern is revealed in the coastal zone as well as in the open sea, where, in addition, it also manifests errors in a shape of “diamonds”, which can be due to data interpolation from satellite tracks. We detected the influence of ageostrophic factors on the current velocities using altimetry in the open sea, and this is also true for the coastal zone. Statistical tests also showed some discrepancies between altimetry data and field (buoys and drifters) data in the open sea. This is manifested in the statistical inequality of variances and in differences of empirical distribution functions. Summing up, we assert that some errors or discrepancies in altimetric data are still observed in high latitudes, both in the coastal zone and in the open part of the Norwegian Sea.