A composable climate model in pure Julia

Coupled general circulation models of atmosphere and ocean form the heart of every earth-system model. Currently,  the only languages used to write a model at that complexity are Fortran and C, which are limited by traditional programming patterns. However, modern languages like Python and Julia shine with interactivity, accessibility, and extensibility, yet a coupled model of that complexity, written in a modern language, has not yet been attempted. Here, we present a coupled climate model implemented entirely in the Julia programming language. The model integrates an atmospheric component based on the SpeedyWeather.jl library with the ClimaOcean.jl package for ocean and sea-ice dynamics. This approach leverages Julia's strengths combining computational efficiency with a high-level programming interface. The result is a climate modeling framework that maintains computational efficiency without sacrificing flexibility. The dynamical core employs high-order numerical methods, combining Weighted Essentially Non-Oscillatory (WENO) advection schemes in the ocean and spectral methods in the atmosphere, ensuring a robust and accurate representation of transport processes. A high-level interface for coupling the codes is introduced, which is distinctly more flexible than traditional couplers. This interface allows running coupled simulations on laptops as well as high-performance computing (HPC) resources. We will present initial results from coupled idealized simulations, highlighting key features such as boundary currents in the ocean, convective patterns in the atmosphere, and air-sea interactions.