The transport by the ocean and the exchanges at the air/sea interface of heat, water and gas are key to regulating the state and evolution of our climate. Recent studies suggest that these phenomena are governed by the ocean small-scale, which has characteristic lengths ranging from 100 m to 100 km and varies in time periods of a few hours to weeks. To initiate and support societal actions as a response to climate change, future projections of the climate system require high-resolution coupled climate model simulations or new parametrization of small-scale processes for coarser-grid models. Indeed, a generic challenge for high-resolution modelling is the need to resolve processes that have typically been parameterized in coarse-grid simulations. Thus, it is becoming evident that a clear lack of process understanding exists to improve and quantitatively evaluate model seamless predictions (from weather to seasonal and decadal forecasts) and projections, and to understand why different models give different answers. 

EUREC⁴A-OA is a large international project that leveraged from and extensively complemented the ElUcidating the RolE of Clouds-Circulation Coupling in ClimAte (EUREC⁴A, www.eurec4a.eu) initiative (Bony et al. 2017) that aims to advance understanding of the interplay between clouds, convection and circulation, and their role in climate change. 

The core of EUREC⁴A-OA (and EUREC⁴A) has been a one-month (Jan/Feb 2020) field study in the western tropical North Atlantic Ocean where high-resolution, synchronized observational data have been collected using cutting-edge technology on airplanes, ships, autonomous vehicles, augmented with the Barbados Cloud Observatory time series (Stevens et al., 2021). EUREC⁴A-OA constitutes the ocean component of EUREC⁴A. It investigates heat, momentum, water and CO₂ transport within the ocean and exchanges across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. EUREC⁴A-OA focuses on ocean dynamics at the small-scale (0.1¿100 km) and related atmospheric boundary layer processes. EUREC⁴A-OA is centered on the tropics where the primary external time scale affecting air-sea exchange is the diurnal cycle. However, the internal ocean and atmosphere dynamics convolute the diurnal, synoptic, seasonal and longer time scales to climate variability. 

EUREC⁴A-OA had made use of significant observing infrastructure investments from the participating countries, augmented with cutting edge third-party autonomous observing platforms (Saildrones, Autonaut, seagliders), to enable sampling of the upper ocean layers and the air/sea interface at temporal and spatial resolutions far higher than could be achieved through traditional observational approaches. Moreover, the success of EUREC⁴A-OA in observing such processes has been intimately linked to the sampling strategy for all the platforms that was based on a day-to-day analyses of various satellite near-real-time products (in particular multi-satellite altimetry, SST and Chl-a produced by CLS, and SSS from SMOS and SMAP). Such a strategy is now allowing a series of in-depth analyses of various combinations of satellite and in-situ data to gain insight in the targeted phenomena and to validate and refine numerical simulations. In fact, the EUREC⁴A-OA consortium has also set up an unparalleled hierarchy of numerical simulations ranging from Large Eddy Simulations (LES), including coupled ocean-atmosphere LES, to global high-resolution ocean-atmosphere simulations and Earth System Models (ESMs). The LES simulations resolve the ocean and the ocean-atmosphere systems explicitly at scales as small as 10 meters and thus allow the direct simulation of the ocean small-scale structures and of their interactions with the atmosphere. These will be used to inform the development and evaluation of the global, coupled Earth System Models.

The EUREC⁴A-OA project connects European and USA experts of ocean and atmosphere observations and modelling to enhance the understanding of key processes and improve the skill of forecasts and future projections. In particular, these experts addresses four key objectives:

1. The assessment of how the diurnal cycle impacts ocean-atmosphere exchanges of energy, water and CO2 and to quantify how the diurnal cycle and related exchanges vary and are influenced at the small ocean scale;
2. The identification and quantification of the processes governing the internal dynamics of the ocean and the ocean-atmosphere exchanges of water, heat, momentum and CO2 at fine scale (0.1-100 km);
3. The characterization of the different surface ocean processes (diurnal cycle, small-scale ocean dynamics, aerosols) responsible for shallow atmospheric convection and cloud formation;
4. Provide data fields and new numerical parameterizations improving numerical models and prediction capabilities.