First, the northerly wind is predominant, so distributions of the current and density are basically similar to those of Animation 1. But there is a faster response as wind stress is significantly higher. After the wind weakens, the highest density region, i.e. upwelling region moves along the coast on its right and reaches the bay head. This is the typical behaviour of free internal Kelvin waves. On 22 July when the southerly wind strengthens, an inward current is produced in the surface layer and outward flow in the lower layer. After a small time lag the density field shows the upwelling along the western coast and downwelling at the eastern coast and the bay head. The downwelling region near the bay head enlarges with time, and after the southerly wind weakens (24 July), the downwelling region dominates. At the same time, the upwelling region also moves along the western coast toward the bay mouth.
Comparison of numerical results with observations
The response of the numerical model to real wind can be compared with the surface temperature distribution obtained by the infrared satellite image (NOAA-AVHRR), and the current and temperature measurements at moored stations in the bay.
Figure 6 shows the sea surface temperature obtained from the satellite images. One image (left) highlights the distribution of low temperature along the eastern side in the bay during the northerly wind prevailing on 16 August 1991. The other shows low temperatures near the bay mouth along the western coast after strong southerly winds. Both distributions agree with the result of the numerical model using real wind (Animation 3). The upwelling caused by the local wind is reproduced by the numerical experiments of our model. There are the two observations at moored stations for validation of our modelling results. One is the TOBEX (Figure 7) obtained in the summer of 1979 (Unoki, 1985) and the current measurements obtained in September of 1989 (Oshima et al., 1990). It is suitable to estimate the cross correlation between the local wind and current variations obtained in the field observation. When the current variations have high correlation with the wind, the currents may be closely connected with the wind-induced circulation. The horizontal scale of wind variation is much larger than the horizontal scale of Tokyo Bay, so that the wind data at Yokohama Meteorological Observation (Figure 1) can be applied as representative of the sea surface wind in the bay. The data of 24-h running average of the wind and 25-h average of the current are used for residual current analysis.
The cross correlation coefficients with time lag for each station in TOBEX are shown in Table 1. The instruments were moored at 3 m below the sea surface and 5 m above the sea bottom, respectively. The correlation coefficients are high, yielding values over 0.5 for most stations. In the surface layer of the bay head and of central region (Stations D7 and 7) the coefficients of both along-bay (υ) and transverse (μ) components are negative and their time lags are within 13 h in the eastern part of the bay except u at Station D7. The time lag of surface layer at the western side (Station J6) is larger than at the eastern side (Station D3). On the other hand, the coefficients in the lower layer from the bay head to the central part (Stations D7 through D3) are negative and their lags are smaller than near the surface. The difference between the eastern and western sides of the maximum lagged coefficients in the surface is shown in Figure 7.
