Missing Argo Float Profiles in Highly Stratified Waters of the Amazon River Plume

Abstract In the western tropical Atlantic Ocean close to the Amazon plume, a large loss rate of Argo-float profiles took place, that is, instances of profiles that should have happened but were not transmitted. We find that APEX and SOLO floats were not ascending to the surface in the presence of low surface practical salinity, typically on the order of 32.5 or less, because of limitations on the surface buoyancy range for those floats. This results in an overall loss of profiles from these floats that is on the order of 6% averaged over the year, with a peak of 12% in July. We also find aborted descents/incorrect grounding detections for ARVOR/PROVOR floats when surface salinity is low and the descending float reaches a strong halocline (2.6% of all the profiles in the June–August season). Altogether, the whole Argo set includes a maximum loss rate of roughly 6% in July. We find a pattern of loss that fits the surface salinity seasonal cycle and the occurrence of low surface salinity investigated from a high-resolution daily satellite salinity product in 2010–21. The agreement is even better when considering surface density instead of surface salinity, with the temperature contribution to density inducing a shift in the maximum occurrence of these events by 1 month relative to the cycle of very low salinity events. Because of changes in the float technology, the loss rate that targets the lowest surface salinities was very large until 2010, with an overall decrease afterward. Significance Statement In the western tropical Atlantic Ocean, some Argo floats were not able to ascend or descend with very low surface salinity, because of buoyancy limitations for some float types and false bottom detection on others. In this region, for surface practical salinity smaller than 32.5, this resulted in a loss of close to one-half of the Argo profiles during the last 20 years. Altogether, this undersampling of the lowest surface salinities by Argo floats modifies the upper-ocean salinity seasonal cycle, as well as longer-term trends portrayed in Argo data–based products. Furthermore, in this region, care must be taken when validating satellite salinity data with Argo data or when adjusting satellite sea surface salinity data to in situ data products.