APEX floats do not
self-correct their pressure measurements, and so the raw pressures returned by APEX
floats can contain errors as a result of pressure drifts. Historically, APEX floats
have used pressure sensors from Paine, Amatek and Druck, all of which have exhibited
pressure drifts of various magnitudes. Therefore APEX pressures are corrected to surface
pressure offsets (SP) in real-time by the DACs and in delayed mode by float experts.
Unfortunately, it is not always possible to correct the raw pressures from
APEX floats. Some APEX controller
boards, APF5-8, restrict SP to values greater than zero with negative SP values
truncated to zero. Thus, if a pressure sensor develops a negative
pressure drift on floats with an APF8 or earlier series controller, the reported SP values
are always zero. Profiles from these periods are labeled as having "Truncating Negative
Pressure Drifts" or TNPDs and are not correctable. Given the fast changing nature
of Argo data, it is best to identify TNDP affected profiles by looking for
the PRES_ADJUSTED_ERROR = 20 dbar.
Before the data set was corrected and these floats identified, the effect of the
APEX pressure biases was to overestimate the temperature in the oceans
and to create errors of about 10% of the magnitude of salinity differences between
Argo and WOA01 datasets. The largest temperature errors occurred in the upper 200 m
of the water column (due to the steep thermocline gradient), prior to 2005. This,
in turn, incorrectly implied a smaller global mean upper-ocean warming and
thermosteric sea level rise from 2003 to 2008. (Barker et al, 2011).
Argo now audits the treatment of pressure biases in the global data set. Most data
are now corrected. TNPD affected profiles can be identified in profile files through
the character string "TNPD" in the SCIENTIFIC_CALIB_COMMENT field for PRES. TNPD
data are labeled with *_ADJUSTED_QC = '2'. The more severe ones have PRES_ADJUSTED_ERROR
The following two papers describe in more detail the pressure biases for APEX floats:
Barker, P. M., J. R. Dunn, C. M. Domingues, and S. E. Wijffels, 2011: Pressure Sensor Drifts in Argo and Their Impacts. Journal of Atmospheric and Oceanic Technology, 28, 1036-1049, http://dx.doi.org/10.1175/2011JTECHO831.1
J.P. Abraham, M. Baringer, N.L. Bindoff, T. Boyer, L.J. Cheng, J.A. Church, J.L. Conroy, C.M. Domingues, J.T. Fasullo, J. Gilson, G. Goni, S.A. Good, J. M. Gorman, V. Gouretski, M. Ishii, G.C. Johnson, S. Kizu, J.M. Lyman, A. M. Macdonald, W.J. Minkowycz, S.E. Moffitt, M.D. Palmer, A.R. Piola, F. Reseghetti, K. Schuckmann,K.E. Trenberth, I. Velicogna, J.K. Willis, A review of global ocean temperature observations: Implications for ocean heat content estimates and climate change, Reviews of Geophysics, (accepted).
In early 2007, it was discovered that Argo profiles from SOLO floats with FSI CTD
(Argo Program WHOI) may have incorrect pressure values. The problem did not affect
any other combination of instrument and sensor. In GTS TESAC messages, potentially
affected instruments can be identified by instrument type 852 (SOLO FSI, see WMO
Code Table 1770
Some profiles can be corrected automatically and some need additional study.
The automatic fix for these profiles was instituted on 10 October, 2007. For profiles
need additional attention, the float will stay on the greylist until the profiles have
While studying the pressure offset errors, a related problem was discovered in a
group of WHOI/SBE profiles. For the affected WHOI/SBE instruments, all profiles have
been corrected and are available on the GDACS as of 14 September 2007.
To learn more details about the pressure problems, click here and to learn which floats have been
fixed, click here