Dynamics of the Kuroshio fluctuations

As mentioned above, the interval of velocity deceleration between Taiwan and the Lan-Yu Island is shorter (20 ~ 30 days) and that east of the Lan-Yu Island is longer (40 ~ 90 days). Looking over the entire period from 2000 to 2005, it appears that the slow-down incidents between Taiwan and the Lan-Yu Island prefer to arise in wintertime, while the events east of the Lan-Yu Island are relevant to cyclonic eddies from the east. The processes responsible for the slow-down (or southward-velocity) incidents around the Lan-Yu Island need further investigation. Because the East Asian Monsoon is the major atmospheric forcing in the region studied, we have tried to relate GSV around the Lan-Yu Island to local wind stress, but found almost no relation between them. Alternatively, the WSC seems to play an important role in the Kuroshio

Figure 4.4 Wind Stress Curl (WSC) around southern Taiwan.

The WSC averaged over 22.5 ~ 23°N is used for east of Taiwan, and that averaged over 21 ~ 21.5°N is used for west of Taiwan. LY and TW represent the Lan-Yu Island and Taiwan, respectively.

Figure 4.5 Zonal geostrophic velocity along 120.75°E in the northern Luzon Strait between 21.5 and 22°N. The positive sign represents the eastward flow.

fluctuations. In the spectrum of WSC off southwest Taiwan (Figure 4.3d), significant peaks are revealed at periods between 20 and 30 days. This result seems to suggest that it is the WSC off southwest Taiwan (west of the Luzon Strait) rather than the WSC in the low-velocity region that is responsible for the intra-seasonal fluctuations of the Kuroshio around the Lan-Yu Island.

Whenever the negative WSC (Figure 4.4) southwest of Taiwan (west of 120.75°E) enhances, GSV near the east coast of Taiwan (Figure 4.2) becomes weaker or even turns southward. In the meanwhile, GSV around the Lan-Yu

Island weakens.

A hypothesis could be applied to address the relationship between the WSC off southwest Taiwan and the flow southeast of Taiwan.Wu et al. [2005]

pointed out that a large and negative WSC would generate a clockwise circulation off southwest Taiwan. Here we also found that a looping branch of the Kuroshio is developed during the episode, resulting in an eastward GSU in the northern Luzon Strait (Figure 4.5). This eastward flow further triggers an

Figure 4.6 Averaged wind field in (a) October-March, (b) May-August, and (c) July-August 2004. Arrows represent wind stress vector and color shading with zero contour line shows wind stress curl.

eastward shift of the main axis of the Kuroshio, which in turn generates a mesoscale cyclone off southeastern Taiwan. As a consequence, the velocity between Taiwan and the Lan-Yu Island slows down. On the contrary, a weakened WSC west of the Luzon Strait leads to a straight path of the Kuroshio that strengthens the velocity between Taiwan and the Lan-Yu Island.

Shown as Figures 4.6a and 4.6b, a strong and negative WSC (centered at 120.25°E, 21.5°N) prevailing in the northeast monsoon season is favorable for the development of anti-cyclonic circulation and thus an eastward current at the southern tip of Taiwan. This process partially explains the reason why the southward-velocity event near the Taiwan coast prevails in wintertime. A particular event is observed in July ~ September 2004 when a strong positive WSC occurs in the region southeast of Taiwan. The strong positive WSC provides positive vorticities and induces a cyclonic (anti-clockwise) circulation in the low-velocity region in the summer of 2004 (Figure 4.6c).

A velocity slow-down incident is analyzed to further demonstrate the scenario between October 2000 and April 2001 when observed velocity data off southwestern Taiwan are available. Based on a pair of moorings, Wu et al.

[2005] suggested that the cyclonic or anti-cyclonic circulation in the region is triggered by the WSC off southern tip of Taiwan. Two episodes with distinct WSC patterns are described in their study. A large and negative WSC extends off southwest Taiwan in December 2000, while between April 1^st and April 20^th in 2001, the curl weakens significantly. We select these two episodes and display the corresponding flow patterns using the simulation of SAT model. In December 2000, the Kuroshio loops into the South China Sea as an anti-cyclone and the eastward flow in the northern Luzon Strait is part of this

clockwise circulation. In the meanwhile, the flow veers southward near the southeast coast of Taiwan (Figure 4.7a). In April 2001, the Kuroshio flows in a rather straight path in the Luzon Strait and reinforces the velocity between Taiwan and the Lan-Yu Island where the core speed of the Kuroshio accelerates from 90 to 140 cm/s (Figure 4.7b). This fact is also revealed in Figure 4.2. With GSU west of Taiwan flowing eastward, a southward velocity of -30 cm/s takes place near the east coast of Taiwan. In the meanwhile, GSV around the Lan-Yu

Figure 4.7 Modeled surface flow (0 ~ 200 m) with shading of speed in (a) December 1 ~ 31, 2000 and (b) April 1 ~ 20, 2001.

Figure 4.8 The 40-90 day band-passed sea level anomaly along 22°N.

