Atlantic Meridional Overturning Circulation (AMOC) geostrophic transport: Comparison of the RAPID time series with hydrography from 2004 to 2011

The AMOC has been measured at 26°N by infrequent hydrographic sections and, since 2004, continuous measurements by the RAPID-MOCHA monitoring system. During the first 4 years of RAPID-MOCHA observations the overturning circulation was, on average, 2.7 Sv stronger than in the subsequent 4 years (Smeed et al., 2014). However, transport inferred from the 2004 and 2010 hydrographic sections yield contrasting results with stronger overturning in 2010 (Atkinson et al., 2012). In this presentation we examine the possible causes of this apparent discrepancy between the RAPID-MOCHA monitoring array and the hydrographic sections. Not only do these two estimates of the overturning circulation use different data but also different methods. We examine the assumptions and approximations used in each method and show that the two datasets provide consistent results. The reasons for the trend calculated from hydrographic sections differing from that calculated with the RAPID-MOCHA time series are down to differing calculation methodologies and the large temporal variability at short timescales that masks the long-term trend when comparing just the hydrographic sections.