A Simulation Study Of Particulate Organic Carbon In The Role Of The Global Ocean Carbon Cycle

The vertical transport and degradation of particulate organic carbon(POC) in the ocean play an important role in the ocean carbon cycle, which can rapidly transfer the absorbed carbon in the upper ocean into the deep ocean, resulting in adjustment of the atmospheric carbon dioxide levels. This study mainly includes two aspects. One is to improve the eddy parameterization of mesoscale tracer transports and to examine the simulation results using passive tracer CFC-11, and the other is to add a prognostic variable(POC) in the original biogeochemistry model IAP-OBM to complete organic carbon reservoir and the carbon cycle process, which has been coupled to the global ocean model LICOM. Using these simulation results, the influences of both mesoscale eddy parameterization schemes and POC on the processes of the carbon cycle are studied. Main conclusions are summarized as follows:1)The model LICOM was used to study the influences of both the eddy diffusion coefficients in the parameterization of mesoscale tracer transports(GM90 scheme) on the physical fields and CFC-11 distribution characteristics. Two numerical experiments were designed. The simulated results show that the simulation ability of the physical field is improved to a certain degree in the mixing scheme with the buoyancy frequency-dependent eddy diffusion coefficients called BUOYANCY. For example, compared to the mixing scheme with constant eddy diffusion coefficients(CONTROL), the Antarctic circumpolar current transport strength in BUOYANCY is increased by about 20%- 30%, which is closer to the observations. Relatively strong strength of the Antarctic Intermediate Water from CONTROL is reduced in BUOYANCY, resulting in the improvement of the simulation of sub-surface potential density in the high-latitude region of the Southern Ocean. Through the analysis of the distribution, storage and transport of CFC-11, it is known that the different values of sub-grid parameters have a relatively great influence on the simulation of the CFC-11 in the Southern Ocean. Specifically, compared to CONTROL, more CFC-11 is taken up in the high-latitude region, and transported from the region near the Antarctic continent to the Southern Ocean’s main storage area, leading to the increase in CFC-11 inventory in the Southern Ocean in BUOYANCY, which is closer to the observations in most regions. In addition, it can be known from the analysis of CFC-11 distributions in the given section that a certain improvement of the simulation of potential density in the upper layers of the Southern Ocean in BUOYANCY makes the vertical structure of CFC-11 closer to the observation, relative to that in CONTROL.2) A prognostic variable called particulate organic carbon(POC) was added into the IAP-OBM model, in which it was an intermediate variable. Three numerical experiments were conducted to study the effects of different physical fields and different treatment of particulate organic carbon on the carbon cycle. Simulation results show that three numerical experiments can all produce the observed distribution characteristics of surface water phosphate, total alkalinity and dissolved inorganic carbon. Using the mixing scheme with the buoyancy frequency-dependent eddy diffusion coefficients, there is no obvious difference in biochemical variables in most regions except in the deep Southern Ocean and 800-1000 m depth near the equator. However, the simulation results are improved in the experiment with POC as a prognostic variable. For example, phosphate is increased to a certain degree in the surface water of the Southern ocean, which is closer to the observations. The vertical structures of phosphate and total alkalinity are also improved significantly, but total dissolved inorganic carbon is reduced in the deep ocean, compared to that from other experiments.