Deep western boundary current east of Abaco: Mean structure and transport

Abstract

To assess whether the southward deep water flow of the Atlantic meridional overturning circulation can be monitored by measuring the deep western boundary current, the structure and transport of the time-averaged deep western boundary current at 26.5N are estimated from long current meter records deployed in a series of 7 mooring deployments over 11 years from March 1986 to June 1997 east of Abaco Island in the Bahamas. Time-averaged meridional velocities for each current meter record are combined into estimates of mean velocity at 3 to 10 depths at 10 mooring locations extending eastward from the continental slope out to an offshore distance of 580 km. The mean deep western boundary current below 1000 m depth exhibits a core of strong southward velocities above 15 cm s Ϫ1 at an offshore distance of about 55 km and extends out to an offshore distance of about 160 km with an estimated total southward transport of 34.6 Sv. We estimate the error in this time-averaged transport to be 3.7 Sv due to uncertainties in the mean velocities due to temporal variability and up to 4 Sv due to the choice of methods to integrate spatially the time-averaged velocities across the section. Offshore from 160 km out to at least 580 km, there is broad, slow northward flow that recirculates deep water northward. While the mooring array is not sufficient to accurately measure the northward recirculation, a simple spatial average of the time-averaged currents indicates a northward recirculation of about 13 Sv in this offshore region. Daily estimates of the deep western boundary current from two arrays with reasonably complete coverage of the deep western boundary current suggest that the instantaneous boundary current has a width of about 100 km, naturally narrower than the mean boundary current. Daily southward transports vary between 5 and 75 Sv but the array does not fully resolve the boundary current during offshore meander events that occur about 20% of the time. We conclude that it is problematic to monitor the net southward flow of the Atlantic meridional overturning circulation with boundary current measurements at 26.5N because the narrow deep western boundary current transport has substantially larger transport than the net southward flow and the offshore northward recirculation is broad and diffuse so that accurate estimates of the recirculation would require an extensive array of moored instruments.

Variability in Deep Western Boundary Current transports: Preliminary results from 26.5°N in the Atlantic

“…the absolute velocity profile is indeterminate. In the region east of Abaco the 800 dbar level has commonly been chosen for a LNM based on long term current meter, LADCP, and Pegasus observations [Molinari et al, 1992;Lee et al, 1990Lee et al, , 1996Hacker et al, 1996;Bryden et al, 2005a]. With the PIES data presented in this paper we can determine the DWBC transport relative to an 800 dbar LNM ('LNM transport') and the transport ''error'' incurred by assuming a LNM due to the actual motion at 800 dbar ('LNM error').…”

“…However, the time-mean bottom velocity cannot be so-determined due to the so-called ''leveling'' problem wherein bottom depth changes cannot be distinguished from deep geostrophic pressure gradients [e.g., Watts et al, 2001b]. If a time-mean bottom-velocity estimate is available (such as from the historical current meter records at 26.5°N, which were recently compiled by Bryden et al, 2005a] then this mean can be added to the time-varying bottom velocities determined from the bottom pressure records and the resulting absolute bottom velocities can be used to reference the IES-derived relative velocity profiles to produce absolute velocity profiles between pairs of PIES.…”

“…For the span between sites D and E (see Figure 1), a comparison between preliminary data from three lowered acoustic Doppler current profiler (LADCP) sections and the temporally-concurrent PIES-based transports indicated that the one-year-mean bottom-velocity was larger by about 2 cm s À1 (northward) than the mean of the historical current meters presented by Bryden et al [2005a] so this correction was applied. No such correction was required at the other sites.…”

Variability of the currents’ vertical structure in the western Subtropical Atlantic and meridian heat transport

“…ш.). Этот район уникален по многим причинам:-он находится вблизи максимума МПТ в Северной Атлантике [1]; -здесь выполнено наибольшее количество гидрологических разрезов [1,3,14]; -МПТ определяется здесь струйными Флоридским (ФТ) и Антильским (АТ) течениями, в остальной части бассейна перенос направлен противопо-ложно [5,13]; -на этих широтах выполнены наиболее долговременные непрерывные наблюдения за Флоридским течением [18 -20].Исследование изменчивости циркуляции и вихрей синоптического мас-штаба в Субтропической Атлантике с использованием данных долгосрочных инструментальных наблюдений за течениями -одна из главных задач много-численных наблюдательных программ MODE, POLYMODE, STACS, WATTS, ACCP,16,[21][22][23][24][25][26]. В указанных работах описаны природа и типичные пространственно-временные масштабы изменчивости вихревых образований, оценены вклады переносов тепла синоптического масштаба в суммарную изменчивость МПТ в северо-западной части ССАК.…”

“…Далее оценивался вклад этого переноса в интеграль-ный МПТ в окрестности 26° с. ш. Субтропической Атлантики, полученный ранее в работе [29]. Такая методика расчета переноса тепла вихрями подроб-но описана в [3,16].Так как большинство синоптических вихрей (~90%) сосредоточено в за-падной части Субтропической Атлантики (на остальную часть бассейна их приходится не более 10% [13,26]МОРСКОЙ ГИДРОФИЗИЧЕСКИЙ ЖУРНАЛ № 3 2015 41западного погранслоя (~0,5·10 6 м), найдем истинный перенос тепла синопти-ческими вихрями, приближенно совпадающий с оценками, полученными по гидрологическим данным [26,29]. …”