Typhoon induced upper ocean cooling off northeastern Taiwan

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L14605, doi: /2008gl034368, 2008 Typhoon induced upper ocean cooling off northeastern Taiwan Yaling Tsai, 1 Ching-Sheng Chern, 1 and Joe Wang 1 Received 16 April 2008; revised 10 June 2008; accepted 19 June 2008; published 22 July [1] The ocean cooling off northeastern Taiwan induced by typhoons is complicated by the presence of the continental shelf of the East China Sea and the Kuroshio. The cooling is primarily due to the upwelling of the Kuroshio s subsurface water, which accompanies the Kuroshio s intrusion onto the shelf, rather than the entrainment mixing generated by the wind. The Kuroshio s intrusion in this region has a seasonal variation, being more substantial in winter than in summer. Previous studies found that the winter northeasterly monsoon creates an intrusion-favored flow pattern, while the reversed summer monsoon opposes it. This study found that the strong northeast wind, accompanied by a typhoon along a certain path, could significantly alter the circulations on the shelf. An intrusion event will be triggered via a similar mechanism as induced by the winter monsoon, but at a faster pace. A numerical study and observation data are provided to illustrate this mechanism. Citation: Tsai, Y., C.-S. Chern, and J. Wang (2008), Typhoon induced upper ocean cooling off northeastern Taiwan, Geophys. Res. Lett., 35, L14605, doi: /2008gl Introduction [2] The upper ocean cooling induced by tropical cyclones in the open ocean has attracted significant attention on the part of researchers. The most striking feature of this cooling is the biased pattern caused by the high correlation between the wind and current on the right side of the storm track in the northern hemisphere. On the other hand, irregular cooling patterns are produced near the continental shelf area because regional geometry and bathymetry interact with the storm-induced circulation. Mitchell et al. [2005], for example, found that greater vertical mixing generated to the right of the storm track hindered the development of the bottom Ekman layer and weakened the onshore flows of the cooler off-shelf water, resulting in stronger cooling to the left of the track. On the continental shelf of the East China Sea (ECS) near northeastern Taiwan, besides the complicated topography, the strong western boundary current, Kuroshio (KS), is another important factor affecting the tropical cyclone induced responses. [3] The KS flows northward along the east coast of Taiwan until it encounters the ECS continental shelf/slope off northeastern Taiwan. The shelf/slope, which is aligned approximately in the zonal direction (Figure 1a), is perpendicular to the flow direction of the KS, such that the KS mainstream then follows the isobaths heading northeast. 1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan. Copyright 2008 by the American Geophysical Union /08/2008GL Some of the KS water may overrun the shelf break and the uplifted subsurface water usually forms a cold pool on the outer shelf. This on-shelf intrusion varies seasonally (Figure 1a), being more substantial in winter [Chuang and Liang, 1994]. The winter northeasterly monsoon not only induces the shoreward migration of the KS by Ekman drift, but also decreases the strength of the Taiwan Strait (TS) outflow. Consequently, a wintertime flow pattern favorable for intrusions is created in this region [Chao, 1991]. In summer, the monsoon reverses to southwesterlies, which enhances the TS outflow and counters the shoreward movement of the KS. Furthermore, a high-pressure region on the shelf formed by the subsurface KS water that intruded during the previous winter effectively blocks the KS off the ECS shelf [Hsueh et al., 1993]. A countercurrent on the inshore side of the KS emerging from the block flows along the shelf break. As it flows over the Mien Hwa Canyon (MHC), a cyclonic circulation is induced that causes upwelling [Hsueh et al., 1993]. [4] Previous studies show that the interaction among the monsoon, KS, and TS outflow can explain the seasonal variation in the scale and frequency of intrusion events near northeastern Taiwan. Yet the cause of summer intrusion events identified from current measurements [Chuang and Liang, 1994] and satellite observations [Chang et al., 2008], needs further study. These summer events coincide with typhoon passages over or further offshore of this area, and they feature intensive cooling and shorter periods as compared to winter events. Since the KS intrusion can contribute significant amounts of salts and nutrients to the ECS, determining how it is triggered by typhoon passages needs to be clarified. [5] In this study, we analyze a post-storm observation near northeastern Taiwan, which shows that the hydrography field on the shelf is strongly modified by the KS intrusion. An examination of sea surface temperature (SST) time series data at MHC indicated that cooling in this area did indeed frequently occur after storm events in summer. A numerical model was then used to study the cooling process. 2. Hydrographic Observations [6] Chern et al. [1990] conducted a hydrographical survey in the study area during September 14 19, 1987, after Typhoon Gerald had passed through the TS. Gerald traveled in the northwest direction from the east of Luzon, passed the southern tip of Taiwan, and continued northwestward through the strait until its landfall (Figure 3c). It maintained typhoon intensity for two days and then decreased to a severe tropical storm when entering the TS. [7] To examine the influence of storm passage on the hydrography field in this area, this study first obtained the background temperature and salinity field from a compiled L of6

