Ocean Science
www.ocean-sci.net
1812-0784
1812-0792
2
2
2006
10.5194/os-2-147-2006
http://www.ocean-sci.net/2/147/2006/
http://www.ocean-sci.net/2/147/2006/os-2-147-2006.html
http://www.ocean-sci.net/2/147/2006/os-2-147-2006.pdf
147
159
2006-10-12
Temporal and spatial characteristics of sea surface height variability in the North Atlantic Ocean
D. Cromwell
ddc@noc.soton.ac.uk
Ocean Observing and Climate, National Oceanography Centre, Southampton (NOCS), Room 254/35, European Way, Southampton SO14 3ZH, UK
We investigate the spatial and temporal variability of sea surface height
(SSH) in the North Atlantic basin using satellite altimeter data from
October 1992–January 2004. Our primary aim is to provide a detailed
description of such variability, including that associated with propagating
signals. We also investigate possible correlations between SSH variability
and atmospheric pressure changes as represented by climate indices.
<P>
We first investigate interannual SSH variations by deriving the complex
empirical orthogonal functions (CEOFs) of altimeter data lowpass-filtered at
18 months. We determine the spatial structure of the leading four modes
(both in amplitude and phase) and also the associated principal component
(PC) time series. Using wavelet analysis we derive the time-varying spectral
density of the PCs, revealing when particular modes were strongest between
1992–2004. The spatial pattern of the leading CEOF, comprising 30% of the
total variability, displays a 5-year periodicity in phase; signal
propagation is particularly marked in the Labrador Sea. The second mode,
with a dominant 3-year signal, has strong variability in the eastern basin.
<P>
Secondly, we focus on the Azores subtropical frontal zone. The leading mode
(35%) is strong in the south and east of this region with strong
variations at 3- and 5-year periods. The second mode (21%) has a
near-zonal band of low variance between 22°–27° N,
sandwiched between two regions of high variance. Thirdly, we lowpass filter
the altimeter data at a cutoff of 30 days, instead of 18 months, in order to
retain signals associated with propagating baroclinic Rossby waves and/or
eddies. The leading mode is the annual steric signal, around 46% of the
SSH variability. The third and fourth CEOFs, 11% of the remaining
variability, are associated with westward propagation which is particularly
dominant in a "waveband" between 32°–36° N.
<P>
For all three cases considered above, no significant cross-correlation is
found between the North Atlantic Oscillation index and the amplitude of the
leading four PCs of interannual SSH variability. The only exception is an
anti-correlation found over the North Atlantic basin between the NAO and the
4th PC. In the subtropical front, the East Atlantic Pattern index is
anti-correlated with the leading PC for SSH variations lowpass filtered at
30 days. Further investigation of forcing mechanisms is suggested using
hindcasts from ocean general circulation models.
Barnston, A. G. and Livezey, R. E.: Classification, seasonality and persistence of low-frequency atmospheric circulation patterns, Mon. Weather Rev., 115, 1083–1126, 1987.
Bellucci, A. and Richards, K. J.: Effects of NAO variability on the North Atlantic Ocean circulation, Geophys Res. Lett., 33, L02612, doi:10.1029/2005GL024890, 2006.
Broecker, W. S.: The great ocean conveyor, Oceanography, 4, 79–89, 1991.
Bryden, H. L., Longworth, H. R., and Cunningham, S. A.: Slowing of the Atlantic meridional overturning circulation at 25° N, Nature, 438, 655–657, 2005.
Chelton, D. B. and Schlax, M. G.: Global observations of oceanic Rossby waves, Science, 272, 234–238, 1996.
Cipollini, P., Cromwell, D., Jones, M. S., Quartly, G. D., and Challenor, P. G.: Concurrent altimeter and infrared observations of Rossby wave propagation near 34° N in the Northeast Atlantic, Geophys. Res. Lett., 24, 889–892, 1997.
Cipollini, P., Cromwell, D., and Quartly, G. D.: Observations of Rossby wave propagation in the northeast Atlantic with TOPEX/POSEIDON altimetry, Adv. Space Res., 22, 1553–1556, 1998.
CLS: SSALTO/DUACS user handbook: (M)SLA and (M)ADT near-real time and delayed time products, Ramonville St-Agne – FRANCE CLS-DOS-NT-04.103, 2004.
Cromwell, D.: Sea surface height observations of the 34° N `waveguide' in the North Atlantic, Geophys. Res. Lett., 28, 3705–3708, 2001.
Eden, C. and Willebrand, J.: Mechanism of Interannual to Decadal Variability of the North Atlantic Circulation, J. Clim., 14, 2266–2280, 2001.
Emery, W. J. and Thomson, R. E.: Data Analysis Methods in Physical Oceanography, Pergamon, Oxford, 1st edition, 1998.
