Structure of the North Atlantic current in stream-coordinates and the circulation in the Newfoundland basin

Accurate knowledge of the absolute North Atlantic Current (NAC) transport has been limited in the past by the di$culty of obtaining absolute velocity measurements. For this study, historical hydrography was combined with time series of round trip travel time from inverted echo sounders, bottom pressure sensor measurements, and measured velocities from deep current meters, via a `Gravest Empirical Modea technique, to yield time series of temperature and absolute velocity sections. These sections were used to determine the mean NAC stream-coordinates structure of temperature and absolute velocity for the period from August 1993 to February 1995. This structure is narrower and stronger than the corresponding Eulerian average, with peak speeds greater than 80 cm s and bottom velocities exceeding 10 cm s . The calculated mean NAC transport was 131$14 ( 10 m s ); temporal variations resulted in a 41 10 m s standard deviation. The stream-coordinates estimate of mean absolute transport was found to be about 10% smaller than the corresponding Eulerian mean transport of 146 10 m s as a result of partially summing the north and south transports within the Mann Eddy, a large semi-permanent anticyclonic eddy normally located just o!shore of the NAC. Most transport sections across the NAC in this region include the northward transport of the inshore side of the Mann Eddy because it is di$cult to discriminate between the two #ows. Historical estimates place the Mann Eddy absolute transport around 50}60 10 m s ; thus, the transport crossing this transect that can be identi"ed with the NAC is about 90 10 m s . The barotropic component of the transport (de"ned as the bottom velocity multiplied by the water depth) is found to contribute at least 30% of the total absolute transport, depending on the choice of baroclinic reference level. Because a time series of transport measurements was made in this study, the accuracy of the mean transport estimate is better than previous estimates, which have all been based on snapshot measurements. From a combination of these measurements with other published measurements, a scheme for the overall circulation and transport in the Newfoundland Basin is suggested. 2001 Elsevier Science Ltd. All rights reserved. 0967-0637/01/$ see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 0 6 3 7 ( 0 0 ) 0 0 1 0 3 5