Missing Argo float profiles in highly stratified waters of the Amazon river plume

In the western tropical Atlantic close to the Amazon plume, a large loss rate of Argo floats profiles took place, that is instances of profiles which 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, due to limitations on the surface buoyancy range for those floats. This results in an overall loss of profiles from these floats 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 to August season). Altogether, the whole Argo set includes a maximum loss rate of roughly 6% in July. We find a pattern of loss which fits the surface salinity seasonal cycle and the occurrence of low surface salinity investigated from a high-resolution daily satellite salinity product in 2010-2021. The agreement is even better when considering surface density instead of surface salinity, the temperature contribution to density inducing a shift in the maximum occurrence of these events by one month compared to the cycle of very low salinity events. Because of changes in the float technology, the loss rate which targets the lowest surface salinities was very large until 2010, with an overall decrease afterwards.