The Luzon Strait is the only deep channel that connects the South China Sea with the northwestern Pacific Ocean. The variability of the upper-ocean flow field inside the strait is mainly controlled by variations in the large-scale East Asian monsoon system and by the different patterns of the strong western boundary current, the Kuroshio intrusion (Hu et al., 2000; Liu et al., 2008). Large changes in ocean dynamics and the marine environment in the upper layer occur over the short term in response to episodic severe weather events such as typhoons, whose extremely strong winds can exert dramatic effects on the upper oceans. According to statistical data published by the World Meteorological Organization, approximately twothirds of the world’s tropical cyclone formations occur in the Northern Hemisphere, especially in the Northwestern Pacific. The proportion and number of these formations that reach typhoon level are higher than anywhere else in the world (Gray, 1968; Webster et al., 2005). During typhoons (or hurricanes) transiting either offshore deep-ocean regions (Babin et al., 2004; Hanshaw et al., 2008; Lin, 2012) or marginal seas (Davis and Yan, 2004; Gierach and Subrahmanyam, 2008; Shi and Wang, 2011), oceanic variabilities in the upper layer in response to typhoon forcing have been widely studied using moored observations (Dickey et al., 1998), airborne measurements (Shay et al., 1992), satellite remote sensing data (Subrahmanyam et al., 2002), and numerical ocean models (Price, 1981; Price et al., 1994; Oey et al., 2007). The effects of riverine inputs, wind mixing, and resuspension are three important factors that affect local biological productivity (Shiah et al., 2000; Zheng and Tang, 2007). There are indications that upwellings induced by episodic typhoon events bring new sources of nutrients from the subsurface water to the euphotic zone, resulting in enhanced nutrient concentrations and production, especially in otherwise typically oligotrophic regions (Chen et al., 2003; Zheng and Tang, 2007).

In the summer and early autumn, typhoons also frequently affect the East China Sea and the South China Sea. The intense winds characteristic of typhoons cause vertical mixing and strong upwellings; both give rise to a diverse range of intensively studied oceanic physical and biogeochemical responses in the upper layer, including bloom events (Lin et al., 2003a, b; Shang et al., 2008; Siswanto et al., 2008; Sun et al., 2010; Chen and Tang, 2012). Although the Kuroshio intrusion generally moves away from the continental shelf during the summer monsoon (Tang et al., 2000), Tsai et al. (2008)found that in some cases a typhoon could create an environment that favors a summertime path for the Kuroshio intrusion northeast of Taiwan Island , which in turn significantly alters circulations along the shelf. Under the influence of a category 4 typhoon, such as Parma in October 2009, there was a strong offshore increase in phytoplankton west of the central Luzon Strait (Zhao et al., 2013), a region generally controlled by nutrient-depleted water even in the presence of adequate light, except in winter. A large body of data obtained from satellites relying on optical, infrared, and microwave remote sensing methods has shown that the passage of typhoons is followed by a rapid drop in the surface temperature and in phytoplankton blooms in the upper oceans. However, direct observations of vertical variations in the ocean before and after typhoons are limited because of the unpredictability of their paths and the paucity of long-term anchored stations.

A self-contained acoustic Doppler current profile (ADCP) mooring was deployed from August 27 to October 6, 2011 at approximately (120.5°E, 20.0°N) and used to record current velocity profiles. During this time, three strong typhoons, Nanmadol, Neast, and Nalgae, passed sequentially through the anchored station. All three typhoons originated in the northwestern Pacific Ocean, reaching maximums of category 4, 3, and 4, respectively, as determined using the Saffir-Simpson tropical cyclone scale. The moving direction of typhoon Nanmadol was roughly parallel to the Kuroshio mainstream east of Luzon Strait (see the absolute dynamic topography contour lines at 220–240 cm in Fig. 1), heading northwest, while typhoons Neast and Nalgae swept over Luzon Island on their way to the South China Sea. Using data recorded during these three typhoons, in the following we describe the short-term temporal variations that occurred in the upper ocean. To assess the influence of these typhoon events on the upper ocean, we used upper-layer current observations and wind data to calculate typhoon-induced upwellings based on wind stresses derived from the maximum sustained wind speeds during the typhoon. The results were then compared with the values estimated from satellite altimeter data. We also examined post-typhoon variations in the biophysical environment corresponding to the vicinity of the typhoon’s path, which showed that chlorophyll-a concentrations were consistently higher after the passage of a typhoon.

