Pressure Sensor Drifts in Argo and Their Impacts

Abstract

In recent years, autonomous profiling floats have become the prime component of the in situ ocean observing system through the implementation of the Argo program. These data are now the dominant input to estimates of the evolution of the global ocean heat content and associated thermosteric sea level rise. The Autonomous Profiling Explorer (APEX) is the dominant type of Argo float (;62%), and a large portion of these floats report pressure measurements that are uncorrected for sensor drift, the size and source of which are described herein. The remaining Argo float types are designed to automatically self-correct for any pressure drift. Only about 57% of the APEX float profiles (or ;38% Argo profiles) can be corrected, but this typically has not been done by the data centers that distribute the data (as of January 2009). A pressure correction method for APEX floats is described and applied to the Argo dataset. A comparison between estimates using the corrected Argo dataset and the publically available uncorrected dataset (as of January 2009) reveals that the pressure corrections remove significant regional errors from ocean temperature, salinity, and thermosteric sea level fields. In the global mean, 43% of uncorrectable APEX float profiles (or ;28% Argo profiles) appear to largely offset the effect of the correctable APEX float profiles with positive pressure drifts. While about half of the uncorrectable APEX profiles can, in principle, be recovered in the near future (after inclusion of technical information that allows for corrections), the other half have negative pressure drifts truncated to zero (resulting from firmware limitations), which do not allow for corrections. Therefore, any Argo pressure profile that cannot be corrected for biases should be excluded from global change research. This study underscores the ongoing need for careful analyses to detect and remove subtle but systematic errors in ocean observations.

The CORA dataset: validation and diagnostics of in-situ ocean temperature and salinity measurements

“…However, only severe negative pressure drifts show observable temperature and salinity anomalies (an error of −20 dbar would cause a positive salinity error of approximately 0.01 PSS-78). Barker et al (2011) estimated a median error of −3 dbar for all TNPD profiles that can be compared with a close good profile. As a consequence they recommended that all the TNPD floats be excluded from studies of oceanic heat content and decadal changes.…”

“…Several widespread problems have also been discovered in the past few years in the Argo data and are known to impact estimates of the global OHC (e.g. Willis et al, 2007;Barker et al, 2011). These data issues have been already corrected by Argo data producers or corrections are still in progress.…”

“…This type of data validation tool is in particular well suited to detecting large scale errors due to measurement drifts and systematic instrumental biases (e.g. Willis et al, 2008;Barker et al, 2011). In this paper, GOIs are estimated for CORA3 using the method introduced in von Schuckmann and Le Traon (2011), and used as a diagnostic of the level of quality reached by the CORA3 database.…”

“…The geographical distribution of the corrections can be compared to the Fig. 5 of Barker et al (2011). In CORA3, more profiles are corrected for a pressure drift than in the GDAC Argo dataset as of January 2009.…”

“…Another reason is the possible compensatory effect in our GSSL estimate of some residual positive and negative biases. For example, Barker et al (2011) noted that negative biases from uncorrectable (and other unusable) APEX profiles nearly compensate positive biases from correctable (but not yet corrected) APEX profiles in the global 0-700 m thermosteric sea level. Further careful comparisons and sensitivity studies are therefore needed to estimate GSSL with the CORA3 dataset and the users should be aware of these limitations.…”