Intra-seasonal variation of the Kuroshio southeast of Taiwan and its possible forcing mechanism
The intra-seasonal variation of the Kuroshio southeast of Taiwan has been studied using satellite data and a numerical model. Superimposed with the main stream of the Kuroshio, two intra-seasonal signals are revealed in the study region. The fluctuation with a period of 1–6 months results from offshore eddies. The westward propagating cyclonic eddies can reduce or reverse the northward flow east of the Kuroshio between 121° and 123° E, but only slightly touch the core velocity of the Kuroshio. The fluctuation with a period of 2–4 weeks is only significant between Taiwan and the Lan-Yu Island (the low-velocity region). Different mechanisms are responsible for the fluctuation in the low-velocity region in different seasons. In winter, the change of negative wind stress curl in the northeastern South China Sea modulates the circulation southeast of Taiwan, while the typhoon-induced intense wind is responsible for the current fluctuation in summer.
KeywordsIntra-seasonal variation Kuroshio southeast of Taiwan
The Kuroshio closes the subtropical gyre on the western boundary of the North Pacific basin. When the North Equatorial Current arrives at the east coast of the Philippines, it bifurcates into two branches. The northern branch is the Kuroshio (e.g., Qu and Lukas 2003). After branching, the Kuroshio continues flowing northward along the east coasts of Luzon and Taiwan, the shelf slope of the East China Sea, and the south coast of Japan, and eventually becomes the Kuroshio Extension off Japan. The flow pattern becomes complicated when the northward-flowing Kuroshio reaches the Luzon Strait, a western land boundary gap. There, the Kuroshio either loops into the South China Sea or bypasses the Luzon Strait in a straight path (e.g., Wu and Chiang 2007; Hsin et al. 2008). Different pathways of the Kuroshio in the Luzon Strait may have distinct effects on the behavior of the downstream Kuroshio, especially the regions southeast of Taiwan. As a result, the Kuroshio southeast of Taiwan is highly variable both in time and space.
Based on in situ hydrographic surveys east of Taiwan, temporal and spatial variations of the Kuroshio have been observed from cruise to cruise (Chu 1974). Its transport varies in a large range of timescales from days to years (e.g., Johns et al. 2001; Gilson and Roemmich 2002; Hwang and Kao 2002; Hsin et al. 2008). In addition to the seasonal variation, the most pronounced signal in the region due to the reversal of East Asian Monsoon, the intra-seasonal variations are also observed. There are several factors contributing to these variations. Among others, the strong influence of intra-seasonal atmospheric forcing has been identified. As a special case of this atmospheric forcing, an intra-seasonal oscillation with a quasi-periodicity of 30–60 days is well-known in the tropical atmosphere as the Madden-Julian-Oscillation. This intra-seasonal signal could be injected into the ocean as a result of local and/or remote air–sea interaction.
Moreover, the intra-seasonal variations on longer timescales of 100∼200 days off eastern Taiwan are often introduced as a result of interaction between the Kuroshio and offshore eddies originating in the interior Pacific. Hsin et al. (2008) suggested that the impingement of mesoscale eddies can alter the velocity and path of the Kuroshio, which in turn can alter the water input into the East China Sea, and is believed to be an important driver of the intra-seasonal variations in the region. Zhang et al. (2001) suggested that these mesoscale eddies have periods between 70 and 200 days. In addition to the direct interaction between the Kuroshio and eddies from the east along the latitude band of 22–25° N, eddies can also be brought by the north-flowing Kuroshio from the further upstream region around the Luzon Island and Strait (Sheu et al. 2010). As Ichikawa (2001) and Ichikawa et al. (2008) have suggested, the mesoscale eddies impinging the Kuroshio south of the East Taiwan Channel (“ETC” in Fig. 1) can also influence the flow field in the southern East China Sea and pass their variability through the Kuroshio to the volume transport at the Takara Strait in the northeastern East China Sea.
This study focuses on the intra-seasonal variation southeast of Taiwan, especially in the low-velocity region (“L” in Fig. 1) between Taiwan and the Lan-Yu Island. By the use of satellite data (altimetry and sea surface wind) combined with simulation from a well-validated numerical ocean model, we attempt to investigate the intra-seasonal variation of the Kuroshio in the upper ocean southeast of Taiwan. In Section 2, the data used in this study are introduced. Section 3 presents and discusses the results of the analyses. Some possible mechanisms related to the Kuroshio intra-seasonal variations are also discussed in this section. The conclusions are given in the final section.
