Changing Expendable Bathythermograph Fall Rates and Their Impact on Estimates of Thermosteric Sea Level Rise

Susan E. Wijffels *Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia

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Josh Willis Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

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Catia M. Domingues *Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia

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Paul Barker *Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia

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Neil J. White *Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia
Antarctic Climate and Ecosystem Cooperative Research Centre, Hobart, Tasmania, Australia

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Ann Gronell *Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia

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Ken Ridgway *Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia

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John A. Church *Centre for Australian Weather and Climate Research, Hobart, Tasmania, Australia
Antarctic Climate and Ecosystem Cooperative Research Centre, Hobart, Tasmania, Australia

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Abstract

A time-varying warm bias in the global XBT data archive is demonstrated to be largely due to changes in the fall rate of XBT probes likely associated with small manufacturing changes at the factory. Deep-reaching XBTs have a different fall rate history than shallow XBTs. Fall rates were fastest in the early 1970s, reached a minimum between 1975 and 1985, reached another maximum in the late 1980s and early 1990s, and have been declining since. Field XBT/CTD intercomparisons and a pseudoprofile technique based on satellite altimetry largely confirm this time history. A global correction is presented and applied to estimates of the thermosteric component of sea level rise. The XBT fall rate minimum from 1975 to 1985 appears as a 10-yr “warm period” in the global ocean in thermosteric sea level and heat content estimates using uncorrected data. Upon correction, the thermosteric sea level curve has reduced decadal variability and a larger, steadier long-term trend.

Corresponding author address: Susan E. Wijffels, CSIRO Marine and Atmospheric Research, GPO 1538, Hobart, 7000, TAS, Australia. Email: susan.wijffels@csiro.au

Abstract

A time-varying warm bias in the global XBT data archive is demonstrated to be largely due to changes in the fall rate of XBT probes likely associated with small manufacturing changes at the factory. Deep-reaching XBTs have a different fall rate history than shallow XBTs. Fall rates were fastest in the early 1970s, reached a minimum between 1975 and 1985, reached another maximum in the late 1980s and early 1990s, and have been declining since. Field XBT/CTD intercomparisons and a pseudoprofile technique based on satellite altimetry largely confirm this time history. A global correction is presented and applied to estimates of the thermosteric component of sea level rise. The XBT fall rate minimum from 1975 to 1985 appears as a 10-yr “warm period” in the global ocean in thermosteric sea level and heat content estimates using uncorrected data. Upon correction, the thermosteric sea level curve has reduced decadal variability and a larger, steadier long-term trend.

Corresponding author address: Susan E. Wijffels, CSIRO Marine and Atmospheric Research, GPO 1538, Hobart, 7000, TAS, Australia. Email: susan.wijffels@csiro.au

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