Characterization of the Arabian Sea warm pool transition layer using a 2-way nested data-assimilative coupled model with supporting in situ and satellite ocean

(Approved for Public Release) The ASTraL stated goal of advancing our understanding of the dynamical mechanisms that control the structure within and the coupling of the ocean and the atmospheric boundary layers in the Arabian Sea (AS) will be addressed in this project using a 2-way nested data-assimilative coupled model. The proposed work is motivated by two current lines of scientific knowledge summarized in this document: (1) The genesis and strong year-to-year AS warm pool variability is determined by the complex interaction of several dynamical processes, and (2) The AS warm pool variability affects the year-to-year variations of the atmospheric boundary layer (ABL) above and, ultimately, monsoon prediction. Based on these insights, we propose developing an analysis system for the Indian Ocean coupled system based on the Regional Ocean Modeling System (ROMS) and the Weather Research and Forecasting (WRF) models. The system will assimilate ocean observations using a 2-way nested configuration with the parent model covering the entire Indian Ocean and the child higher resolution grid targeting the AS region. Coupling the ocean model with WRF will allow us to simultaneously model the AS warm pool evolution and its impacts on the regional and large-scale atmospheric circulation. This synergy of modeling capabilities will enable us to understand the main mechanisms modulating the shape and evolution of the AS warm pool. The general scientific question we will address is #What are the physical processes controlling the AS warm pool variability and evolution,from genesis to collapse, and how do these processes affect the ABL above and ultimately monsoon prediction?#. Specific questions motivating the science objective are: (1) What are the most relevant dynamical processes controlling the shape, maintenance, and dissipation of the AS warm pool? (2) Can a higher model resolution in the AS region better represent the critical processes of the warm pool evolution? (3) How are the ocean-atmosphere fluxes and the ABL above affected by the variations of the warm pool, from genesis to collapse? (4) Are coupled ocean-atmosphere feedbacks relevant in reproducing the observed warm pool evolution? (5) How does the coupling of the warm pool with the ABL affect monsoon prediction? and (6) How sensitive is the coupled model to different coupling strategies for improved monsoon prediction and cool/dry SST/ABL bias reduction? Data assimilation of ocean observations provided by the ASTraL field campaign, complemented by other observational datasets, will offer a unique opportunity to constrain the ocean dynamics in the AS warm pool for the scientific inquiry summarized in this proposal. The proposed approach leverages on very recent enhancements to the ROMS code that allows for (i) the relatively easy coupling of ROMS with atmospheric models, (ii) the implementation ofdata assimilation in one-way and two-way nested applications, and (iii) the inclusion of coupled ocean-atmosphere feedbacks in the assimilation of ocean observations. In addition, the modeling system, including its variational tools, will be used to guide the ASTraL team in developing strategies to deploy observational assets.