2 Satellite data and model description
The gridded multi-satellite altimeter-based mean sea level anomaly (SLA) and mean geostrophic velocity are used in this study. The products were distributed by Archiving, Validation and Interpretation of Satellite Oceanographic (AVISO) data and obtained from http://www.aviso.oceanobs.com/. They were merged from five altimeter missions: ENVISAT, ERS-1 and ERS-2, GFO, Jason-1, and TOPEX/Poseidon, and have resolutions of 1/3° in space and 7 days in time. The gridded altimeter-based products are available from October 1992 to the present. The wind stress data used in this study were derived from a time and space blend of the QuikSCAT-DIRTH satellite scatterometer observations and the National Center for Environmental Prediction (NCEP) analyses (QSCAT/NCEP; Milliff et al. 1999) and have a horizontal resolution of a half degree with a time interval of 6 h for the period after July 1999. The data were distributed by the Data Support Section of the National Center for Atmospheric Research (http://dss.ucar.edu/). Combined with the NCEP reanalysis version 2 wind product with a horizontal resolution of 2.5°, we extended the time of wind data back to 1993. Firstly, the NCEP reanalysis wind product is linear-interpolated onto the grid of QSCAT/NCEP wind.
Altimeter-based geostrophic velocity is used to investigate the temporal and spatial variation of the Kuroshio in the upper ocean of the area studied. Figure 1 shows the annual mean surface flow from the shipboard-ADCP (Sb-ADCP) observations (green arrows) and the geostrophic velocity (red arrows) derived from the satellite altimetry SLA. The mean flow pattern of the altimeter-based velocity is quite similar to that of Sb-ADCP-based velocity, which was composited with observations between 1991 and 2005, except for some quantitative differences in magnitude. This is particularly true in the regions around the Luzon Strait and eastern Taiwan, including the intrusion position and pathway of the Kuroshio. A low-velocity region on the shore side of the Kuroshio (shown in the drifter map of Centurioni et al. (2004); indicated by letter “L” in Fig. 1) is also well reproduced by this satellite product. Since the altimeter-based geostrophic velocity resolves temporal and spatial scales of interest, we use the data to discuss the variability of the surface Kuroshio off southeast Taiwan.
In addition, the East Asian Marginal Seas (EAMS) model based on the Princeton Ocean Model (POM; Mellor 2004) is also used in this study. This model has been used to investigate the variations of volume transport and flow field in different depth ranges in the seas east and west of Taiwan (Wu and Hsin 2005; Hsin et al. 2008). Hsin et al. (2008) also used this model to study the impact of the westward-propagating eddies on the flow east of Taiwan on the timescale of about 100 days. The EAMS model covers a domain of 99–140° E and 0–42° N with a resolution of 1/8° in horizontal and has 26 sigma levels in vertical. The model is forced by the 6-hourly half-degree resolution blended QSCAT/NCEP wind product, which is gained after July 1999, and the weekly 1° AVHRR sea surface temperature. A North Pacific Ocean Model, which has an extended domain of 99° E–77° W and 16° S–60° N with a horizontal resolution of 1/4° × 1/4° and the same vertical sigma levels as the EAMS model, serves the open boundary conditions of the EAMS model at a 1-day time interval using one-way nesting method. Careful validations for the EAMS model have been made in Wu and Hsin (2005) and Hsin et al. (2008), respectively, for the regions west and east of Taiwan. The simulation period spans from 2000 to 2005. The detailed information about this model is referred to Hsin et al. (2008), and the basic hydrodynamic content of POM is consulted in Mellor (2004).
