Progressing in the understanding of small-scale nonlinear ocean dynamics and ocean-atmosphere interactions by innovative joint observing and modelling approaches

Author:

Sabrina Speich - (LMD)

Co-Investigator(s):

Xavier Carton (LOPS)
Gilles Reverdin (LOCEAN)

Abstract:

The transport by the ocean and the exchanges at the air/sea interface of heat, water and gas is key to regulating the state and evolution of our climate. Recent studies suggest that these phenomena are governed by the ocean smallscale, 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 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. EUREC4A-OA constitutes the ocean component of EUREC4A. It investigates heat, momentum, water and CO2 transport within the ocean and exchange across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. EUREC4A-OA focuses on meso- and submesoscale ocean dynamics 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 (Saildrone), 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 will also allow 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 specialists of ocean, atmosphere physical and biogeochemical observations and numerical modelling as well as scientists working on numerical parameterization and future projections to address four key objectives: 1) To assess the impact of the diurnal cycle on oceanatmosphere 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) To identify and quantify the processes governing the internal dynamics of the ocean and the ocean-atmosphere exchanges of water, heat, momentum and CO2 at these scales; 3) Identify 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 for global models (ocean-atmosphere and ESM). The project has already received fundings for the realization of the field experiment (from INSU-CNRS, ENS, CNES-TOSCA –within the TOEddies project–, Ifremer, GEOMAR, NOAA, NASA and from the French and German research vessels fleets), and to initiate and run the various ocean and coupled ocean-atmosphere simulations (JPI Ocean & Climate and NOAA). The field experiment that took place in January-February 2020 has been very successful in collecting a wide variety of ocean-atmosphere data resolving the ocean small scales across a vast domain of the Tropical North Atlantic Ocean. This has been possible because of the availability on board of the ships of real-time satellite data as well as numerical atmosphere and ocean predictions that enabled an original adaptive routing of the ships and autonomous vehicles on a daily basis. In particular, satellite altimetry data have been essential in locating the largest mesoscale features. These data have been used together with other satellite NRT fields (SST and Chl-a, surface salinity, wind, sea state etc) distributed by CLS, ODATIS, AERIS, CMEMS, SMOS, SMAP, CATDS, PODACC to locate, also with very high precision, the submesoscale features surrounding mesoscale eddies and fronts. The entire set of satellite data we used during the field experiment are therefore key to the analyses of the collected observations and in understanding the key processes. The analyses of these data used in parallel to the in situ ones is at the core of this project.

The project gathers several teams from 10 different French laboratories (LMD, LOCEAN, LATMOS and IPGP in Ile de France, LOPS and Lab-STICC in Brest, MIO and IRPHE in Marseille, LEGOS, and CNRM in Toulouse). CLS, the AERIS and ODATIS "database" infrastructures, and the Coriolis service enabled us to obtain all the data (satellite and in situ) we needed in near-real time and agreed to collaborate in the implementation of the project analyses. In addition, the data acquired by EUREC4A and therefore EUREC4A-OA are already available on DataTerra's AERIS database. The ocean data will be migrated after validation and calibration to the ODATIS database (and will remain mirrored on AERIS) EUREC4A-OA will deliver novel knowledge to improve seamless ocean-atmosphere numerical predictions and climate projections and will have a significant impact on science and society.

Supported by CNES.