The impact of historical biases on the XBT‐derived meridional overturning circulation estimates at 34°S

Abstract

An observational system based on high-density expendable bathythermograph (XBT) data has provided the longest record of the South Atlantic meridional overturning and heat transport estimates across 34°S. Measurement biases are a point of concern for the capability of an XBT system to capture long-term trends in volume and heat transports, and the impact of such biases on the meridional overturning estimates has never been quantified. In the present study, the sensitivity of the meridional overturning circulation and heat transport to uncertainties in XBT measurements is quantified under the framework of an eddy-resolving model simulation. Results show that XBT measurement biases after 2010 can translate into small meridional overturning errors on the order of 3% or 0.38 Sv (1Sv = 10 6 m 3 s À1), and 0.025 PW (1 PW = 10 15 W) or 8% of the meridional heat transport in the model. Historical XBT-derived trends in transport estimates across 34°S are stronger and statistically significant after the late 1990s, 0.3 Sv decade À1 and 0.02 PW decade À1 . These trends are mostly due to the XBT linear depth bias, with smaller contributions associated with temperature and depth offsets from the historical record. Long-term trends calculated from Simple Ocean Data Assimilation reanalysis, estimated as 0.1 Sv/decade and 0.006 PW/decade, are 3 times smaller than the XBT-derived historical trends. Therefore, an adequate correction of historical XBT data is necessary for an early detection of trends in the meridional overturning circulation and heat transport.

More Than 50 Years of Successful Continuous Temperature Section Measurements by the Global Expendable Bathythermograph Network, Its Integrability, Societal Benefits, and Future

“…Using the three XBT data performance methods (Levitus et al, 2009;Gouretski and Reseghetti, 2010;Cheng et al, 2014) results in near-identical global OHC changes in the upper 2000 m since 1966 based on the mapping method proposed in Cheng and Zhu (2016) (Figure 12). In addition, temporal, and spatial variability of locations and transports of ocean currents, estimates of MHT and MOC, and the determination of mixed layer depths are robust for any XBT data improvement scheme (Goes et al, 2015b;Houpert et al, 2015).…”

Observational Advances in Estimates of Oceanic Heating

“…Undersampled areas, particularly located in the southern Hemisphere, may have significantly biased low the estimates of global OHC trends between 1970 and 2004 [16]. The uncertain nature of the multi-decadal record was further highlighted by the difficulty of correcting significant biases in expendable bathythermograph measurements, which represented the main source of upper-ocean temperature profiles before the launch of Argo [23,40]. Overall, the OHC curves prior to the mid 2000s have large error-bars, and the year-to-year variations typically show limited agreement with the net TOA fluxes estimated from satellite products [38,64].…”

XBT Effects on the Global Ocean State Estimates Using a Coupled Data Assimilation System

“…XBT relies on a FRE to estimate depth by recording the descending time because it does not have a pressure sensor. FRE has an error of about ±5 m or 2% depth that yields a significant uncertainty in temperature measurements (Goes et al 2015a). Hanawa et al (1995) first suggested a correction for XBT FRE biases, and many subsequent studies have continuously proven the negative impact of XBT biases on ocean climate studies (Kizu et al 2005;Gouretski and Koltermann 2007;Domingues et al 2008;Wijffels et al 2008;Levitus et al 2009;DiNezio and Goni 2011;Cowley et al 2013;Goes et al 2013;Cheng et al 2014Cheng et al , 2015.…”