Island decreases its speed from 60 cm/s to 20 cm/s. After flow west of Taiwan reverses its direction from eastward to westward in April 2001, GSV around the Lan-Yu Island rebounds to a value larger than 50 cm/s (Figure 4.2).

Furthermore, a possible cause for the velocity slow-down around the Lan-Yu Island on a longer timescale of 40 ~ 90 days is discussed. Shown in the evolution of the 40-90 day band-passed SLA (Figure 4.8), the slow-down events are associated with the impingement of the westward propagating cyclonic eddies. In Figures 4.2 and 4.8, as a cyclonic eddy with negative SLA propagates westward to the neighborhood of the Lan-Yu Island, the flow between 122 and 123°E becomes weaker or even turns to the south. Counting the number of eddies, these cyclonic eddies collide with the Lan-Yu Island at an approximate period of 60 ~ 90 days (2 ~ 3 times in six months) and propagate westward at a speed of 6 ~ 7 km/day, which is close to the speed proposed by Hwang et al. [2004]. The cyclonic eddy decelerates the northward flow near the Lan-Yu Island on occasion. For example, as a cyclonic eddy collides with the Lan-Yu Island in November ~ December 2000, the GSV reverses from a northward flow of 50 cm/s to a southward flow of -20 cm/s at 122°E and decreases from a stronger speed of 70 cm/s down to a weaker speed of 20 cm/s near the Lan-Yu Island. The decrease in the velocity of the Kuroshio southeast of Taiwan might influence its behavior in the downstream region around the East Taiwan Channel, e.g. the pathway around the Ryukyu Islands and the transport through the East Taiwan Channel.

4.4 Conclusions

Several significant peaks are revealed from the spectral analysis of meridional geostrophic velocity estimated by satellite altimeter. The longer intra-seasonal signal is on the period of 40 ~ 90 days. The peak at 20 ~ 30 days is only revealed between Taiwan and the Lan-Yu Island. Two different mechanisms attributing to these intra-seasonal variations have been proposed in this study. Evidence exists to suggest that the WSC west of the Luzon Strait is a major process responsible for variations in the flow between Taiwan and the Lan-Yu Island at a time interval of 20 ~ 30 days in winter. The clockwise circulation induced by an enhanced negative WSC generates an eastward current in the northern Luzon Strait that further weakens the current west of the Lan-Yu Island. On the other hand, cyclonic eddies originating in the interior Pacific Ocean appear to be primarily responsible for variations in the flow southeast of Taiwan at a longer time interval of 40 ~ 90 days. These variations are large enough to reverse the velocity between 122 and 123°E from a strong northward flow to a weak southward flow, but not large enough to completely diminish the intense velocity at 121.5°E, the core of the Kuroshio.

4.5 References

Centurioni, L. R., P. P. Niller, and D. - K. Lee (2004), Observations of inflow of Philippine Sea surface water into the South China Sea through the Luzon Strait, Journal of Physical Oceanography, 34, 113 - 121.

Chu, T. - Y. (1974), The fluctuations of the Kuroshio current in the eastern sea area of Taiwan, Acta Oceanographica Taiwanica, 4, 1 - 12.

Gilson, J., and D. Roemmich (2002), Mean and temporal variability in Kuroshio geostrophic transport south of Taiwan (1993 – 2001), Journal of Oceanography, 58, 183 - 195.

Hsin, Y. - C., C. - R. Wu, and P. - T. Shaw (2008), Spatial and Temporal Variations of the Kuroshio East of Taiwan, 1982 - 2005: A numerical study, Journal of Geophysical Research, 113, C04002, doi: 10.1029/

2007JC004485.

Hwang, C., and R. Kao (2002), TOPEX/Poseidon-derived space-time variations of the Kuroshio Current: Applications of a gravimetric geoid and wavelet analysis, Geophysical Journal International, 151, 835 - 847.

Johns, W. E., T. N. Lee, D. Zhang, R. Zantopp, C. - T. Liu, and Y. Yang (2001), The Kuroshio east of Taiwan: Moored transport observations from WOCE PCM-1 array, Journal of Physical Oceanography, 31, 1031 - 1053.

Kessler, W. S. (2005), The oceans, in Intraseasonal variability in the atmosphere-ocean climate system, edited by W. K. M. Lau and D. E.

Waliser, p. 175 - 212, Praxis Publishing Ltd, Chichester, UK.

Liang, W. D., T. Y. Tang, Y. J. Yang, M. T. Ko, and W. - S. Chuang (2003), Upper-ocean currents around Taiwan, Deep Sea Research II, 50, 1085 - 1105.

Milliff, R. F., W.G. Large, J. Morzel, G. Danabasoglu, and T. M. Chin (1999), Ocean general circulation model sensitivity to forcing from scatterometer

winds, Journal of Geophysical Research, 104, 11337 - 11358.

Qu, T., and R. Lukas (2003), The bifurcation of the North Equatorial Current in the Pacific, Journal of Physical Oceanography, 33, 5 - 18.