3.1 Mean current structure off southeastern Taiwan
As shown in Fig. 1, when the Kuroshio arrives in the vicinity of the Luzon Strait, a branch of it intrudes westward into the northern South China Sea from the middle of the Luzon Strait around 20.75° N and 120.75° E. Most of the Kuroshio water circulates in a saddle-shape and returns to the east at the southern tip of Taiwan. The main stream of the Kuroshio flows straightly along the west side of the Batan Islands to the east coast of Taiwan. After leaving the Luzon Strait, the core of the surface Kuroshio always passes around the Lan-Yu Island at 121.5° E, 22° N to the north and reaches its maximum speed around 24° N. Between the Kuroshio and southeastern coast of Taiwan, a low-velocity region is visible in both surface flow products. Some speedy flow passes east of the Lan-Yu Island and forms the Kuroshio “offshore path” as proposed by Hsin et al. (2008). The mean offshore path extends to 123° E at 22° N (Hsin et al. 2008).
3.2 Variability of geostrophic velocity off southeastern Taiwan
The GSV at 121° E, 22° N and local meridional wind stress (120–123° E, 21–24° N) around eastern Taiwan are generally in good correlation on the seasonal timescale (shown as the green and orange bold lines in Fig. 3), despite some quantitative discrepancies in some years (e.g., 1996 and 2004). In summer (winter) when the wind prevails in the northward (southward) direction, the GSV tends to have northward (southward) velocities (Fig. 3). However, they do not show a good correlation in the higher frequency time series (black and red lines), in which the intra-seasonal variation is expected to play a more significant role.
The southward-flow events also take place farther seaward of the Lan-Yu Island between 122° and 123° E (Fig. 2), roughly coinciding with the mean eastern bound of the Kuroshio at this latitude (Liang et al. 2003; Hsin et al. 2008). However, the southward-flow incident at 122.75° E, 22° N seems to have a longer lapse (49 events in 15 years) and a shorter duration between 7 and 80 days, with an average of 22 days, in comparison with that of 26 days near the Taiwan coast. The occurrence of the southward-flow events east of the Lan-Yu Island does not have a distinct seasonal variation and the southward-velocity signals seem to originate from the eastern boundary (Fig. 2), inferring the impact from the east. Satellite observations have shown the abundance of eddies in the Pacific subtropical regions, especially north of 10° N (Hwang et al. 2004). These westward-propagating eddies reach the study area where they interact with the Kuroshio and modulate the Kuroshio’s transport and path. The impact of eddies on the Kuroshio east of Taiwan has been reported in the literature (e.g., Yang et al. 1999; Johns et al. 2001; Zhang et al. 2001; Hwang and Kao 2002; Hsin et al. 2008; Ichikawa et al. 2008).
3.3 Forcing mechanism for the Kuroshio intra-seasonal fluctuations in winter
As mentioned above, the lapse of the meridional velocity reversal between Taiwan and the Lan-Yu Island is shorter than that east of the Lan-Yu Island. Looking over the entire observational period from 1993 to 2007 (Fig. 2), much more southward-flow incidents between Taiwan and the Lan-Yu Island have been observed during winter. One possible process responsible for these incidents is related to the variation of the GSU southwest of Taiwan (Fig. 5). In contrast, the events east of the Lan-Yu Island may heavily depend on the impingement of the westward propagating cyclonic eddies from the interior ocean. In use of data from in situ field works, mooring array, satellite altimeter, and numerical modeling, Yang et al. (1999), Yang and Liu (2003), Johns et al. (2001), and Hsin et al. (2008) have discussed the Kuroshio–eddy interaction processes around eastern Taiwan, as described in the Section 1.
Because the East Asian Monsoon is the major atmospheric forcing in the study region, the GSV and the meridional wind stress are highly correlated on the seasonal timescale. On the intra-seasonal timescale, on the other hand, their correlation is relatively weak (above-mentioned in Section 3.2). To further look into the relationship between the wind field and geostrophic current field, the composite plots are made. For the period between 1993 and 2007, composites of northward- and southward-velocity events based on the altimeter-based meridional geostrophic velocity are depicted in Fig. 6b, c, e, and f. The critical values of velocity selected for the composites are −10 cm s−1 (mean − 1 standard deviation) for the southward-velocity events and 24 cm s−1 (mean + 1 standard deviation) for the northward-velocity events.
