Transport through the Taiwan Strait

Figure 2.5 shows the model-derived transport together with some limited observed transports through the Taiwan Strait. Blue stars and red circles represent strait-wide volume transports calculated from sb-ADCP observations by Dr. Ruo-Shan Tseng (unpublished data) and by Chung et al. [2001]. Similar to the volume transport through the Penghu Channel, the model-derived transports through the Taiwan Strait are comparable to the observed values. The average strait-wide transport of 1.09 Sv is about double the average transport through the Penghu Channel although the cross-sectional area of the Penghu Channel is only one-fifth of that of the Taiwan Strait. Therefore, currents are much stronger in the Penghu Channel than in the rest of the strait. The model result is consistent with the well-known statement that the Penghu Channel is the major pathway for the northward flow entering the Taiwan Strait (e.g. Wang and Chern [1988] and Jan and Chao [2003]).

Strong seasonal variation in the Penghu Channel is evident in Figure 2.5 as well. The volume transport is northward and largest in summer. It is minimum and even southward in fall and winter. The trend of the volume transport is related to the seasonal reversal of the monsoons. The occasional strong northeasterly wind bursts in fall or winter drive currents southward and produce large and negative transport up to -5 Sv (Figure 2.5). These events of winter fronts are seldom observed because of severe weather, but are important to balance the nutrient budget.

In addition to the seasonal variation, inter-annual variation also exists.

The purple line in Figure 2.5 shows the smoothed transport. The mean transport

Figure 2.5 Model-derived transport and observed transports in the Taiwan Strait. Blue stars and red circles represent strait-wide volume transports calculated from sb-ADCP measurements by Dr. Ruo-Shan Tseng (unpublished data) and by Chung et al. [2001], respectively. Purple line represents monthly mean model-derived transport.

Figure 2.6 Relationship between model-derived transport through the Taiwan Strait and the along-strait wind stress.

in summer 2003 is largest in the 5-year period. On the other hand, southward transport is smallest in fall and winter of 2002. Furthermore, variations of volume transport in summer are much smaller than those in winter, indicating that flow conditions in the Taiwan Strait are more complex during wintertime.

Several factors could account for the phenomenon, e.g., local effects of the northeast monsoon and sea level differences between the East China Sea and South China Sea. The annual average transport of 1.09 Sv is much smaller than most observed values, which contain uncertainties due to coarse spatial resolution and lack of winter measurements.

Figure 2.6 shows the relationship between volume transport through the Taiwan Strait and along-strait wind stress obtained by averaging the QuikSCAT/NCEP wind stress in the domain from 118°E to 120°E and from 23°N to 25°N. Unlike the two regression lines shown in Figure 2.4, only one simple regression line suffices in the figure, and its correlation coefficient (γ) is equal to 0.82. The equation is:

Transport (Sv) = 1.06 × wind stress (dyne/cm²) + 1.99 (4)

The volume transport is 1.99 Sv northward when wind stress is zero. The result suggests that the strait-wide volume transport is contributed by not only wind stress but also a northward pressure gradient force.

Figure 2.7 shows the model-derived pressure gradient force between north and south entrances of the Taiwan Strait during the period from 1999 to 2003. The pressure gradient is calculated from model sea surface height difference. The sea surface height at the north entrance is averaged over the domain from 119°E to 121.5°E and from 25°N to 25.25°N and that at the south

entrance is averaged over the domain from 117.25°E to 120.25°E and from 23.5°N to 23.75°N. The pressure gradient is always negative because sea surface is always lower at the north entrance of the Taiwan Strait than at the south end. This pressure gradient forces a mean current northward year round and is not driven by local winds. However, the large-scale monsoon is still responsible for building up the sea level in the south. The monsoon wind field is affected by coastline, resulting in the large difference between the sea surface height in the East China Sea and South China Sea. The Kuroshio intrusion through the Luzon Strait is also a possible source of higher sea level south of the Taiwan Strait.

There is significant seasonal variation in Figure 2.7. For example, the pressure gradient force is generally larger in winter than in summer. This seasonal variation of the pressure gradient force might affect the volume transport as well. However, it seems that its effect is not significant in this study.

Figure 2.7 Model-derived pressure gradient force between north and south entrances of the Taiwan Strait during the period from 1999 to 2003. The pressure gradient is calculated from model sea surface height difference.

We have excluded the volume transport caused by the pressure gradient force and have done regression analysis with the along-strait wind stress. The regression line is similar to that in Figure 2.6 with correlation coefficient (γ) increasing slightly from 0.82 (Figure 2.6) to 0.84.

Chuang [1985] proposed a simple equation to describe the balance of along-channel momentum for friction dominated steady flow in a straight

Figure 2.8 Comparison between model-derived transport (red line) and transport estimated using a resistance coefficient (blue line) during the period from 1999 to 2003.

Figure 2.9 Comparison of upper-layer temperatures (0 ~ 50 m) to the east (black line) and west (red line) of the Penghu Island during the period from 1999 to 2003. The temperatures are averaged over the regions of black and red rectangles shown in the upper panel.

channel. Using the regression equation (4) as well as Chuang’s [1985] equation, a resistance coefficient was estimated. An approximate transport was thereafter calculated from the wind stress, resistance coefficient, and model sea surface height based on Chuang’s simple equation. The agreement between the model-derived transport (red line) and the approximate transport (blue line) in Figure 2.8 supports the friction dominant scenario.

From Figures 2.4 and 2.6, model-derived transports through the Taiwan Strait and the Penghu Channel, respectively, are -6 and -1 Sv at WS = -8 dyne/cm², and are -2 and -0.5 Sv at WS = -4 dyne/cm². The results suggest that under strong northeast monsoon, most southward transport in the Taiwan Strait is to the west of the Penghu Island, where the current along the coast of China transports cold coastal water southward. The modeled temperature distribution confirmed this statement. Figure 2.9 shows a comparison of upper-layer temperatures to the east and west of Penghu Island during the period from 1999 to 2003. The upper-layer temperatures were averaged vertically from surface to 50 m deep. All five-year data show that temperature is 1 ~ 2 °C lower west of the Penghu Island than east of it in winter. The results are consistent with earlier observations that cold, fresh China coastal water in the western portion of the Taiwan Strait was forced southward in winter under the strong and steady northeast monsoon.