Today, ocean modeling is fast developing as a versatile tool for the study of earth’s climate, local marine ecosystems and coastal engineering applications. Though the field of ocean modeling began in the early 1950s along with the development of climate models and primitive computers, even today, the state-of-the-art ocean models have their own limitations. Many issues still remain such as the uncertainity in the parameterisation of essential processes that occur on spatial and temporal scales smaller than that can be resolved in model calculations, atmospheric forcing of the ocean and the boundary and initial conditions. The advent of data assimilation into ocean modeling has heralded a new era in the field of ocean modeling and oceanic sciences. “Data assimilation” is a methodology in which observations are used to improve the forecasting skill of operational meteorological models. The study in the present thesis mainly focuses on obtaining a four dimensional realization (the spatial description coupled with the time evolution) of the oceanic flow that is simultaneously consistent with the observational evidence and with the dynamical equations of motion and to provide initial conditions for predictions of oceanic circulation and tracer distribution. A good implementation of data assimilation can be achieved with the availability of large number of good quality observations of the oceanic fields as both synoptic and in-situ data. With the technology in satellite oceanography and insitu measurements advancing by leaps over the past two decades, good synoptic and insitu observations of oceanic fields have been achieved. The current and expected explosion in remotely sensed and insitu measured oceanographic data is ushering a new age of ocean modeling and data assimilation. The thesis presents results of analysis of the impact of data assimilation in an ocean general circulation model of the North Indian Ocean. In this thesis we have studied the impact of assimilation of temperature and salinity profiles from Argo floats and Sea Surface height anomalies from satellite altimeters in a Sigma-coordinate Indian Ocean model. An ocean data assimilation system based on the Regional Ocean Modeling System (ROMS) for the Indian Ocean is used. This model is implemented, validated and applied in a climatological simulation experiment to study the circulation in the Indian Ocean. The validated model is then used for the implementation of the data assimilation system for the Indian Ocean region. This dissertation presents the qualitative and quantitative comparisons of the model simulations with and without subsurface temperature and salinity profiles and sea surface height anamoly data assimilation for the Indian Ocean region. This is the first ever reported data assimilation studies of the Argo subsurface temperature and salinity profile data with ROMS in the Indian Ocean region.

Les évènements de convection profonde sont importants car ils forment les masses d'eau intermédiaires et profondes qui nourrissent la circulation globale. La mer du Labrador, qui forme la Labrador Sea Water (LSW), est le site le plus documenté de l'océan Atlantique Nord. La mer d'lrminger a également été citée mais n'est pas entièrement reconnue à cause du manque d'observations directes. Cette thèse fournit la première description de la convection profonde en mer d'lrminger à l'échelle du bassin grâce aux données Argo. Trois évènements de convection se sont produits en mer d'lrminger depuis 2010. Au cours de l'hiver 2O11-2O12, la convection atteint 1000 m et est expliquée par la séquence d'apparition des tip jets groenlandais.La convection de l'hiver 2O13-2O14, qui atteint 1300 m, est caractérisée par un préconditionnement particulièrement important et un forçage par les tip jets faible. La convection de l'hiver 2O14-2O15, qui atteint 1700 m, montre des tip jets très nombreux et persistants. L'advection de LSW provenant de la mer du Labrador explique les profondeurs exceptionnelles observées au cours de ces deux derniers hivers. Les résultats montrent que la convection n'est pas rare en mer d'lrminger et qu'elle joue un rôle non négligeable sur l'équilibre climatique. / The deep convection events are important because they form the intermediate and deep water masses feeding the global circulation. The Labrador Sea is the main site of deep convection in the North Atlantic Ocean and produces the intermediate Labrador Sea Water (LSW). The lrminger Sea was also cited but was forgotten during decades because of the lack of direct observations. This thesis provides the first description of the lrminger Sea deep convection at basin scale, thanks to the Argo data. Three convective events occurred in the lrminger Sea since 2010. During the 2011-2012 winter, the convection reached 1000 m and is explained by the sequence of the Greenland tip jets. The event of the 2O13-2O14 winter, reaching 130O m, is characterized by a strong preconditioning and a weak forcings by the Greenland tip jets.The convection event of the 2O14-2015 winter, reaching 1700 m, shows many of persistant tip jets. The advection of LSW from the Labrador Sea explains the deepest mixed layers observed during the last two winters. The results show that deep convection in the lrminger Sea is not a rare isolated event and plays a significant role on the climate balance.