2 Figure 1. (a) Temperature (shaded contour) and salinity (white contour, unit PSU) fields at 50-m depth in the study area in summer (July September) averaged between Dotted contours are bathymetry in unit m. Kuroshio paths in winter and summer and Taiwan Strait outflow are drawn schematically. (b) As in Figure 1a except for the deviations of Chern et al. [1990] from Figure 1a. (c) As in Figure 1b except for the meridional transect ( E, the dot-dashed line in Figure 1a). dataset collected from by the National Center for Ocean Research in Taiwan. Figure 1a shows the mean summertime (July to September) hydrography fields at the 50-m depth as calculated from this dataset. The cold eddy corresponds to the upwelling area centered at MHC and has the KS subsurface water characteristics (21 C, 34.4 PSU). To the west of the cold eddy on the shelf region, hightemperature and low-salinity water originating from the inner shelf and TS are observed. To the south of the cold eddy, the KS front can be identified as a temperature front aligned in the southwest-northeast direction. [8] Figure 1b, obtained by subtracting the 20-year mean from the post-gerald measurement, shows that the temperature and salinity anomalies at the 50-m depth resulted from storm passage. The water on the shelf is generally colder and more saline, so that deviations of 2 C and 0.2 PSU are found near the cold eddy center. This anomaly implies a cold and saline water source away from the shelf because water with salinity higher than 34.6 PSU normally does not appear on the southern ECS shelf in summer. Hence, cooling and salinity increases of this magnitude are unlikely to have been contributed from the water below through some process involving vertical mixing of continental shelf water. A filament of low salinity water, which stems from the mixing of the shelf water and the subsurface KS water [Chern et al., 1990], appeared southeast of the cold eddy. This indicates that some shelf water flowed seaward along the north shore of Taiwan and rejoined the KS east of the cold eddy. Further south, the temperature and salinity of the KS water increased in most regions east of Taiwan. Another comparison of the transect data along the E meridian across the center of the cold eddy shows that the post-storm ocean in the upper 30 m is evidently colder and saltier, except on the shelf break at around N (Figure 1c). This area corresponds to the filament water which extends to about 120 m. To the south of the shelf, the water is warmer and more saline below 30 m, which indicates the shoreward movement of the KS. [9] In summary, the analysis may provide only a snapshot of an event, yet the deviations of the water mass characteristics on the shelf from the 20-year mean suggest that the KS must have intruded onto the ECS in conjunction with the passage of Typhoon Gerald. 3. Sea Surface Temperature Data [10] The observed temperature decrease in the cold eddy region may be an indicator of a summer intrusion event. 2of6

3 Figure 2. (right) Tracks of storms with warnings issued in Taiwan from May to September in the years The number denotes the order of appearance. (left) Time series of daily SST in the cold eddy (marked by the cyan dot in the track plot) with the period of corresponding storm events marked with thick line segments. Note red and blue lines denote, respectively, warming and cooling periods. This study then acquired the time series data of daily satellite measured SST in this area from and examined possible typhoon induced cooling events in the years The SST data in Figure 2 corresponds to the period when there were typhoon-warnings issued by the Central Weather Bureau in Taiwan. Figure 2 also shows that the prevailing track of these typhoon events is a relatively straight line, heading west or northwest, sweeping across the vicinity of Taiwan. [11] The SST data in this area generally retained a value of around 28 C in summertime, but variations larger than 2 C have also been observed. It is noteworthy that corresponding storm events can be found for such significant cooling [e.g., Haitang 2005 and Bilis 2006]. Although the cooling magnitude depended on the storm intensity, which varied among these cases, the data showed that in most events the SST decreased, particularly