Interannual-to-decadal variability of the North Atlantic from an ocean data assimilation system

• Published in: Climate Dynamics
• Main Authors: Masina, S., Di Pietro, P., Navarra, A.
• Other Authors: Masina, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia, Di Pietro, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia, Navarra, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia, Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia
• Format: Article in Journal/Newspaper
• Language: English
• Published: Springer Verlag GMBH Germany 2004
• Subjects: Ocean dynamics; Ocean modelling; Ocean Reanalysis; data assimilation; Interannual variability; decadal variability; 03. Hydrosphere::03.01. General::03.01.04. Ocean data assimilation and reanalysis; North Atlantic; North Atlantic oscillation
• Online Access: http://hdl.handle.net/2122/8947; https://doi.org/10.1007/s00382-004-0453-6

An ocean analysis, assimilating both surface and subsurface hydrographic temperature data into a global ocean model, has been produced for the period 1958–2000, and used to study the time and space vari- ations of North Atlantic upper ocean heat content (HC). Observational evidence is presented for interannual-to- decadal variability of upper ocean thermal fluctuations in the North Atlantic related to the North Atlantic Oscillation (NAO) variability over the last 40 years. The assimilation scheme used in the ocean analysis is a uni- variate, variational optimum interpolation of tempera- ture. The first guess is produced by an eddy permitting global ocean general circulation forced by atmospheric reanalysis from the National Center for Environmental Prediction (NCEP). The validation of the ocean analysis has been done through the comparison with objectively analyzed observations and independent data sets. The method is able to compensate for the model systematic error to reproduce a realistic vertical thermal structure of the region and to improve consistently the model estimation of the time variability of the upper ocean temperature. Empirical orthogonal function (EOF) analysis shows that an important mode of variability of the wintertime upper ocean climate over the North Atlantic during the period of study is characterized by a tripole pattern both for SST and upper ocean HC. A similar mode is found for summer HC anomalies but not for summer SST. Over the whole period, HC variations in the subtropics show a general warming trend while the tropical and north eastern part of the basin have an opposite cooling tendency. Superimposed on this linear trend, the HC variability explained by the first EOF both in winter and summer conditions reveals quasi- decadal oscillations correlated with changes in the NAO index. On the other hand, there is no evidence of cor- relation in time between the NAO index and the upper ocean HC averaged over the whole North Atlantic which exhibits a substantial and monotonic warming trend ...