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 0.38Sv (1Sv=106m3s-1), and 0.025PW (1PW=1015W) 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, 0.3Svdecade-1 and 0.02PWdecade-1. 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 0.1Sv/decade and 0.006PW/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. Key Points The XBT biases account for 3 to 8% error in MOC and MHT Impact of XBT biases are small relative to MOC natural variability Historical biases produce a positive trend on MOC/MHT estimates after the 1990s