Fig. 1 Map of the study area The red star indicates the location of the moored ADCP. The thick solid lines show the paths of typhoons Nanmadol, Neast, and Nalgae, and the circles the central locations of each typhoon over a 6-h time interval. The colors of the circles represent the intensities of each typhoon. TD: tropical depression; TS: tropical storm; Cat.: category. The September climatology within the absolute dynamic topography (unit in cm) is shown by the dashed gray lines and is based on data collected from 1993 to 2010 by the Aviso project. The contour interval is 10 cm. Data from regions shallower than 200 m were excluded.

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This paper is organized as follows. Section 2 presents the data used in this study. Section 3 compares the geostrophic and Ekman currents and examines the physical and biophysical responses of the upper ocean during typhoon passage. Our conclusions are presented in Section 4.

An upward-facing 150-kHz ADCP (Workhorse Quartermaster, Teledyne Instruments), with a 20° transducer and 300-m ideal measuring range, was deployed in deep water (~3 850 m) in the central Luzon Strait (Fig. 1) on August 27, 2011 from R/V Dongfanghong NO 2. The ADCP was mounted on neutrally buoyant glass balls supplying ~200 kg of buoyancy. Horizontal current (u, v) profile data in the form of a time series were collected until October 6, 2011. The observational range extended from the near surface to as close to the depth of the ADCP transducer as feasible. According to the ADCP’s design requirements, it should be moored 3 550 m above the ocean bottom, at a water depth of ~300 m, to measure the vertical profile of current velocity every 5 min, with a vertical interval of 4 m and a total of 74 observed layers. In fact, the pressure sensor onboard the ADCP transducer (5-m accuracy) indicated that the actual depth of the moored ADCP varied quasisinusoidally, with a maximum crest of 260 m and a maximum trough of 460 m (shown in Fig. 2). The equilibrium depth during the observation period was ~310 m. Power spectral analysis of the pressure data confirmed that the dominant tidal constituents around the moored site were the principal lunar diurnal (O₁ : period 25.82 h) and the principal lunar semidiurnal (M₂ : period 12.42 h). Consequently, in this study all of the velocity data were low-pass filtered using the inverse Fourier transform method with the cut-off period set at 33 h. This filtering window eliminated both major tidal information and higher frequency oscillations from the original time series. The selected window width was similar to that used by Beardsley et al. (2004) but shorter than that used by Liang et al. (2008).

Fig. 2 Time series of ADCP transducer depth The dashed line shows the raw data acquired from the ADCP pressure sensor, and the solid line the corresponding low-pass filtered results after removal of the fluctuations for frequencies > 0.030 3 cycles per hour.

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The near-real-time merged products of the altimeter data from Jason-1, Jason-2, and Envisat are used to collect geostrophic velocity (GV) and sea-level anomaly (SLA) data. The latter are available daily from the AVISO project and have a 1/3° resolution. The monthly chlorophyll-a (CHL) concentration as measured by the Moderate Resolution Imaging Spectroradiometer (MODIS; 4-km resolution) onboard the Aqua satellite was also used in our study. The CHL data were processed by the NASA Ocean Biology Processing Group.

