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- Title
Tropical Pacific Air‐Sea Interaction Processes and Biases in CESM2 and Their Relation to El Niño Development.
- Authors
Wei, Ho‐Hsuan; Subramanian, Aneesh C.; Karnauskas, Kristopher B.; DeMott, Charlotte A.; Mazloff, Matthew R.; Balmaseda, Magdalena A.
- Abstract
Coupled processes and associated subsurface dynamics near the eastern edge of the Indo/western Pacific (WP) Warm Pool are important for air‐sea interactions involved in tropical Pacific dynamics. We seek to shed light on the physical mechanisms governing air‐sea interactions in the region and the impacts of their biases in models. In this study, we use the Ocean ReAnalysis System 5 (ORAS5) to identify mean‐state biases in the National Center for Atmospheric Research Community Earth System Model version 2 (CESM2) with a particular focus on upper ocean properties and air‐sea interaction processes. We show that the CESM2 has warm and fresh surface biases in the tropical Pacific Ocean, a barrier layer that is too thin in the WP, and an isothermal layer depth (ILD) that is too deep in the eastern Pacific (EP). These biases impact air‐sea interaction processes involved in El Niño development. We compare the strong El Niño events in ORAS5 and CESM2 and show that biases in barrier layer thickness in the WP and in ILD in the EP are significant before the onset of the El Niño events. These biases then influence vertical mixing and entrainment processes, resulting in mixed layer cooling biases. Biases in the sea surface temperature seasonal cycle in the CESM2 also influence the development of the El Niño. We emphasize how the El Niño progression in models can be influenced by its sensitivity to the mean state biases in both subsurface ocean structure and seasonal cycle through local as well as the large‐scale physical processes. Plain Language Summary: The western tropical Pacific has warm ocean surface temperatures that extend to over 100 m depth due to the prevailing trade winds. In contrast, the eastern tropical Pacific has cold ocean surface temperatures due to the upwelling of cold water from below. Changes to the trade winds can lead to changes in these temperatures and the whole ocean‐atmosphere coupled system. We shed light on the important physical processes that govern the ocean‐atmosphere interactions in the western tropical Pacific and how models have limitations in representing some of the processes, which potentially leads to errors in the simulation of important climate events such as El Niño. We show that the Community Earth System Model, a widely used global climate model, has warmer and fresher ocean surface waters compared to observations in this region which then influences the ocean‐atmosphere interactions adversely in this region. We also show that the so‐called "barrier" layer, which restricts the cold, deep water from reaching the surface when mixing occurs, is too thin in the climate model, which significantly hinders the model from reproducing the observed evolution of large El Niño events. Key Points: Coupled processes and subsurface dynamics near the eastern edge of the western Pacific (WP) Warm Pool are important for air‐sea interactionsThe CESM2 has warm and fresh surface biases in the tropical Pacific Ocean, a too thin barrier layer bias in WPBarrier layer thickness biases in the west Pacific and isothermal layer depth biases in east influence the development of El Niño
- Subjects
PACIFIC Ocean; UNDERGROUND storage; OCEANOGRAPHY; EARTH system science; ISOTHERMAL flows
- Publication
Journal of Geophysical Research. Oceans, 2021, Vol 126, Issue 6, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016967