Institute of Hydrological and Oceanic Sciences Research

Modeling Taiwan's Hydro-Environment  

    In the island of Taiwan and the surrounding seas, human, industrial and economic activities are intimately linked to the water cycles and associated resources and disasters. It is therefore important to establish ocean-atmosphere-land models to make short-to-long-term predictions of winds, sea levels, ocean currents, hydrography, and water cycles.  The short-term (nowcast) system can be used for disaster mitigation, rescue and search, and fishery and marine resources management.  The long-term system can be used for climate prediction, drought warning, and water resources managements. 

    The atmosphere models are based on existing regional models.  Currently, a version of the model nesting down to 2.2-km horizontal grid spacing with a proper treatment of topographic forcing, ice-phase microphysics, and the assimilation of radar data can successfully predict 24-hour rainfall in selected typhoons.  The model will be furthered modified by improving the treatment of precipitation, boundary layer, and land surface processes.   (M.-J. Yang)

    The ocean models will be developed in house.  The validation and improvement requires ship observations, moored and satellite instrumentmeasurements.
  These will be done jointly with collaborating scientists.   (S. Jan)

    Distributed basin-scale runoff models are also being developed to estimate the river flow in response to typhoon rains.  The upstream basin of Reservoir Shihmen is selected in the figure as a target domain of interest.  Ground-based hourly observations of both stream flows and gage rainfalls are crucial measurements.  Selected typhoons  are used for calibrating the runoff parameters and the basin landuse conditions of the distributed model.    (M.-H. Li)


Air-Sea Exchanges

    High- frequency (daily) momentum and heat exchanges are important for coupled ocean-atmosphere variability but are generally  treated  insufficiently in many models. The study from an ocean circulation model shows such exchanges primarily enhance vertical mixing that reduces the vertical shear of the upper ocean.  It also changes the net heat into the ocean through two contrasting processes: one is the increased surface latent heat loss induced by transient winds and the other is colder SST due to stronger mixing, which reduces further heat loss at the surface.  As a result, the momentum and heat balance in the upper ocean are affected by both local and remote responses.    (C.-H. Sui)

    The effect of ais-sea exchanges in typhoons, intraseasonal oscillations, and the oceanic mesoscale eddies may cause locally large variations in the Kuroshio transport as those observed at the World Ocean Circulation Experiment PCM-1 section east of Taiwan.  An eddy-resolving, general circulation model, with a horizontal resolution of 1/6 degrees, is adopted to study the Kuroshio-eddy interactions.  The figure shows flow fields before and after a westward-propagating anticyclonic eddy impinges to the Kuroshio.  The scenario runs qualitatively mimic observational evidences, suggesting mesoscale eddies with a relative vorticity of 0.0001/s would considerably affect the path, flow volume transport and vertical hydrographic structure of the Kuroshio.   (S. Jan)

     A "numerical laboratory" of the surface ocean driven by winds shows the thermal structure within the aqueous boundary layer. The picture enables a visualization of the "surface renewal events" which have been widely employed in estimating the air-sea fluxes (so called bulk parameterizations) but remain to be conceptual.  Such a thermal structure also produces the "cool-skin effect" important in validating satellite-derived sea surface temperature using the in situ bulk temperature.    (W.-T. Tsai)

    The ubiquitous physicochemical effect of surfactants in the global oceans is another process of micro length-scale but can substantially influence the global air-sea exchanges. The figure shows the distribution of the annually-averaged global atmosphere-ocean CO2  flux with the transfer reduction by the presence of the surfactants considered in the calculations.  The major reduction in absorption fluxes occurs in the northern Pacific and Atlantic, where the water is known to be highly productive, while the decrease in outgassing occurs in the eastern equatorial and subtropical Pacific and Atlantic, where nutrient-rich water wells up.  
(W.-T. Tsai)

Biogeochemical Cycles in Natural Environment

    These cycles affect water quality in fresh waters and coastal waters on a local scale; they also affect ocean productivity and climate on the regional to global scale.  The approaches include field surveys, isotopic and elemental analyses of water samples, and numerical modeling to investigate the coupled physical-biogeochemical processes controlling the elemental cycles.  The example shows the comparison between the modeled chlorophyll distribution and the image of pigment concentration derived from satellite ocean color data in the South China Sea in winter.  The patches of high chlorophyll result from upwelling of nutrient-rich seawater from the deeper ocean, which was induced by the strong northeast monsoon.    (K.-K. Liu)