The 6-h typhoon-tracking data used in this study are available from the Unisys Weather website. The data included the maximum sustained wind speed near the typhoon center and the longitude and latitude of the typhoon center. A bulk formula with a high wind-speed-limited drag coefficient (Oey et al., 2006) was used to calculate wind stresses, after which the potential contribution of typhoons to the current field in the upper ocean was assessed. In addition to the typhoon-tracking data issued by the Joint Typhoon Warming Center in the USA, we used surface wind data from the real-time along-track global wind products at 10-m height above the ocean surface (hereafter U₁₀). Daily wind data, including ascending and descending passes, were collected by the advanced scatterometer (ASCAT) instrument onboard the Meteorological Operational (MetOp) polar satellites launched by the European Space Agency. The ASCAT L2 wind dataset covers an effective swath width of 512.5 km with a 12.5-km spatial resolution; it consists only of data on ocean vector winds, with no information on soil moisture.

As the 14th named tropical storm of the northwestern Pacific during the 2011 typhoon season, Nanmadol was first recorded on August 22nd at 18:00 UTC. It moved slowly northwestward along the Luzon Strait and swept the southern tip of Island , eventually making l and fall in Fujian Province (China) on August 31st at 00:00 UTC. On August 25th at 18:00 UTC, Nanmadol strengthened into a category 4 typhoon (124.0°E, 16.5°N), at which time it was located at a distance of ~540 km away from the moored ADCP. Thereafter, Nanmadol became slower moving, but remained stronger than a category 1 typhoon, and lingered in the vicinity of Luzon Strait for more than 3 days. The 20th and 22nd named tropical storms, Neast and Nalgae, formed as tropical depressions in the deep interior of northwest Pacific Ocean (~138.0°E) on September 23rd at 12:00 UTC and on September 27th at 06:00 UTC, respectively. Both storms moved westward along a straight path, during which time they progressively intensified to category 3 and 4 typhoons, on September 26th and 30th, respectively. They quickly traversed Luzon Island at their highest wind speeds and entered the South China Sea, with second l and falls in the east coastal area of Hainan Island. Table 1 summarizes the characteristics of each typhoon (the data are available at http://weather.unisys.com). As it passed over the Luzon Strait, Nanmadol had a relatively slow translation speed of only ~3.1 m/s and a decreasing wind speed, albeit still >33.0 m/s. Neast and Nalgae had translation speeds almost twice that of Nanmadol but lower wind speeds. This allowed observations of the short-term temporal variations in the displacement of the thermocline and the enhancement of phytoplankton during the passage of the three typhoons.

Table 1 Physical and other characteristics of the typhoons

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The mean and relevant st and ard deviation of the low-pass filtered time series of u and v at each depth are shown as (U_mean, V_mean) in Fig. 3. U_mean was nearly constant, with a mean value of -4.0 cm/s over the whole water column, while the mean value of V_mean was 6.6 cm/s in the upper 150 m of the water layer and 10.1 cm/s in the lower water column. There was no zero crossing point for U_mean and V_mean. The st and ard deviations decreased with increasing water depth, and the variation of u in the upper layers was much larger than that of v. The maximum st and ard deviation of u was >22.9 cm/s above the 50-m water layer, whereas for v it was nearly 16.9 cm/s at around 110 m. The significant variability of u and v in the upper water column during the short observational period probably reflected short-term events, such as typhoon transitions.

Fig. 3 The vertical distributions of the time-averaged and standard deviation of a) Umean and b) Vmean Solid lines denote the mean velocity and dashed lines the standard deviation.

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The progressive vector diagram in Fig. 4 clearly shows the general trend of the ADCP-measured current vector time series (Chuang and Liang, 1994; Liu et al., 2010). The x-and y-axes represent the time integral of each velocity component of u and v, with the time integration step fixed at 24 h. The first ADCP measuring point, on August 27th, was defined as the coordinate origin while the integral curves display the current directions until October 6th, a period of ~41 days. According to the Euler method, the diagram derived from the ADCP data reflects the temporal variance in the current directions at each of the analyzed water layers. The current at 250 m resembled an inflow into the South China Sea that typically headed northwest. Owing to the lack of hydrological data for the same period, we were unable to determine whether the current was part of the South China Sea circulation west of Luzon Island or if it derived from the Kuroshio intrusion from Luzon Strait. However, this general trend was not the same at 50 m, where two outflow events occurred: one that first moved south and then turned east and lasted from the beginning of the record to September 7th, and the other that traveled northeast beginning in late September and persisting until the end of the record. These two cases were in good agreement with the passage of the Nanmadol, Neast, and Nalgae typhoons, implying the significant influence of typhoons on the upper ocean.

