Variations of the surface arrival time of Park and Profile APEX floats due to their depth control scheme

Abstract

The envelope method introduced by Park et al. (2005) is used to estimate the surface arrival time of APEX-type floats. This is one of the most important parameters for estimating subsurface velocities accurately from float trajectories. However, the method does not work for APEX with the Park and Profile (PnP) observation scheme, at least in its original form, because each cycle of the APEX surface arrival time varies due to its depth control scheme: APEX's surfacing in PnP scheme is classified into two modes, earlier or later, which correspond to cycles with measurements deeper or shallower respectively than the preset profile depth. It is possible to improve the envelope method for APEX with the PnP scheme. However, obtaining additional information for improving the estimate is difficult considering the numerous gaps in the Argo data at present. Furthermore, it is expected that the results will be less accurate even if an improvement is made. The authors strongly recommend that the principal investigators (or Data Assembly Centers) identify the surface arrival times at data decoding, which is the easiest way to resolve this problem, and the method by which this can be achieved is explained herein.