Impact of the winter North-Atlantic weather regimes on subtropical sea-surface height variability
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Interannual variability of subtropical sea-surface-height (SSH) anomalies, estimated by satellite and tide-gauge data, is investigated in relation to wintertime daily North-Atlantic weather regimes. Sea-level anomalies can be viewed as proxies for the subtropical gyre intensity because of the intrinsic baroclinic structure of the circulation. Our results show that the strongest correlation between SSH and weather regimes is found with the so-called Atlantic-Ridge (AR) while no significant values are obtained for the other regimes, including those related to the North Atlantic Oscillation (NAO), known as the primary actor of the Atlantic dynamics. Wintertime AR events are characterized by anticyclonic wind anomalies off Europe leading to a northward shift of the climatological wind-stress curl. The latter affects subtropical SSH annual variability by altered Sverdrup balance and ocean Rossby wave dynamics propagating westward from the African coast towards the Caribbean. The use of a simple linear planetary geostrophic model allows to quantify those effects and confirms the primary importance of the winter season to explain the largest part of SSH interannual variability in the Atlantic subtropical gyre. Our results open new perspectives in the comprehension of North-Atlantic Ocean variability emphasizing the role of AR as a driver of interannual variability at least of comparable importance to NAO.
KeywordsAtlantic Meridional Overturning Circulation North Atlantic Oscillation Subtropical Gyre Baroclinic Mode Weather Regime
NCEP Reanalysis data were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site (http://www.esrl.noaa.gov/psd/). The altimeter products were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (http://www.aviso.oceanobs.com/duacs/). Tide gauge data were obtained from the Permanent Service For Mean Sea Level website (http://www.psmsl.org/). The authors acknowledge Cécile Cabanes for having provided us the matlab routines of the baroclinic component of her model and for fruitful discussions. Nicolas Barrier is supported by a PhD grant from Unniversité de Bretagne Occidentale, Ifremer and Europôle Mer. Anne-Marie Treguier, Christophe Cassou and Julie Deshayes acknowledge the CNRS.
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