[1] We present the concept of the Carbon Cycle Data Assimilation System and describe its evolution over the last two decades from an assimilation system around a simple diagnostic model of the terrestrial biosphere to a system for the calibration and initialization of the land component of a comprehensive Earth system model. We critically review the capability of this modeling framework to integrate multiple data streams, to assess their mutual consistency and with the model, to reduce uncertainties in the simulation of the terrestrial carbon cycle, to provide, in a traceable manner, reanalysis products with documented uncertainty, and to assist the design of the observational network. We highlight some of the challenges we met and experience we gained, give recommendations for operating the system, and suggest directions for future development.

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In this paper, we present a new linear system solver for use in a fully-implicit ocean model. The new solver allows to perform bifurcation analysis of relatively high-resolution primitive-equation ocean-climate models. It is based on a block-ILU approach and takes special advantage of the mathematical structure of the governing equations. In implicit models Jacobian matrices have to be constructed. Analytical construction is hard for complicated but more realistic representations of mixing. This is overcome by evaluating the Jacobian in part numerically. The performance of the new implicit ocean model is demonstrated using (i) a high-resolution model of the wind-forced double-gyre flow problem in a (relatively small) midlatitude spherical basin, and (ii) a medium-resolution model of thermohaline and wind-driven flows in an Atlantic size single-hemispheric basin.

Agulhas Leakage is an important link in the global ocean circulation, as it transfers a significant volume of relatively warm and salty water from the Indian Ocean to the Atlantic Ocean. The main route of this transfer is through the shedding of large Agulhas rings from the Agulhas retroflection. In this paper we study the dynamics of the ring formation process by analyzing the stability of the Indian/Atlantic supergyre in a reduced gravity model. We show that the ring‒shedding process results from a barotropic instability of the steady circulation in the Agulhas retroflection region. The destabilizing mode appears to be linked to a Rossby basin mode of the combined South Indian/Atlantic basin, which is localized in the retroflection region by the background flow.

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