Understanding Causes of Climate Model Biases in the Southeastern Tropical Atlantic

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Recent studies have revealed that among all the tropical oceans, the Atlantic has experienced the most pronounced warming trend over the 20th century. Many extreme climate events are influenced by oceanic conditions over the Atlantic. It is therefore imperative to have accurate simulations of the Atlantic Ocean climate to be able to make credible projections of future climate in the surrounding continental regions. Unfortunately, current-generation models, including those used by the Intergovernmental Panel on Climate Change (IPCC), exhibit severe biases in their simulations of tropical Atlantic climate. An infamous example is a persistent warm sea surface temperature bias of at least 6°C off the coast of Namibia and Angola. This research aims at investigating the causes of this bias which will ultimately lead to major improvements in the simulation and projection of future climate changes within the Atlantic sector.Intellectual Merit: This project will explore oceanic and atmospheric biases, as well as amplification of biases from the coupled ocean-atmosphere system. The sources of oceanic biases will be analyzed and identified from oceanic observations, ocean reanalysis products, IPCC model simulations, and eddy resolving regional ocean simulations. Similarly, atmospheric biases will be evaluated using atmospheric reanalysis data and convection-resolving regional atmospheric model simulations. The role of ocean-atmosphere interactions in amplifying the oceanic or atmospheric biases will be examined from high-resolution coupled regional climate model (CRCM) simulations. The use of high-resolution (up to 3 km) regional climate models will allow processes such as oceanic mesoscale eddies, fronts and upwelling dynamics off the west coast of southern Africa, as well as atmospheric convection and steep orography effects, and thus will minimize model systematic errors due to uncertainties associated with parameterizations of subgrid-scale processes. This modeling approach will isolate sources of the biases from other ocean basins and will help to pinpoint the causes of the biases within the Atlantic basin. The study will utilize an array of fine temporal and spatial resolution data sets produced by recent intense observational programs within the Atlantic sector. Furthermore, the project will coincide with the launch of two major European research projects in the study region: a German project "Southwest African Coastal Upwelling System and Benguela Niño" (SACUS) led by Brandt, and an EU project "Enhancing prediction of tropical Atlantic climate and its impacts" (PREFACE) led by Keenlyside. Both of these projects will generate new arrays of observations along the southern Africa coast. Close collaboration with the European colleagues will allow their new observations to be combined with US-based detailed modeling that will likely lead to new breakthroughs in solving tropical Atlantic bias problems.Broader Impacts: This research will enhance our understanding of the uncertainties in projecting impacts of global climate change in the Atlantic region, which has enormous socio-economic implications for countries in the region. The CRCM is being developed within the framework of the Coupler (CPL7) software that is part of the National Center for Atmospheric Research Community Earth System Model (CESM). This framework allows easy transfer of parameterizations between CESM and the CRCM, and potential partial coupling between the CESM and the CRCM. We envision that the CRCM will become a community-modeling tool and be made available to the broader climate research community. Findings from this study on reducing climate model biases will feedback to community model development activities. The project will support one graduate student and one postdoc at Texas A&M University (TAMU) and provide valuable professional development for all young scientists involved, including Co-PI Patricola.