Biases in Expendable Bathythermograph Data: A New View Based on Historical Side-by-Side Comparisons

Abstract

Because they make up 56% of ocean temperature profile data between 1967 and 2001, quantifying the biases in expendable bathythermograph (XBT) data is fundamental to understanding the evolution of the planetary energy and sea level budgets over recent decades. The nature and time history of these biases remain in dispute and dominate differences in analyses of the history of ocean warming. A database of over 4100 side-by-side deployments of XBTs and conductivity–temperature–depth (CTD) data has been assembled, and this unique resource is used to characterize and separate out the pure temperature bias from depth error in a way that was not previously possible. Two independent methods of bias extraction confirm that the results are robust to bias model and fitting method. It was found that there is a pure temperature bias in Sippican probes of ~0.05°C, independent of depth. The temperature bias has a time dependency, being larger (~0.1°C) in the earlier analog acquisition era and being likely due to changes in recorder type. Large depth errors are found in the 1970s–80s in shallower-measuring Sippican T4/T6 probe types, but the deeper-measuring Sippican T7/Deep Blue (DB) types have no error during this time. The Sippican T7/DB fall rate slows from ~1990 onward. It is found that year-to-year variations in fall rate have a bigger effect on corrections to the global XBT database than do any small effects of ocean temperature on fall rate. This study has large implications for the future development of better schemes to correct the global historical XBT archive.

Heat transport variation due to change of North Pacific subtropical gyre interior flow during 1993–2012

“…Kizu et al (2011) reported that Hanawa et al's correction method yields positive and negative fall rates for TSK T-7 and Sippican Deep Blue probes; in the present case, these depth biases result in temperature biases of up to 0.07 and −0.03 • C, respectively. Furthermore, data collected by Sippican Deep Blue XBT are known to be subject to time-varying biases in both depth and temperature (e.g., Cowley et al 2013), which are reported to cause positive temperature biases of up to 0.1 • C for the study period. Therefore, by integrating the vertically uniform error added to the observed temperature profiles, we estimated the error in H XBT to be approximately 2 cm.…”

Reconciling Estimates of Ocean Heating and Earth’s Radiation Budget

“…The time-space varying temperature biases associated with XBTs arise from a number of sources, including the fallrate equation that is used to estimate the probe depth as a function of time [25]. While the first attempts to correct XBT biases were able to eliminate major differences among observed and simulated global ocean heat uptake and sea level rise [35], new correction schemes continue to be developed [36,37] under ongoing research by the international community.…”

“…For example, approximately 50% of XBTs in the historical databases are of unknown type. This has required researchers to make intelligent guesses about the likely probe type, based on information from individual profiles, such as the maximum recorded depth, country of origin and profile date [36,37]. Efforts to devise more comprehensive approaches to missing metadata and assess its impact on ocean heating rates are being fostered through international collaborative projects, such as the IQuOD (www.iquod.org; [33]).…”

“…Coverage and depth sampling of the ocean improved over time as XBT instruments were developed that could sample to 1 km and deeper ( Fig. 2b, c; [37]). The early 2000s saw a dramatic improvement in ocean sampling with the development of the Argo array of autonomous profiling floats ( Fig.…”

Velocity and Density Effect on Impact Force during Water Entry of Sphere

“…For devices such as these, the depth is not a measured quantity; rather it is inferred from a calibrated fall rate equation which is developed during controlled experiments. It has been known that the actual depth depends on a variety of factors such as probe shape and rate of rotation [2][3][4], drop height and water temperature [5] or on other factors [6][7].…”