Reevaluation of historical ocean heat content variations with time-varying XBT and MBT depth bias corrections
- 1.2k Downloads
As reported in former studies, temperature observations obtained by expendable bathythermographs (XBTs) and mechanical bathythermographs (MBTs) appear to have positive biases as much as they affect major climate signals. These biases have not been fully taken into account in previous ocean temperature analyses, which have been widely used to detect global warming signals in the oceans. This report proposes a methodology for directly eliminating the biases from the XBT and MBT observations. In the case of XBT observation, assuming that the positive temperature biases mainly originate from greater depths given by conventional XBT fall-rate equations than the truth, a depth bias equation is constructed by fitting depth differences between XBT data and more accurate oceanographic observations to a linear equation of elapsed time. Such depth bias equations are introduced separately for each year and for each probe type. Uncertainty in the gradient of the linear equation is evaluated using a non-parametric test. The typical depth bias is +10 m at 700 m depth on average, which is probably caused by various indeterminable sources of error in the XBT observations as well as a lack of representativeness in the fall-rate equations adopted so far. Depth biases in MBT are fitted to quadratic equations of depth in a similar manner to the XBT method. Correcting the historical XBT and MBT depth biases by these equations allows a historical ocean temperature analysis to be conducted. In comparison with the previous temperature analysis, large differences are found in the present analysis as follows: the duration of large ocean heat content in the 1970s shortens dramatically, and recent ocean cooling becomes insignificant. The result is also in better agreement with tide gauge observations.
KeywordsOcean temperature XBT fall-rate equation oceanographic observation ocean heat content objective analysis
Unable to display preview. Download preview PDF.
- Boyer, T. P., J. I. Antonov, H. E. Garcia, D. R. Johnson, R. A. Locarnini, A. Mishonov, M. T. Pitcher, O. K. Baranova and I. V. Smolyar (2006): World Ocean Database 2005. NOAA Atlas NESDIS 60, ed. by S. Levitus (available at http://www.nodc.noaa.gov/OC5/WOD05/docwod05.html).
- Conkright, M. E., R. A. Locarnini, H. E. Garcia, T. D. O. Brien, T. P. Boyer, C. Stephens and J. I. Antonov (2001): World Ocean Atlas 2001: Objective Analyses, Data Statistics, and Figures. NOAA Atlas NESDIS 42, 17 pp., CD-ROM, U.S. Government Printing Office, Washington, D.C.Google Scholar
- Fahrbach, E. (1989): The use of electronic digital thermometers and pressure meters. WOCE Newsletter No. 8, 12–13.Google Scholar
- Levitus, S. and J. Antonov (1997): Limatological and Interannual Variability of Temperature, Heat Storage, and Rate of Heat Storage in the Upper Ocean. NOAA Atlas NESDIS 16, 186 pp.Google Scholar
- Levitus, S., C. Stephens, J. I. Antonov and T. P. Boyer (2000): Yearly and Year-Season Upper Ocean Temperature Anomaly Fields, 1948-1998. NOAA Atlas NESDIS 40 (available from http://www.nodc.noaa.gov/OC5/PDF/ATLAS/ nesdis40.pdf).
- Levitus, S., J. I. Antonov, T. P. Boyer, H. E. Garcia and R. A. Locarnini (2005): Linear trends of zonally averaged thermosteric, halosteric, and total steric sea level for individual ocean basins and the world ocean, (1955–1959)–(1994–1998). Geophys. Res. Lett., 32, L16601, doi: 10.1029/2005GL023761.CrossRefGoogle Scholar
- Quadfasel, D., N. Verch and J. Langhof (1990): Are mercury deep-sea reversing thermometers out of date? Ocean Dynamics, 145–152.Google Scholar
- Sakamoto, T., H. Hasumi, M. Ishii, S. Emori, T. Suzuki, T. Nishimura and A. Sumi (2005): Responses of the Kuroshio and the Kuroshio Extension to global warming in a highresolution climate model. Geophys. Res. Lett., 32, doi:10.1029/2005GL023384.Google Scholar
- Willis, J. K., J. M. Lyman, G. C. Johnson and J. Gilson (2008): In situ data biases and recent ocean heat content variability. J. Atmos. Oceanic Tech. (in press).Google Scholar
- Wyatt, B., R. Still, D. Barstow and W. Gilbert (1967): Hydrographic Data from Oregon Waters 1965. Department Oceanography, School of Science, Oregon State Univ. Date Report No. 27.Google Scholar