Ocean state reconstruction using the analog method and in situ observations : application to tropical Pacific variability

– Erwan Oulhen, Postdoc au LOPS, Brest –

Résumé :

In the tropical Pacific, the El Niño – Southern Oscillation (ENSO) represents a major component of climatic variability.Precise analysis of ocean temperature and salinity observations could improve our understanding of ENSO, its complexity, and its evolution in relation to Tropical Pacific Decadal Variability (TPDV) and climate change associated with the increase in greenhouse gases. TPDV is linked to multiple oceanic and atmospheric phenomena, influencing ENSO and making it more challenging to estimate changes linked to anthropogenic forcing. Understanding TPDV is a major challenge for climate research. Over the past two decades, Argo floats have produced an unprecedented database of temperature and salinity observations, significantly reducing uncertainties in their analyses. However, before the Argo period, observations were scarcer and more challenging to analyze. In this thesis, Argo data are used to estimate the spatial covariance between ocean temperature and salinity, and to build a dynamic model based on the Analog method, propagating information from the future to the past. These elements are combined in RedAnDA (Reduced-space Analog Data Assimilation), a data-driven analysis method applied to pre-Argo observations in the tropical Pacific. The development, implementation, and validation of RedAnDA represent a major part of the work carried out. Validation was conducted using model outputs and synthetic observations, as well as comparing the results obtained with real data to other widely used temperature and salinity data analysis products. Reconstructions of salinity and temperature in the tropical Pacific, for the 20th century, were thus obtained, with improved representation of the interannual variability associated with ENSO. Using these reconstructions and preliminary analyses, phenomena associated with low-frequency variability were studied, enabling their description and the establishment of their mutual modulations. In particular, the climate change due to anthropogenic emissions appears to have led to cooling at the equator in the subsurface and warming of the Pacific, more pronounced at depths of 100 m. Surface salinity seems to be subject to an increase in the northwestern tropical Pacific, while in the southeast it appears to decrease. TPDV has been isolated from the signal, enabling its description in terms of salinity and temperature anomalies. These anomalies are mainly due to the vertical displacements of isopycnals (heave), but with a significant contribution from compensated temperature and salinity anomalies (spiciness).