for those of prevailing tracks. Another type of storm that originated from the South China Sea and traveled northeastward along the continental shelf also induced distinct cooling [e.g., Conson 2004, Chanchu 2006]. The SST in the cold eddy changed abruptly during these events, whether the storm passed by Taiwan from the east or the west. For storms that traveled further south or north of Taiwan [e.g. Sanvu and Khanun 2005], the corresponding SST variation was small and may appear to increase. Lastly, storm events with curved tracks [Meari 2004 and Shanshan 2006] showed a different trend in SST variation. Such variation may be attributed to the differences in their intensity, translation speed, and nearest distance to Taiwan. [12] The SST data revealed that significant cooling off northeastern Taiwan occurred frequently and coincided with the storm passages in summer. Such cooling may have resulted from a KS intrusion event, as suggested by the previous analysis of the hydrographic data. To further illustrate this process, a process-oriented numerical study on the ocean response to a storm passage case was conducted and is discussed below. 4. Numerical Study [13] A 3-dimensional hydrostatic primitive equation model based on work by Semtner and Mintz [1977] was adopted to study the cooling events pertaining to typhoon passage off northeastern Taiwan. The model had realistic bottom topography but a rigid-lid on the top. The rigid-lid approximation was applied based on the evaluation of the ratio in this area: f 2 L 2 /gh =(510 5 sec) 2 ( m) 2 / (9.8 m/sec 2 ) (100 m) 0.1 1, where f is the Coriolis parameter; g is the acceleration due to gravity; L is the width of the continental shelf; and H is the averaged depth on the shelf [Gill and Schumann, 1974]. [14] The model domain covers most of the western North Pacific and the South China Sea (Figure 3a), with a horizontal resolution of 0.2. There were 19 non-uniformly spaced z-levels in the model with a total depth of 4 km. To resolve the response of the upper ocean, the upper 250 m in the model consisted of eight 12.5-m and six 25-m layers. The initial field (mean state in July) was calculated using a larger model domain (North Pacific), driven by the monthly mean SST and wind, and integrated until a quasi-steady state was attained. The salinity in the model was kept constant (35 PSU) throughout the entire calculation. In the initial field, the TS current flowed north and skewed to the east as described by Jan et al. [2002], and the KS east of Taiwan also showed reasonable scale and patterns (Figure 3b). [15] The storm wind was calculated based on the Rankine Vortex theory [Holland, 1980] and imposed upon the monthly mean wind field. A northwestward straight path 3of6

4 Figure 3. (a) Initial surface temperatures and currents in the model. (b) Upper 50-m mean temperatures and currents of the initial field near Taiwan. The x and the dashed line denotes, respectively, the cold eddy site and the transect line in Figure 4. (c) Paths of typhoon Gerald (black) and the storm in the numerical study (orange). Colors indicate the bathymetry. (d f) Same as Figure 3b except for the deviations from Figure 3b at different times. was chosen to represent the prevailing typhoon track in this region (Figure 3c). The storm had a steady translation speed of 5 m/s and constant intensity (a radius of maximum wind of 45 km and a maximum stress of 2 Pa). The modeled storm wind in the north of the TS was generally a strong easterly/northeasterly before the storm center arrived. In reality, the strait may channel the wind into a northeasterly, which however is not considered in the model. [16] Figures 3d 3f reveal the series evolution for the averaged temperature and horizontal velocity anomalies in the upper ocean of the model around Taiwan. Before the storm center arrived at Taiwan, it had induced a strong divergence flow that forced the KS water to move shoreward to the east coast of Taiwan. Therefore, positive temperature anomalies in the downwelling area appeared along the coast and were transported downstream by the KS (Figure 3d). Note that the current in the TS already contained a large southward anomaly. The cooling in the study area occurred as the currents in the TS began to flow southward, bringing the KS across the slope (Figure 3e). At the same time, the storm also induced negative temperature anomalies along the west coast of Taiwan. After the storm passed, the currents in the TS gradually reverted to the prestorm direction and the cooling weakened (Figure 3f). The cold water formed by the intrusion in the study area then flowed northeastward with the mean flow. [17] The decline of the northward TS outflow would certainly modify the circulation to the north of Taiwan that the blocking high pressure region, mentioned by Hsueh et al. [1993], would become weaker. A flow field similar to that induced by winter monsoon, but reacting faster due to stronger storm wind, emerges on the shelf. In consequence, the shoreward KS can intrude onto the shelf. Figure 4a shows the time series of the wind stress magnitude and horizontal current velocity at the 25 N zonal transect. The direct influence of the wind persisted for about a day in the TS and the westward flow velocity was greatly enhanced within that period. As the storm passed, the TS outflow gradually resumed its direction with a small modification from the inertial oscillation. While the currents in the TS were mainly wind-driven, the KS off the east coast of 4of6

