Using altimeter and Argo data to estimate biases in XBT fall rate equations

1. Using altimeter and Argo data to estimate biases in XBT fall rate equations Josh K. Willis joshua.k.willis@jpl.nasa.gov Jet Propulsion Laboratory

2. Overview • Ocean “cooling” – why overall accuracy in the XBT network is important • Argo and altimeter data as calibration tools • Time evolution of recent XBT biases • Remaining XBT errors

3. Cooling!!! 2003 to 2005 cooling: -1.0 ± 0.33 W/m2 (Averaged over Earth’s surface) Upper-Ocean “cooling” from Lyman et al. (GRL, 2006)

4. XBT Data – Signals and Errors Eddies • Big signals in “isotherm displacement” • Lots of averaging required for large-scale • Average signals susceptible to systematic error

5. WHOI float biases

6. A correction to “recent cooling” Ocean Heat Content from 2004 to 2006 Removing the bad float data reduces the cooling but does not completely eliminate it. From Willis et al., GRL, in prep.

7. A correction to “recent cooling” Ocean Heat Content from 2004 to 2006 Another bias: XBTs are biased warm, which also causes spurious cooling. From Willis et al., GRL, in prep.

8. Isotherm Displacement: Dz = Tclim – T dTclim /dz XBT bias & fall-rate errors – pair analysis Comparison of Isotherm Displacements XBT/Argo pairs ~12,000 Argo/CTD pairs ~2,000 From Willis et al., GRL, in prep.

9. The “pseudo pair” Coefficient of regression, r between SSH anomaly and T(400 m) • Much of upper ocean T variaibility is contained in SSH anomaly • Use AVISO SSH maps to make “pseudo” temperature anomalies: • Tpseudo = a(z) * SSHA From Wijffels et al., manuscript in prep.

10. “Pseudo-pair” comparison Comparison of Sippican Deep Blue probes with nearby Argo pairs (2004 – 2006), ~12,000 • Pseudo-pairs give same bias, but have narrower distribution • More comprehensive means of test XBT bias because of SSH data availability From Wijffels et al., manuscript in prep.

11. “Pseudo-pair” analysis of other data(a.k.a. sanity check) Argo profile data CTD data • CTD data show no significant bias during any time period • Argo floats show little bias except for WHOI/FSI floats From Wijffels et al., accepted

12. Time dependence of XBT bias Time dependence of bias in Sippican Deep Blue XBT probes • Bias increases over time • Hi bias in later years may reflect “double” application of Hanawa et al. (1995) correction From Wijffels et al., manuscript in prep.

13. Stretching factor by probe type

14. Revised ocean heat content estimate

15. Skewness: significant difference between mean and median Could be a sign of wire stretch Remaining Errors

16. Remaining Errors Gaussian Tail for positive depth error is much bigger