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KLIMAFORSK-Stort program klima

Improving the representation of small-scale nonlinear ocean-atmosphere interactions in Climate Models

Awarded: NOK 5.9 mill.

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. An important challenge for high-resolution modelling is the need to resolve processes that have typically been parameterized in coarse-grid simulations. The exchange of heat, water and gas at the air-sea interface is key to regulating the state and evolution of our climate. Large exchanges can occur at the air-sea interface on short time and small spatial scales. There is therefore an urgent need to understand the processes governing these exchanges, so that we can quantitatively evaluate model predictions and projections and understand why different models give different answers. EUREC4A-OA will address this issue through advancement of understanding of non-linear and small-scale ocean-atmosphere exchange processes and, in parallel, investigate their representation in coupled climate models of the CMIP Earth System Models (ESMs) family. EUREC4A-OA makes use of, and contributes to, the ElUcidating the RolE of Clouds- Circulation Coupling in ClimAte (EUREC4A) 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 is a one-month (Jan/Feb 2020) field study in the western tropical North Atlantic Ocean where high-resolution, synchronized observational data will be collected using cutting-edge technology on airplanes, ships, autonomous vehicles, augmented with the Barbados Cloud Observatory time series. EUREC4A-OA will add the ocean component to EUREC4A by investigating heat, momentum and CO2 exchange across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. Our focus is on meso- and submesoscale ocean dynamics and related atmospheric boundary layer processes. EUREC4A-OA is focused on the tropics where the primary external time scale affecting air-sea exchange is the diurnal cycle. We have developed a new, high-resolution version of the Norwegian coupled global climate model, NorESM, that has ¼ degree resolution in both the ocean and atmosphere. This model setup includes a framework for testing higher vertical resolution in the atmosphere to better resolve the atmospheric boundary layer processes, including the formation of low-level clouds, which are crucial for surface exchanges between ocean and atmosphere. We have designed our model experiments to use data assimilation in the ocean and spectral nudging in the atmosphere so that we can constrain both the slow ocean processes and the large-scale atmospheric circulation. This allows us to focus on how well the NorESM can reproduce the ocean-atmosphere coupling in the tropical Atlantic on short timescales. Our next step will be to perform sensitivity analysis of the cloud and boundary layer physics schemes to identify the largest sources of model error, and the sensitivity of this performance to vertical resolution.

The exchange of heat, water and gas at the air/sea interface is key to regulating the state and evolution of our climate. Sizeable air-sea exchanges of energy and ocean-atmosphere boundary layer processes can occur on short time and small spatial scales. 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. A generic challenge for high-resolution modelling is the need to resolve processes that have typically been parameterized in coarse-grid simulations. EUREC4A-OA will address this issue thorough advancement of understanding of non-linear and small-scale ocean-atmosphere exchanges processes and investigate their representation in the CMIP Earth System Models (ESMs) family. EUREC4A-OA will leverage from, and contribute to, the ElUcidating the RolE of Clouds-Circulation Coupling in ClimAte (EUREC4A) 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 is a one-month (Jan/Feb 2020) field study in the western tropical North Atlantic Ocean where high-resolution, synchronized observational data will be collected using cutting-edge technology on airplanes, ships, autonomous vehicles, augmented with the Barbados Cloud Observatory time series. EUREC4A-OA will add the ocean component to EUREC4A by investigating heat, momentum and CO2 exchange across the air/sea interface using innovative high-resolution ocean observations and a hierarchy of numerical simulations. Our focus is on meso- and submesoscale ocean dynamics and related atmospheric boundary layer processes. EUREC4A-OA is focused 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, seasonal and longer time scales to climate variability.

Funding scheme:

KLIMAFORSK-Stort program klima