Farge, M.: Wavelet transforms and their applications to turbulence, Annual Review of Fluid Mechanics, 24, 395–457, 1992.
Foufoula-Georgiou, E. and Kumar, P.: Wavelets in Geophysics, San Diego, Academic Press, 1994.
Fu, L.-L.: The interannual variability of the North Atlantic Ocean revealed by combined data from TOPEX/Poseidon and Jason altimetric measurements, Geophys Res. Lett., 31, L23303, doi:10.1029/2004GL021200, 2004.
Gill, A. E.: Atmosphere-Ocean Dynamics, San Diego, Academic Press, 662pp., 1982.
Gould, W. J.: Physical Oceanography of the Azores Front, Progress in Oceanography, 14, 167–190, 1985.
Häkkinen, S.: Variability in sea surface height: A qualitative measure for the meridional overturning in the North Atlantic, J. Geophys. Res., 106, 13 837–13 848, 2001.
Häkkinen, S. and Rhines, P. B.: Decline of Subpolar North Atlantic Circulation During the 1990s, Science, 304, 555–559, 2004.
Jacobs, G. A., Hurlburt, H. E., Kindle, J. C., Metzger, E. J., Mitchell, J. L., Teague, W. J., and Wallcraft, A. J.: Decade-scale trans-Pacific propagation and warming effects of an El Niño anomaly, Nature, 370, 360–363, 1994.
Killworth, P. D., Cipollini, P., Uz, B. M., and Blundell, J. R.: Physical and biological mechanisms for planetary waves observed in sea-surface chlorophyll, J. Geophys. Res., 109, C07002, doi:10.1029/2003JC001768, 2004.
Le Traon, P. Y. and Ogor, F.: ERS-1/2 orbit improvement using TOPEX/Poseidon: the 2 cm challenge, J. Geophys. Res., 103, 8045–8057, 1998.
Le Traon, P. Y., Nadal, F., and Ducet, N.: An improved mapping method of multi-satellite altimeter data, J. Atmos. Oceanic Technol., 25, 522–534, 1998.
Levermann, A., Griesel, A., Hofmann, M., Montoya, M., and Rahmstorf, S.: Dynamic sea level changes following changes in the thermohaline circulation, Clim. Dyn., 24, 347–354, 2005.
Marshall, J., Kushnir, Y., Battisti, D., Chang, P., Czaja, A., Hurrell, J., McCartney, M., Saravanan, R., and Visbeck, M.: North Atlantic climate variability, Int. J. Clim., 21, 1863–1898, 2001.
Pingree, R. D.: The eastern Subtropical gyre (North Atlantic): Flow, Rings, Recirculations, Structure and Subduction, Journal of the Marine Biological Association of the United Kingdom, 77, 573–624, 1997.
Pingree, R. D.: Ocean structure and climate (Eastern North Atlantic): in situ measurement and remote sensing (altimeter), Journal of the Marine Biological Association of the UK, 82, 681–707, 2002.
Pingree, R. D: North Atlantic and North Sea Climate Change: curl up, shut down, NAO and Ocean Colour, Journal of the Marine Biological Association of the United Kingdom, 85, 1301–1315, 2005.
Pingree, R. D. and Sinha, B.: Westward moving waves or eddies (\textitStorms) on the Subtropical/Azores Front near 32.5° N? Interpretation of the Eulerian currents and temperature records at moorings 155 (35.5° W) and 156 (34.4° W), J. Marine Syst., 29, 239–276, 2001.
Schiermeier, Q.: Gulf Stream probed for early warnings of system failure, Nature, 427, 769, 2004.
Schiermeier, Q.: A sea change, Nature, 439, 256–260, 2006.
Tokmakian, R. T. and Challenor, P. G.: Observations in the Canary Basin and the Azores Frontal Region Using Geosat Data, J. Geophys. Res., 98, 4761–4773, 1993.
Torrence, C. and Compo, G. P.: practical guide to wavelet analysis, Bulletin of the American Meteorological Society, 79, 61–78, 1998.
Tourre, Y. M., Kushnir, Y., and White, W. B.: Evolution of Interdecadal Variability in Sea Level Pressure, Sea Surface Temperature, and Upper Ocean Temperature over the Pacific Ocean, J. Phys. Ocean., 29, 1528–1541, 1999.
Tychensky, A., Le Traon, P. Y., Hernandez, F., and Jourdan, D.: Large structures and temporal change in the Azores Front during the SEMAPHORE experiment, J. Geophys. Res., 103, 25 009–25 027, 1998.
von Storch, H. and Zwiers, F. W.: Statistical Analysis in Climate Research, Cambridge University Press, 1999.