Abstract
Abstract. The Rapid instrument array across the Atlantic Ocean along 26° N provides unprecedented monitoring of the basin-scale circulation. A unique feature of the Rapid array is the combination of full-depth moorings with instruments measuring temperature, salinity, pressure time series at many depths with co-located bottom pressure measurements so that dynamic pressure can be measured from surface to bottom. Bottom pressure measurements show a zonally uniform rise (and fall) of bottom pressure of 0.015 dbar on a 5 to 10 day time scale, suggesting that the Atlantic basin is filling and draining on a short time scale. After removing the zonally uniform bottom pressure fluctuations, bottom pressure variations at 4000 m depth against the western boundary compensate instantaneously for baroclinic fluctuations in the strength and structure of the deep western boundary current so there is no basin-scale mass imbalance resulting from variations in the deep western boundary current. After removing the mass compensating bottom pressure, residual bottom pressure fluctuations at the western boundary just east of the Bahamas balance variations in Gulf Stream transport. Again the compensation appears to be especially confined close to the western boundary. Thus, fluctuations in either Gulf Stream or deep western boundary current transports are compensated in a depth independent (barotropic) manner very close to the continental slope off the Bahamas. In contrast, compensation for variations in wind-driven surface Ekman transport appears to involve fluctuations in both western basin and eastern basin bottom pressures, though the bottom pressure difference fluctuations appear to be a factor of 3 too large, perhaps due to an inability to resolve small bottom pressure fluctuations after removal of larger zonal average, baroclinic, and Gulf Stream pressure components. For 4 tall moorings where time series dynamic height (geostrophic pressure) profiles can be estimated from sea surface to ocean bottom and bottom pressure can be added, there is no general correlation between surface dynamic height and bottom pressure. Dynamic height on each mooring is strongly correlated with sea surface height from satellite observations and the variability in both dynamic height and satellite sea surface height decrease sharply as the western boundary is approached.
Highlights
The extensive Rapid instrument array deployed across the Atlantic at 26◦ N (Kanzow et al, 2008) provides a unique opportunity to examine the dynamics of the large-scale ocean circulation. 26◦ N was selected as the location for monitoring the Atlantic meridional overturning circulation for several reasons
The first year of boundary mooring observations has been used to monitor the temporal variability in mid-ocean layer transports (Fig. 1) and the southward mid-ocean thermocline transport has been combined with northward Gulf Stream and Ekman transports to produce a time series of the Atlantic meridional overturning circulation (Fig. 2, Cunningham et al, 2007)
The geostrophic transport is based on Rapid time series of dynamic height at the eastern and western boundaries of the 25◦ N section and the reference level velocity is set by forcing the geostrophic transport to equal the Florida straits plus
Summary
The extensive Rapid instrument array deployed across the Atlantic at 26◦ N (Kanzow et al, 2008) provides a unique opportunity to examine the dynamics of the large-scale ocean circulation. 26◦ N was selected as the location for monitoring the Atlantic meridional overturning circulation for several reasons. In addition to the ongoing Gulf Stream and Ekman transport measurements, the key elements of the Rapid array to monitor the mid-ocean meridional flow components are time series dynamic height profiles over the full water column near the eastern and western boundaries and co-located bottom pressure time series. The first year of boundary mooring observations has been used to monitor the temporal variability in mid-ocean layer transports (Fig. 1) and the southward mid-ocean thermocline transport has been combined with northward Gulf Stream and Ekman transports to produce a time series of the Atlantic meridional overturning circulation (Fig. 2, Cunningham et al, 2007). The geostrophic transport is based on Rapid time series of dynamic height at the eastern and western boundaries of the 25◦ N section and the reference level velocity is set by forcing the geostrophic transport to equal the Florida straits plus. We will examine how dynamic height and bottom pressure relate to sea surface height variations measured by satellites
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