Tang, T. Y., J. H. Tai, and Y. J. Yang (2000), The flow pattern north of Taiwan and migration of the Kuroshio, Continental Shelf Research, 20, 349 - 371.

Wu, C. - R., H. - F. Lu, S. - Y. Chao (2008), A numerical study on the formation of upwelling off northeast Taiwan, Journal of Geophysical Research, In press .

Wu, C. - R., T. Y. Tang, and S. - F. Lin (2005), Intra-seasonal variation in the velocity field of the northeastern South China Sea, Continental Shelf Research, 25, 2075 - 2083.

Zhang, D., T. N. Lee, W. E. Johns, C. - T. Liu, and R. Zantopp (2001), The Kuroshio east of Taiwan: Modes of variability and relationship to interior ocean mesoscale eddies, Journal of Physical Oceanography, 31, 1054 – 1074.

CHAPTER 5 Conclusions

Using numerical model simulations combined with the observational data from satellites and drifters, the circulations around Taiwan have been investigated. Nesting model systems are built up to perform the works in the thesis. The nesting method provides suitable initial and boundary conditions for the finer resolution models from a larger domain model. Among these models, the 1/8°×1/8° East Asian Marginal Seas (EAMS) model is the major model adopted to study the temporal and spatial variations of current around Taiwan.

The EAMS model is well validated in the Taiwan Strait and the Kuroshio region. The 1/20°×1/20 Seas Around Taiwan (SAT) model coupled to the EAMS model is used to study the processes of the intra-seasonal variability of current around southern Taiwan.

From the experiments with three different global wind data sets, the QuikSCAT/NCEP blend wind is the best surface forcing for driving the circulation in the Taiwan Strait. Forced by the QuikSCAT/NCEP blend wind, the variations of the transports through the Taiwan Strait and the Penghu Channel, which is the major entry of the Taiwan Strait, are well reproduced.

The averaged modeled volume transports (1999 ~ 2003) through the Taiwan Strait and the Penghu Channel are 1.09 Sv and 0.55 Sv, respectively. The proposed transport is smaller than that in the literature because transports with strong southward flow in severe winter were rarely involved in the calculation

of mean transport. In addition, the model result reveals that the cold China coastal water can be transported southward in the western Taiwan Strait under strong northeasterly wind bursts in winter. Different dynamics for the response of transport through the Taiwan Strait and the Penghu Channel are demonstrated from the regression of the volume transport and wind stress. Two linear regression lines proposed for the relationship between wind stress and transport through the Penghu Channel is an innovative finding based on the present study.

In the region east of Taiwan, the variations of the Kuroshio are studied using the EAMS and SAT models combined with satellite-based data and surface drifter data. The Kuroshio in the region studied is highly influenced by westward-propagating mesoscale eddies from the interior Pacific Ocean on time scales of 40 ~ 200 days, especially in the seas without lands blocking on the eastside. The mean state of the Kuroshio is well simulated by the EAMS model. The modeled downstream Kuroshio transport is 32.7 Sv. With contribution from the countercurrent flowing to the south, the transport is estimated at 28.4 Sv. South of the Ryukyu Islands, a subsurface westward current below 200 m is revealed in the model results, and it plays an important part in the variations of the southward countercurrent in the subsurface near the Taiwan coast and the currents in the Okinawa Trough. However, the existence of this westward current should be testified with more observations.

One of the important findings is the two paths of the Kuroshio off southeast Taiwan, the inshore and offshore path. Based on the EAMS model simulation, the Kuroshio is mostly in the top 300 m in the inshore path but extends to 600 m in the offshore path. Only about 20% transport attributes to

the inshore path at 22°N. The two paths are significantly influenced by the upstream current in the Luzon Strait, in which the Kuroshio may either loop into the South China Sea or flow straightforward along the Batan Islands.

Revealed from the altimetric data, the flows southeast of Taiwan have two intra-seasonal variations on the periods of 20 ~ 30 days and 40 ~ 90 days.

The shorter variation can be only observed between Taiwan and the Lan-Yu Island on the inshore path while the longer one is revealed off southeast Taiwan between 120.75°E and 123°E. Here, two possible mechanisms are proposed to explain the flow variability on these two intra-seasonal timescales. The 20 ~ 30 days’ variation is contributed from the wind stress curl variability west of the Luzon Strait. A clockwise circulation off southwest Taiwan is profitable to be developed under the strengthening of negative wind stress curl in winter.

Meanwhile, an eastward current in the northern Luzon Strait is developed as a part of the clockwise circulation and shifts the Kuroshio main stream to the offshore path. When the negative wind stress curl retracts, the Kuroshio in the Luzon Strait has a rather straight path, and strengthens the current between Taiwan and the Lan-Yu Island. This fact is also evident in the SAT model simulation. Cyclonic eddies originating from the interior Pacific Ocean slow down the current around the Lan-Yu Island on the intra-seasonal variation of 40

~ 90 days. As a cyclonic eddy impinges east of the Lan-Yu between 121.5°E and 123°E, the northward current in the area has been decelerated, or even been reversed from northward to southward.