The Indo-Pacific region is the convergence zone of multi-earth-spheres with largest biodiversity in the global ocean. However, previous models were not refined enough to resolve the complex strait and topography, so the understanding of local biogeochemical dynamics is still limited.
Recently, the research team led by Prof. YIN Baoshu from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has constructed a high-resolution physical-biogeochemical model which covers the whole indo-pacific region. The physical model is able to characterize the mulita-scale ocean dynamics process and reproduce the seasonal variation of transport at main straits.
The study was published in Journal of Geophysical Research: Oceans on Oct. 29.
Large uncertainties exist in the ecosystem model, as the function forms describing the biological process are empirically determined via observation. The accuracy of biogeochemical dynamics in numerical modeling largely depends on the setting of biological parameters, which must be specifically tuned in various regions. Generally, tens of sensitivity experiments must be conducted to optimize only one biological parameter. A physical-biogeochemical model often consists of several key parameters with high sensitivities, therefore, theoretically, ten thousand or more numerical experiments are required to achieve a set of optimal biological parameters. Obviously, the corresponding computing resources are unaffordable when running a three-dimensional and high-resolution physical-biogeochemical model. Thus, the conditional nonlinear optimal perturbation (CNOP) method was applied in a realistic physical-biogeochemical model run, which optimized the parameters in remarkably fewer experiments.
Based on the improved model results, the research term examined the physical mechanisms controlling the seasonal variation of surface chlorophylls in the Lombok Strait region. The mixed-layer depth varies significantly and the ocean dynamics are heterogeneous in/below the mixed layer, so the results from the mixed-layer and fixed-depth budget are different and should be combined to discuss the behind physical mechanism. Under the joint influence of equatorial Kelvin waves and local wind forcing, the coastal current varies and affects the nutrient transport semi-annually. In the mixed layer, the high nutrient water was formed mainly by the offshore Ekman transport rather than direct local upwelling.
"This study is the first high-resolution biogeochemical simulation optimized by CNOP method," said Dr. GAO Guandong, first author of the study.
"This study has unraveled the physical mechanisms of high primary productivity in the Lombok Strait region and laid the model and theory foundation for understanding the large biodiversity in indo-pacific region," added Prof. YANG Dezhou, the corresponding author.
This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, etc.
Fig. 1 (a) The study region; (b) model validation of transport at main straits; (c) model validation of surface chlorophylls.
Fig. 2 Schematic plot of the physical drivers behind the nutrient transport that affect the chlorophyll blooms.
Gao, G., Yang, D., Xu, L., Zhang, K., Feng, X., & Yin, B. (2022). A biological-parameter-optimized modeling study of physical drivers controlling seasonal chlorophyll blooms off the southern coast of Java Island. Journal of Geophysical Research: Oceans, 127, e2022JC018835.
(Text by GAO Guandong)
Institute of Oceanology
(Editor: ZHANG Yiyi)