Sensitivity tests for Improvement of Chlorophyll Simulation with NEMO-TOPAZ

Hyomee Lee1, Byung-Kwon Moon1, Jong-Yeon Park1, Jieun Wie2, Hyun-Chae Jung3, Hyojin Park4, Young-Hwa Byun5, Yoon-Jin Lim6 and Johan Lee7, (1)Chonbuk National University, Jeonju, South Korea, (2)Chonbuk National University, South Korea, (3)Mirae Climate, South Korea, (4)Gimje Girls' High School, South Korea, (5)National Institute of Meteorological Sciences, Seogwipo-si, South Korea, (6)National Institute of Meteorological Sciences, Climate Research Division, Seogwipo, South Korea, (7)National Institute of Meteorological Sciences, Seogwipo, South Korea
The process of ocean-biogeochemistry plays a vital role to the earth system through material exchange with the atmosphere including the carbon cycle. Owing to this, recently ocean models and earth system models are under development with respect to the ocean biogeochemical process. We have incorporated ocean models, Nucleus for European Modelling of the Ocean (NEMO), and Tracers of Ocean Phytoplankton with Allometric Zooplankton (TOPAZ), a biogeochemistry module, to attain the NEMO-TOPAZ model. To evaluate the biogeochemistry simulation function of the model and to improve the simulation, a sensitivity experiment was conducted. For this, observed atmospheric forcing COREv2 was used to prescribe and the period from 1981 to 2009 was analysed. Variables used in the analyzed are chlorophyll, nitrate, phosphate, silicate, dissolved oxygen (DO), alkalinity, and dissolved inorganic carbon (DIC).

NEMO-TOPAZ well represents the major biogeochemical variables; however, chlorophyll was over-estimated (CTL). Chlorophyll generally represents the amount of phytoplankton, and the growth of phytoplankton is largely affected by the trace amount of iron in iron-limited regions. The model takes into account the amount of iron deposition supplied from the atmosphere to seawater, desorption parameter relating to the supply process of iron from particulates in the ocean to seawater, and scavenging parameter being removed from seawater after being absorbed to particles for impact of iron. Iron deposition of NEMO-TOPAZ was over-diagnosed compared to the other biogeochemistry model NEMO-PISCES; however when these data were changed to reduced iron in the NEMO-PISCES model, the concentration of chlorophyll improved (PFE; Equatorial Pacific RSME reduced by 59% compared to CTL).

As a result of adjusting parameters affecting the concentration of iron in seawater, scavenging of iron had a large impact on reducing the concentration of iron in seawater (PFE_S1.5; Equatorial Pacific RSME reduced by 65% compared to CTL). However, these results demonstrate an increase in silicate in seawater, which can be caused by a reduction of phytoplankton that decreased consumption of nutrients. This error was somewhat improved when the dissolution coefficient related to silicate removal was reduced (PFE_S1.5_D0.5), as chlorophyll errors did along with it. However, various sensitivity experiments such as adjusting the plankton rate related to silicate consumption should be conducted in addition to improving over-simulated silicate and chlorophyll that are still in presence.

This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI (KMI2018-03513). The main calculations were performed by using the supercomputing resource of the Korea Meteorological Administration (National Center for Meteorological Supercomputer)