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Improved Understanding of air-sea interaction processes and biases in the Tropical Western Pacific using observation sensitivity experiments and global forecast models

We propose to determine physical mechanisms governing air-sea interactions in the tropical west Pacific at the eastern edge of the warm pool by isolating coupled feedback processes through analyses of short-term coupled and uncoupled forecasts. Climate model forecasts of the Madden–Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO) experience a systematic climate drift resulting in biases of the modeled tropical western Pacific climatology. Global models tend to have an excess rainfall in the warm pool region and a deficiency in rainfall at the eastern edge of the warm pool. We propose to increase understanding of the dynamics and thermodynamics in the region by utilizing the Community Earth System Model (CESM) global runs as well as high-resolution Massachusetts Institute of Technology general circulation model (MITgcm) regional uncoupled simulations.
Studying coupled vs uncoupled forecasts initialized with different ocean reanalysis products reveals the impact of ocean data assimilation on forecast error growth in the Tropical West Pacific region. The European Centre for Medium-Range Weather Forecasts (ECMWF) has recently completed several observation sensitivity experiments (OSE) ocean analyses by individually assimilating different ocean observation platforms (Argo, moorings, satellite, and others) into ocean analyses state estimates. Initializing our coupled forecasts from these OSE experiments will show the impact of different ocean observations on forecasts in the Tropical Pacific region. The PI (Subramanian) collaborates closely with Dr. Magdalena Balmaseda (ECMWF) who has given permission to use these OSE solutions. The proposed work will inform the role of forecast sensitivity to ocean state and air-sea feedbacks in the warm pool eastern edge region. Also the experiments with different ocean initializations will show the impact of various ocean observation platforms in constraining regional biases. The proposed analyses will help prioritize process study field campaigns and ocean observation platforms to best constrain modeling and parameterization development efforts. Analyzing air-sea interactions and the oceanic mixed layer heat and salt budgets in the model forecasts and in the reanalyses fields will reveal discrepancies in components of the budgets. This budget and flux analysis will yield a process-based understanding of the regional mixed layer, barrier layer, and MJO dynamics, feedbacks, and sensitivities.
This research will identify the physical processes that lead to the mean and intraseasonal variability biases in the Tropical West Pacific and, more specifically, the key biases controlling MJO and ENSO background states. This understanding prioritizes process studies and observational field campaigns in the region as a part of TPOS 2020 on which processes to prioritize for observations. The work will also inform parameterization improvements for use in CESM for real-time forecasts of the climate-scale processes in the tropical Pacific.

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