Vertical structure and transport on a transect across the North Atlantic Current near 42°N: Time series and mean

Abstract

A new method is presented for combining historical hydrography with the acoustic travel time measurements from an inverted echo sounder (IES) to obtain time series of full water column profiles of temperature T and specific volume anomaly. Hydrography is used to demonstrate that profiles constructed from the IES measurement account for more than 90% of the T and δ variance through the main thermocline range. Horizontal gradients between neighboring pairs of instruments, when combined with measurements from bottom pressure sensors and deep current meters, provide accurate estimates of absolute geostrophic velocity profiles. A section of IESs and current meters across the North Atlantic Current (NAC) along World Ocean Circulation Experiment line ACM6 near 42.5°N is analyzed using these methods, and the Eulerian mean temperature and absolute velocity sections are calculated. The mean transport for the combined NAC and northward flow within the inshore portion of the Mann Eddy is 146±13 (× 106 m3 s−1). The temporal standard deviation of the northward transport is 41×106 m3 s−1, arising not only from changes in the transport of the NAC but also from lateral shifts of the currents and eddies captured by the fixed span of moorings. Assumption of a level of no motion at the bottom and at 2000 dbar underestimates the true absolute transport by 33 or 60%, respectively. Historical measurements indicate that 50–60×106 m3 s−1 circulates within the Mann Eddy adjacent to the North Atlantic Current, thus 86–96×106 m3 s−1 appears to be throughput at the ACM6 line. Because the throughput observed on the ACM6 line considerably exceeds the combined historical estimates of northward and eastward outflow from the Newfoundland Basin of ≈ 30×106 m3 s−1, these measurements suggest a substantial (≈ 50 × 106 m3 s−1) recirculation or southward flow within the Newfoundland Basin in addition to the historical recirculation within the Mann Eddy.