Nuno Serra, Detlef Stammer, and Armin Köhl

The goal of the CLIVAR project is a quantitative definition of all observation and modeling components that are necessary for detecting the influence of large-scale changes on regional conditions in the North Atlantic area.

For understanding future or ongoing changes in the North Atlantic, syntheses methods are necessary that combine dynamical concepts as formulated by numerical models with observed data. This optimal description of the changing circulation and their associated forcing fields is also called reanalysis. A goal of this project is a dynamically consistent estimation of the North Atlantic circulation and the associated surface forcing fields by means of data assimilation over the last 50 years, which can serve as basis for studies of the ocean circulation and its interaction with the atmosphere.

The assimilation is based to MIT model and uses the adjoint method to bring the trajectory of the model into consistency with all available oceanic observations over the last 50 years. To this end, the initial conditions of the model, the surface forcing fields as well as freshwater fluxes from rivers runoff and ice sheet melting, become adjusted in an optimal way.

Scientific applications of the syntheses comprise in the comprehensive description of changes of the meridional overturning circulation (MOC) and the meridional heat and freshwater transports, the exchange between subpolar/subtropical/tropical Atlantic as well as the fluxes between ocean and atmosphere in the Atlantic region. A special focus is on the determination of dominant variability patterns of the Atlantic circulation.

A further focus is on the identification of optimal observation strategies, which will be necessary for detection of the predicted changes and their associated climatic processes in the Atlantic region. Sensitivity studies allow studying the response of the circulation and the sea level in the North Atlantic to a drastic fresh water flux from Greenland ice sheet melting. These sensitivity studies form together with measuring campaigns the basis for studying the reaction of the North Atlantic circulation, including the strength of the MOC, to changes of the northern boundary condition or surface fluxes. Adjoint the model can also be used to identify key regions for measurement campaigns especially designed to identify in the future rapid changes of the MOC and other critical characteristics of the North Atlantic circulation.

Nuno Serra