Revealed from Fig. 6, the wind patterns are out of phase between seasons, southwesterly wind in summer and northeasterly wind in winter. However, regardless of seasons, the composite geostrophic current maps show a similar pattern around southern Taiwan. When the surface current off southeast Taiwan flows to the south (the southward-event composite), a clockwise loop (anti-cyclone) appears off southwest Taiwan, and the Kuroshio with a larger intrusive angle is present in the middle of the Luzon Strait (Fig. 6b, e). On the other hand, when the surface current off southeast Taiwan flows to the north (the northward-event composite), the surface current flows in a straight path in the Luzon Strait and forms a cyclonic circulation off southwest Taiwan (Fig. 6c, f). The fact indicates that the negative WSC off southwest Taiwan in winter is favorable for the development of Kuroshio loop current in the Luzon Strait (Fig. 6b, c), as well as for the generation of the Kuroshio offshore path southeast of Taiwan.
Furthermore, the magnitudes of WSC in winter (Fig. 6a–c) are always much larger than those in summer (Fig. 6d–f), implying that the magnitude of WSC is not the key factor influencing the flow off southern Taiwan. The change of magnitude of WSC is critical in altering the GSU off southwestern Taiwan and further influence the GSV off southern Taiwan in winter. On the other hand, the composites of WSC in summer (Fig. 6e, f) are not similar to those in winter, when the southward-event composite WSC off southwest Taiwan (Fig. 6b) is larger than the northward-event composite one (Fig. 6c). Therefore, in summer, different processes seem to work on causing the current to flow southward in the low-velocity region (Fig. 6e, f), which is discussed in Section 3.4.
3.4 Forcing mechanism for the Kuroshio intra-seasonal fluctuations in summer
Unlike the situation in winter, the correlation between velocity components southeast and southwest of Taiwan is fairly poor in summer. This indicates that the WSC off southwest Taiwan and the intrusion of the Kuroshio in the Luzon Strait cannot explain the variation of the GSV between Taiwan and the Lan-Yu Island in this season. Due to the speedy translation of typhoons in a short period (hours to days), it is hard to find close relations between the typhoon-induced wind and geostrophic current in the composite fields (Fig. 6e). Of interest is a northerly wind component to the north of the low-velocity region. Between 1993 and 2007, there were 14 summertime southward-velocity events selected to compose Fig. 6e. Among them, 12 events were formed when the tropical cyclones or typhoons passed around Taiwan and induced northerly wind east of Taiwan. That is to say, that ∼85% (12/14) of summertime southward-velocity events can be attributed to the influence of typhoons, suggesting that the formation of southward-velocity events is highly related to the local typhoon-induced wind around the eastern Taiwan.
A series of numerical experiments are performed to further investigate the relationship between variations in local typhoon-induced wind and surface flow. In summer, the short-term fluctuations of wind speed around Taiwan take place rather frequently. During these high-frequency fluctuations, the wind can reverse its direction within a short time period. These changes in wind can immediately affect the surface flow and temperature southeast of Taiwan. A single event of wind reversal can sometimes make the northward surface current in the low-velocity region slow down (when the original flow velocity is high) or even turn southward (when the original flow velocity is low) in a few days. Another immediate consequence of these wind fluctuations is a temperature drop near the sea surface. A sequential of wind reversal events would persistently weaken the surface flow and sustain a cold surface temperature in the low-velocity region for 1–2 months.
The second example is concerning the impact from a distinct typhoon track. The super typhoon Haitang went from southeast to northwest, which was different from the nearly meridional path of Mindulle (2004), and made landfall at central eastern Taiwan on 18 July 2005 (Fig. 10a). As shown in Fig. 10b, the evolutions of local meridional wind, modeled meridional velocity, and sea surface temperature caused by Haitang are similar to those caused by Mindulle. Distinct from the pre-wind condition of Mindulle in June 2004, the surface Kuroshio formed a loop current into the northern South China Sea with a slowly southward flow in the low-velocity area, and the strength of the Kuroshio (∼1.5 m s−1) in 2005 (Fig. 10c) was somewhat weaker than that (∼2.0 m s−1) in 2004 (Fig. 9c). When Haitang approached the eastern coast of Taiwan on 17 July (Fig. 10b, d), the wind direction reversed from northwestward to southeastward, and the southward surface flow in the low-velocity area became stronger in 17–18 July (Fig. 10d, e). With Haitang landing and passing through Taiwan, the wind turned northward again on 19 July (Fig. 10f). A few days later, the flow in the low-velocity area also revived to a weak northward current (Fig. 11c). It is noteworthy that, unlike the case of Mindulle, the surface Kuroshio east of Taiwan had an obvious alteration in this event (Fig. 9c–f). In 17–18 July, the Kuroshio east of Taiwan was invisible in the surface layer. It seems that the impact of typhoon heavily depends on the strength of the Kuroshio. When the Kuroshio is stronger as the first example above, the impact of typhoon could be much more reduced, and vice versa as the second example above.