Fig. 4 The progressive vector diagram of the upper-layer currents as recorded by the ADCP from August 27 to October 6, 2011 The empty and solid circles denote the current direction trends in the 50-m and 250-m layers, respectively. The x-axis represents the easterly and the y-axis the northerly direction.

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The monsoon transition in September was dominated by the northern South China Sea (Lu and Chan, 1999). The three typhoons swept through the ADCP station sequentially during the observation period, which allowed us to examine the relationship between wind forcing and water movement in the upper ocean. The low-pass filtered velocity data derived from the ADCP still consisted of two components, the geostrophic and Ekman velocities: u = U_g + U_E. Figure 5 shows the vertical profiles of the cross-correlation between the wind stresses from U₁₀ and the daily mean velocities from the ADCP. As is typical of Ekman flows, the zonal wind stress (τ_x) correlated negatively with v, whereas the meridional wind stress (τ_y) correlated positively with u. Moreover, above the 95% significance level, the influence depth of τ_x reached ~200 m, which was much deeper than the ~120 m of τ_y. At a significance level of 85%, the influence of τ_x was still deeper than that of τ_y. The cross-correlation between τ_x and v reached a maximum negative value of -0.67 at ~70 m, while the crosscorrelation between τ_y and u had a relatively low positive value of 0.4 at 50 m. The cross-correlations were both limited to 0 with increasing water depth, indicative of the absence of a correlation between wind stresses and velocities at a depth below 250 m. Table 2 provides rough estimates of U_g based on the AVISO GV fields and of U_E from the ASCAT U₁₀ products during passage of the typhoons. These estimates show that U_g is of approximately the same order of magnitude as U_E. Together, the results suggest that, during a typhoon, the Ekman flow mainly concentrates in the upper 200 m and that the Ekman velocity component (U_E) is not negligible.

Fig. 5 Vertical distributions of the cross-correlation a) between the meridional wind stress (τy) and u, and b) between the zonal wind stress (τx) and v at each ADCP observation layer The red and blue curves were calculated for a cross-correlation coefficient with >95% and >80% significance, respectively.

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Table 2 Parameters derived from the equations

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The upward displacement of the thermocline resulting from the typhoon-induced upwelling could be calculated as η _typhoon = τ _t / (ρ₀·f· U _tran), where τ _t is the wind stress obtained from the maximum sustained wind speed (shown in Table 1), ρ₀ is the water density, with a nominal value of 1 024 kg/m³, f is the local Coriolis force, and U_tran is the typhoon transit speed (Babin et al., 2004; Walker et al., 2005). Table 2 shows the η_Typhoon variations around the ADCP station in the central Luzon Strait during the passage of each typhoon. The upwelling was more effective for the slowly moving typhoon Nanmadol. Using the reduced gravity approximation η_SLA = -(g/g′)·SLA (Shay et al., 2000), we estimated the depth variation of the isotherm η_SLA from the change in the SLA data before and after each typhoon. Based on a stratification of the historical hydrographic (WOA09) data, the value of g′ was 0.04 m/s² which is representative of the ADCP station. The estimated down-welling of η_SLA was -24.5 m for every 10-cm change in SLA. The results are summarized in Table 2. The η_typhoon values derived from the maximum sustained wind speed agreed well with the differences in the η_SLA before and after the typhoon.

