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A winning proposal for the Innovative Research Program, 2006: Can phytoplankton change SST and upper ocean circulation?Investigators: Toshiaki Shinoda (CIRES, University of Colorado), Weiqing Han (ATOC, University of Colorado) Research Theme, Background and Importance. It is well known that the turbidity of upper ocean water is primarily determined by phytoplankton distribution. Hence the phytoplankton pigment concentration in the upper ocean significantly affects the mixed layer absorption and penetration of solar radiation. Given that solar radiation is the largest component of surface heat fluxes that determine the sea surface and upper ocean temperature in tropical oceans, variation of phytoplankton distribution can directly influence sea surface temperature (SST) and upper ocean thermal structure. As a result, upper ocean circulation can also be altered due to its sensitivity to the thermal structure. In most state-of-the-art ocean general circulation models (OGCM), however, turbidity of water is parameterized by a constant water type, and therefore spatial and temporal variations of phytoplankton are neglected. The proposed research aims at understanding the role played by phytoplankton in causing variabilities of SST and upper ocean circulation in the tropical Pacific and Indian Oceans. The approach for this project is to perform OGCM experiments by including and excluding phytoplankton concentration, which can be inferred from chlorophyll-a (chl-a) concentration of satellite-derived ocean color data. The penetrative component of solar radiation (Qpenet) will be calculated from the ocean color data using a solar transmission parameterization based on chlorophyll concentration. To help test the sensitivity of penetrative radiation to vertical resolution, a 1-D mixed layer model will also be used. Completion of the proposed research is expected to improve the simulation of SST and upper ocean circulation by advancing the parameterization of penetrative radiation in OGCMs, and ultimately improve climate model simulations and predictions. What makes this innovative? While the satellite-derived ocean color data have been widely used to examine the upper ocean biological processes, their effects on SST and upper ocean circulation are not emphasized. The proposed research is unique and innovative in that it will qualify and quantify the impact of spatial and temporal variations of phytoplankton pigment concentration on upper ocean physical processes, and thus will build a foundation for understanding the physical-biological interactions and their impact on climate variability. Objectives. Specific objectives of the proposed research are:
Short Description of the Project. A parameterization of solar transmission based on the upper ocean chl-a concentration along with remotely sensed ocean color data will be used to calculate Qpenet in ocean model experiments. Net irradiance profiles are expressed as a sum of two exponential terms. We will use Moderate-Resolution Spectroradiameter (MODIS) products which include the chl-a concentration data from recent satellite measurements (Aqua and Terra). Solar radiation at the surface will be obtained from the new satellite-based earth’s radiation budget (ERB) 3-hourly data. The OGCM is the HYbrid-coordinate ocean model (HYCOM). The fine resolution global HYCOM will be operational for the Navy and the regional HYCOM (US east and west coast) will be operational at NCEP with both scheduled for operational deployment around 2006-2007. The OGCM will be first integrated for the 6 year period from February 2000 to 2005 after the high quality ocean color data are obtained. The model will be then forced with same surface fluxes but penetrative solar radiation calculated based on a constant water type in space and time. The difference between the two experiments estimates the impact of the use of satellite-derived ocean color data on upper ocean variability. The model SST from these experiments will be compared with a variety of SST data (e.g., TRMM Microwave Imager SST) to determine to what extent the simulation of upper ocean variability is improved by the use of satellite-derived ocean color data. We will further conduct a number of model experiments in order to isolate the effect of Qpenet variations on different time scales using low-pass filtered chl-a. 1-D mixed layer model experiments will be also conducted in order to determine how the vertical resolution of the model affects the impact of Qpenet. 1-D model results with OGCM experiments also isolates the impact on only vertical processes. A particular emphasis in 1-D model experiments is given to the diurnal cycle of SST. The spatial variation of the amplitude of the diurnal cycle in each experiment will be described. Expected Outcome and Impact. Completion of the proposed research will lead to better solar transmission parameterization in ocean models and identify its impact on the upper ocean variability. Thus it will help climate model development and improvement. Also, the proposed study is considered to be a "jump start" for further model diagnoses of climate variability. There are a variety of climate variability in the tropical Indian and Pacific Oceans which can be diagnosed from the ocean model output. For instance, the Indian Ocean dipole variability in recent years can be investigated. Also, a coupled ecosystem model, which can examine physical/biological feedback processes, could be developed in future projects as extension of the proposed research. Research Plan. T. Shinoda will conduct most of the experiments and diagnoses of ocean models, and W. Han will work with Shinoda to perform some of the model diagnoses especially in the Indian Ocean region. The proposed project will be completed in 12 months. Surface flux datasets will be created in the first 2 months. Then OGCM and 1-D model experiments will be conducted in the next 6 months. The model output will be analyzed in the next 2 months. The results will be thoroughly documented in the last 2 months. |
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