The typhoon Matsa (2005), which is a much weaker (maximum velocity is 46 m s−1) than the above two super typhoons (maximum velocities are 64 and 72 m s−1 for Mindulle and Haitang, respectively), is selected as the third example (Fig. 11). Matsa had a significant impact on the current east of Taiwan between 31 July and 06 August 2005. Since Matsa came just 2 weeks after Haitang left, the pre-condition of Matsa (Fig. 11c) was similar to that of Haitang (Fig. 10c). During August 03–04, when Matsa generated southward winds, the surface current in the low- velocity area had a southward velocity of as large as 0.3 m s−1 (Fig. 11b). In the mean time, SST dropped and reached its minimum around August 08. Unlike the case of Haitang (2005), the surface Kuroshio east of Taiwan remained visible when Matsa passed around Taiwan (Fig. 11d, e). After Matsa landed on mainland China, the current returned to the pre-typhoon condition again (Fig. 11f).
To further check the velocity variation in vertical for these cases above-mentioned, the Kuroshio core between southern Taiwan and the Lan-Yu Island located at the sea surface on the pre-typhoon condition. When the typhoon came to the neighborhood of Taiwan, the typhoon-induced wind slowed down the surface flow and then made the Kuroshio core sink into the subsurface of about 50–100 m. Several days later, the core returned to the sea surface. To the flow below 300 m, there was a fair light impacted by the typhoon-induced wind. Furthermore, when local wind forcing was turned off in the EAMS model, the southward current and temperature drop also vanished in the low-velocity region, indicating again that typhoon-induced wind is largely responsible for the southward surface current.
Another interesting phenomenon is related to the reversal of surface current in the Taiwan Strait. Both mooring observations and numerical models have suggested that transport through the Taiwan Strait can reverse its direction when the northerly wind bursts in winter (e.g., Wu and Hsin 2005). Here, we found that southward current can also appear in the Taiwan Strait when a super typhoon sweeps around the region and inserts a large amount of southward momentum into the Taiwan Strait. Figures 10 and 11 show two examples of this. When southward wind blows, the current in the Taiwan Strait gets weaker. If this typhoon-induced wind is strong enough as in the case of Haitang (2005), the southward flow will become dominant in the Taiwan Strait. After the typhoon goes away, the current in the Taiwan Strait will return to its normal condition to flow northeastward in only 1–2 days. Though not confirmed by observations yet, this model result raises an important issue for typhoon and coastal water interaction study, which needs to be investigated further by research.
Two significant intra-seasonal signals in the Kuroshio southeast of Taiwan are revealed by the combined use of satellite data and results from a numerical model. According to the prior studies, the 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 period of 30–180 days. Revealed in the study, these variations are large enough to reverse the surface flow 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.
In the low-velocity region between Taiwan and the Lan-Yu Island, where the shorter period (14–28 days) fluctuations are significant, two forcing mechanisms are proposed to explain the intra-seasonal variations in summer and winter, respectively. In winter, the eastward flow, triggered by the enhanced negative wind stress curl off southwest Taiwan, produces an eastward shift of the main axis of the Kuroshio, thus weakening the current velocity in the low-velocity region. In summer, the typhoon-induced wind seems to play an important role in modulating the intensity and pathway of the surface Kuroshio southeast of Taiwan.
The valuable comments given by Dr. Yasumasa Miyazawa of JAMSTEC, and the reviewers greatly improved both science and the presentation. Authors YCH and CRW were supported by the National Science Council, Taiwan, ROC, under grants NSC 98-2811-M-003-012 and NSC 98-2628-M-003-001. T. Qu was supported by the National Science Foundation through grant OCE06-23533 and by the JAMSTEC, the NASA, and the NOAA through their sponsorship of the International Pacific Research Center (IPRC). SOEST contribution number 7925 and IPRC contribution number IPRC-690.
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