5 Figure 4. (a) Time-longitude plot of wind stress magnitude (shaded contour) and the model produced upper 50-m mean currents (arrows) at the 25 N transect (marked by the dashed line in Figure 3b). Contour lines west and east of 121 E are velocity anomalies for the U and V components, respectively. The time axis is also labeled with units of local inertial period (IP). (b) Model produced time series of upper ocean velocity (arrows, with components of U and W 360) and temperature anomalies (contours) near the cold eddy (marked by the x in Figure 3b). Shaded area indicates cooling greater than 6 C with 1 C interval. Taiwan responded differently to the storm. In the windforced period, the northward component of the KS increased to about 150 cm/s in the upper ocean (Figure 4a). Such increase is in response to both the wind and the withdrawal of the TS outflow. The inertial oscillations took place as the storm left and the current speed variation was distinct. Overall, the storm induced a shoreward movement of the KS and its speed was reduced afterward. [18] Figure 4b shows a time series of the ocean temperature anomalies and velocity vectors near the cold eddy. The prominent cooling was clearly associated with the KS advance up to the shelf. It occurred mainly at the upper thermocline, with the magnitude once reaching 8 C. Poststorm inertial currents occurred and persisted for a few days. The cooling near the 60 m depth decayed quickly and the temperature below 100 m increased by about 1 C with the storm induced mixing. 5. Discussions [19] The results of the model proposed by this study indicated that the complicated flow field off northeastern Taiwan could be dramatically altered by typhoons. The observed cooling there may then be due to the upwelling of the KS subsurface water, accompanied by its intrusion onto the continental shelf, while the intrusion occurs because the northward TS outflow and hence, the blocking high becomes too weak to block the shoreward KS out of the shelf in the storm event. Note that the cooling was observed whether or not the storm center traveled directly over this area, although it may be much enhanced with a local passage of the typhoon over the area due to stronger local wind stresses. The diversity of the tracks among the cooling events showed that the related mechanism is rather complicated. [20] To conclude, typhoon passage provides one mechanism of creating an environment favorable for the KS intrusion from the northeast of Taiwan in summer. Other factors such as: KS instability, internal tide, impinging meso-scale eddies, etc., may induce intrusions as well, and these factors are also worth exploring. More experiments are necessary to fully understand their effects on the KS-TS-ECS flow system. 5of6

6 [21] Acknowledgments. This study is sponsored by the grant from the National Science Council of the Republic of China, NSC M MY3. References Chang, Y., H.-T. Liao, M.-A. Lee, J.-W. Chan, W.-J. Shieh, K.-T. Lee, G.-H. Wang, and Y.-C. Lan (2008), Multisatellite observation on upwelling after the passage of Typhoon Hai-Tang in the southern East China Sea, Geophys. Res. Lett., 35, L03612, doi: /2007gl Chao, S.-Y. (1991), Circulation of the East China Sea: A numerical study, J. Oceanogr. Soc. Jpn., 46, Chern, C.-S., J. Wang, and D.-P. Wang (1990), The exchange of Kuroshio and East China Sea shelf water, J. Geophys. Res., 95(C9), 16,017 16,023. Chuang, W.-S., and W.-D. Liang (1994), Seasonal variability of intrusion of the Kuroshio water across the continental shelf northeast of Taiwan, J. Oceanogr., 50, Gill, A. E., and E. H. Schumann (1974), The generation of long shelf waves by the wind, J. Phys. Oceanogr., 4, Holland, G. J. (1980), An analytic model of the wind and pressure profiles in hurricanes, Mon. Weather Rev., 108, Hsueh, Y., C.-S. Chern, and J. Wang (1993), Blocking of the Kuroshio by the continental shelf northeast of Taiwan, J. Geophys. Res., 98(C7), 12,351 12,359. Jan, S., J. Wang, C.-S. Chern, and S. Y. Chao (2002), Seasonal variation of the circulation in the Taiwan Strait, J. Mar. Syst., 35, Mitchell, D. A., W. J. Teague, E. Jarosz, and D. W. Wang (2005), Observed currents over the outer continental shelf during Hurricane Ivan, Geophys. Res. Lett., 32, L11610, doi: /2005gl Semtner, A. J., and Y. Mintz (1977), Numerical simulation of the Gulf Stream and mid-ocean eddies, J. Phys. Oceanogr., 7, C.-S. Chern, Y. Tsai, and J. Wang, Institute of Oceanography, National Taiwan University, P. O. Box 23-13, Taipei 106, Taiwan. (cschern@ntu.edu.tw) 6of6