Centennial Scale Elemental And Isotopic Variability In The Tropical And Subtropical North Atlantic Ocean

Author : Nicolas Barrier
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (899 download)

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Download or read book Variability of the Ocean Circulation in the North-Atlantic in Response to Atmospheric Weather Regimes written by Nicolas Barrier and published by . This book was released on 2013 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The aim of the PhD is to investigate the impacts of the large-Scale atmospheric variability on the North- Atlantic ocean circulation. This question has already been addressed in a large number of studies, in which the atmospheric variability is decomposed into modes of variability, determined by decomposing sea-Level pressure anomalies into Empirical Orthogonal Function (EOFs). These modes of variability are the North-Atlantic Oscillation (NAO), the East-Atlantic Pattern (EAP) and the Scandinavian Pattern (SCAN). EOF decomposition assumes that the modes are orthogonal and symmetric. The latter assumption, however, has been shown to be inadequate for the NAO. Hence, a different framework is used in this study to assess the atmospheric variability, the so-Called weather regimes. These are large-Scale, recurrent and quasi-Stationary atmospheric patterns that have been shown to capture well the interannual and decadal variability of atmospheric forcing to the ocean. Furthermore, they allow to separate the spatial patterns of the positive and negative NAO phases. Hence, these weather regimes are a promising alternative to modes of variability in the study of the ocean response to atmospheric variability. Using observations and numerical models (realistic or in idealised settings), we have shown that the Atlantic Ridge (AR), NAO- and NAO+ regimes drive a fast (monthly to interannual) wind-Driven response of the subtropical and subpolar gyres (topographic Sverdrup balance) and of the meridional overturning circulation (MOC, driven by Ekman transport anomalies). At decadal timescales, the subpolar gyre strengthens for persistent NAO+ and Scandinavian Blocking (BLK) conditions via baroclinic adjustment to buoyancy fluxes and slackens for persistent AR conditions via baroclinic adjustment to wind-Stress curl anomalies. The latter mechanism also accounts for the strengthening of the subtropical gyre for persistent NAO+ conditions and its weakening for persistent AR conditions. The gyres response to persistent NAO- conditions reflects the southward shift of the gyre system (the intergyre gyre). The MOC spins-Up for persistent NAO+ and BLK conditions via increased deep water formation in the Labrador Sea, and conversely for the NAO- and AR regimes. Last, heat budget calculations in the subpolar gyre and the Nordic Seas have been performed using four global ocean hindcasts. The winter averaged heat convergence in the western subpolar gyre is positively correlated with the NAO- winter occurrences, which is due to the intergyregyre circulation, while it is negatively correlated with AR winter occurrences, because of the wind-Driven reduction of both gyres. Downward surface heat flux anomalies are negatively correlated with NAO+ occurrences, and conversely for the NAO-. In the Nordic Seas, they are positively correlated with BLK and to a lesser extent AR occurrences. Furthermore, we suggest that the heat content variability in the western subpolar gyre is the signature of the delayed response (6-Year lag) to the time-Integrated NAO+ forcing, due to the combination of the immediate (0-Lag) response of surface heat flux and the lagged (3 year lag) response of ocean heat convergence.