OS Ocean Science OS Ocean Sci. 1812-0792 Copernicus Publications Göttingen, Germany 10.5194/os-5-421-2009 Adjustment of the basin-scale circulation at 26° N to variations in Gulf Stream, deep western boundary current and Ekman transports as observed by the Rapid array Bryden H. L. 1 Mujahid A. 1 2 Cunningham S. A. 1 Kanzow T. 1 3 National Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia Leibniz-Institut für Meereswisschenschaften, Düsternbrooker Weg 20, 24105 Kiel, Germany 19 10 2009 5 4 421 433 Copyright: © 2009 H. L. Bryden et al. 2009 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://os.copernicus.org/articles/5/421/2009/os-5-421-2009.html The full text article is available as a PDF file from https://os.copernicus.org/articles/5/421/2009/os-5-421-2009.pdf

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.

 References Baringer, M. O. and Larsen, J. C.: Sixteen years of Florida Current transport at 27° N, Geophys. Res. Lett., 28, 3179–3182, 2001. Brown, W., Munk, W., Snodgrass, F., Mofjeld, H., and Zetler, B.: MODE Bottom Experiment, J. Phys. Oceanogr., 5, 75–85, 1975. Bryden, H. L., Johns, W. E., and Saunders, P. M.: Deep western boundary current east of Abaco: Mean structure and transport, J. Mar. Res., 63, 35–57, 2005. Chelton, D. B., Schlax, M. G., Samelson, R. M., and de Szoeke, R. A.: Global observations of large oceanic eddies, Geophys. Res. Lett., 34, L15606, https://doi.org/10.1029/2007GL030812, 2007. Cunningham, S. A., Kanzow, T., Rayner, D., Baringer, M. O., Johns, W. E., Marotzke, J., Longworth, H. R., Grant, E. M., Hirschi, J. J.-M., Beal, L. M., Meinen, C. S., and Bryden, H. L.: Temporal Variability of the Atlantic Meridional Overturning Circulation at 26.5° N, Science, 317, 935–938, 2007. Cunningham, S. A., Rayner, D., and Marotzke, J., et al.: Rapid mooring cruise report April–May 2005, RRS Charles Darwin Cruise CD170 and RV Knorr Cruise KN182-2, National Oceanography Centre Southampton Cruise Report 2, Southampton, UK, 147 pp., 2006. Hirose, N., Fukumori, I., and Ponte, R. M.: A non-isostatic global sea level response to barometric pressure near 5 days, Geophys. Res. Lett., 28, 2441–2444, 2001. Johns, W. E., Beal, L. M., Baringer, M. O., Molina, J. R., Cunningham, S. A., Kanzow, T., and Rayner, D.: Variability of shallow and deep western boundary currents off the Bahamas during 2004–05: Results from the 26° N RAPID-MOC Array, J. Phys. Ocean., 38, 605–623, 2008. Kanzow, T., Cunningham, S. A., Rayner, D., Hirschi, J. J.-M., Johns, W. E., Baringer, M. O., Bryden, H. L., Beal, L. M., Meinen, C. S., and Marotzke, J.: Observed flow compensation associated with the MOC at 26.5° N in the Atlantic, Science, 317, 938–941, 2007. Kanzow, T., Flechtner, F., Chave, A., Schmidt, R., Schwintzer, P., and Send, U.: Seasonal variation of ocean bottom pressure derived from Gravity Recovery and Climate Experiment (GRACE): Local validation and global patterns, J. Geophys. Res., 110, C09001, https://doi.org/10.1029/2004JC002772, 2005. Kanzow, T., Hirschi, J. J.-M., Meinen, C., Rayner, D., Cunningham, S. A., Marotzke, J., Johns, W. E., Bryden, H. L., Beal, L. M., and Baringer, M. O.: A prototype system for observing the Atlantic meridional overturning circulation – scientific basis, measurement and risk mitigation strategies, and first results, Journal of Operational Oceanography, 1, 19–28, 2008. Kanzow, T., Johnson, H., Marshall, D., Cunningham, S. A., Hirschi, J. J.-M., Mujahid, A., Bryden, H. L., and Johns, W. E.: Observing basin-wide integrated volume transports in an eddy-filled ocean, J. Phys. Ocean., https://doi.org/10.1175/2009JPO4185.1, in press, June 2009. Kanzow, T., Send, U., Zenk, W., Chave, A., and Rhein, M.: Monitoring the deep integrated deep meridional flow in the tropical North Atlantic: Long-term performance of a geostrophic array, Deep-Sea Res. Pt. I, 53, 528–546, 2006. Longworth, H. R.: Constraining variability of the Atlantic meridional overturning circulation at 25° N from historical observations, 1980–2005, Ph.D. thesis, School of Ocean and Earth Science, University of Southampton, Southampton, UK, 200 pp., 2007. Luther, D. S.: Evidence of a 4–6 day barotropic, planetary oscillation in the Pacific Ocean, J. Phys. Oceanogr., 12, 644–657, 1982. Woodworth, P. L., Windle, S. A., and Vassie, J. M.: Departures from the local inverse barometer model at periods of 5 days in the central South Atlantic, J. Geophys. Res., 100, 18281–18290, 1995. Wunsch, C.: Mass and volume transport variability in an eddy-filled ocean, Nat. Geosci., 1, 165–168, https://doi.org/10.1038/ngeo126, 2008.