(b)One analytical solution of a diffusion equation when diffusivity is a function of time and space（Jamstec report, 1997）

Hydrographic surveys and surface current observations using satellite tracked buoys were conducted from December 1992 to February 1993 near the spawning ground of Southern bluefin tuna to study the oceanographic conditions. Surface of the Timor sea during this period was covered by warm tropical water. Warm water was also observed along the west coast of Australia extending from the North West Cape. Below the surface, there appears that tropical east Indian Ocean water was spreading in the northern part of observation area(north of 19 degree S) while subtropical east Indian ocean water was spreading in the southern part of observation area. The thickness of the surface mixed layer was about 20 to 50m and chlorophyll was very low. Chlorophyll started increasing rapidly from slightly below the bottom of the surface mixed layer and reached maximum at about 75m. Below this depth, it decreased rapidly. Surface geoptential anomalies along the observation line west of Australia showed eastward geostrophic flow. In general, these results qualitatively agree with past research in the region.

Trajectories of satellite tracked drifting buoys show mean westward current at about 0.06m/s. Meridional and zonal dispersion coefficients were almost identical. They both increased almost linearly at first and became almost constant at about 3610m^2 /s after about 150 hours from release. Concentrations of eggs at the position of North West Cape are computed using simple advection/dispersion analytical model to study the effects of the values of dispersion coefficient, advection speed and mortality. The result shows that about 1% of eggs are trapped within the distance of internal Rossby radius of deformation from the coast for the standard case. Number of eggs trapped initially increases as the value of dispersion coefficient increases but starts decreasing if dispersion becomes too large due to the effect of the coast. Similarly, increase of eggs trapped in the coastal region by slowing advection has a limit.

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