Chen et al. (2003) and Shiah et al. (2000)described the effects of entrainment, vertical mixing, and upwelling in the upper ocean after the passage of a typhoon. Both groups concluded that severe tropical storms or typhoons may play an important role in phytoplankton blooms, by promoting biological activity in typical oligotrophic deep-ocean water, where the effect of terrestrial runoff becomes very weak. In this study we used the monthly images of MODIS-Aqua CHL, representative of the biological index, to characterize the biological response after the typhoons Nanmadol, Neast, and Nalgae. However, because the study areas along the path of Nanmadol and in the vicinity of Luzon Strait had heavy cloud cover before and after the typhoon, only the variabilities in CHL in the inner northern South China Sea subsequent to Neast and Nalgae are presented herein.

Shen et al. (2008)measured very low CHL levels in the South China Sea and Luzon Strait during the summer, and found that CHL correlated negatively with the local sea surface temperature in the inner part of the northern South China Sea. The average CHL concentration inside the sample box in September 2011 was generally low (<0.08 mg/m³) before the typhoon but it was much higher (>0.15 mg/m³) along the typhoon tracks of Neast and Nalgae (black square area shown in Fig. 6a) in October. The pattern of CHL enhancement reflected the asymmetrical biological response. A northwest-southeast-oriented large patch of maximum CHL, with a monthly mean value >0.43 mg/m³, was detected at 117.0°E, 18.8°N, ~140 km away from the right side of the track of typhoon Neast. Another small patch of CHL, with a monthly mean value of 0.46 mg/m³, occurred at 116.6°E, 17.6°N, to the right of the path of typhoon Nalgae. Thus, the ocean’s biological responses to these two typhoons were right-biased.

Fig. 6 a. CHL distribution after the typhoons Neast and Nalgae in October 2009; b. monthly means of CHL every October from 2002 to 2014; c. 9-year mean CHL distribution from 2002 to 2014 except in 2009– 2011 and 2013 The sample box (113°–119°E, 16°–20°N) is indicated as a black square in a and c. The optical remote sensing data inside the sample box were used to calculate the time series of the monthly mean CHL shown in b, where the black and red circles denote low and high levels of CHL, respectively.

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Within the time series of the average CHL concentration in the sample box (the black square in Fig. 6a) every October from 2002 to 2014, four CHL enhancement events, from 2009 to 2011 and in 2013 (red circles in Fig. 6b), could be distinguished. The total CHL concentration during the 4 years was as high as 0.64 mg/m³, accounting for ~42% of the total CHL concentration (1.54 mg/m³) every October from 2002 to 2014. All four CHL blooms appeared after the passage of a typhoon, specifically, Parma (category 4) in 2009, Megi (category 5) in 2010, Neast (category 3) and Nalgae (category 4) in 2011, and Wutip (category 2) in 2013. In the northern South China Sea, the sea surface temperature in October was generally too warm to promote phytoplankton blooms (Shen et al., 2008). The physical mechanism of the rapid growth of the blooms could be mainly attributed to typhoon-induced vertical mixing and upwelling in the upper ocean, such that both subsurface CHL and nutrients were transported into the euphotic zone (Subrahmanyam et al., 2002; Babin et al., 2004; Zheng and Tang, 2007; Sun et al., 2010).

The 9-year average CHL distribution (Fig. 6c), excluding 2009–2011 and 2013, indicated an oligotrophic offshore deep-sea region of the northern South China Sea. The averaged CHL was <0.10 mg/ m₃ in the absence of an impact of typhoons, resulting in an “oceanic desert.”

In this study, a moored ADCP was deployed in the central Luzon Strait from August 27 to October 6, 2011 and data on the upper ocean currents were collected at high frequency. During the observation period, three typhoons sequentially swept the Luzon Strait, which afforded the opportunity to examine the physical and biological responses of the upper ocean to the passing of a typhoon.

The impact of the strong winds on the upper-layer flow field was prominent, especially in the upper 200 m. The Ekman velocity (U_E) was nearly comparable to the geostrophic flow (U_g) during typhoon passage. Using data on the maximum sustained wind speed of the typhoon, we were able to calculate the upward displacement of the thermocline in the vicinity of Luzon Strait. The results were in agreement with those derived from satellite SLA data. Typhoon-induced upwellings were detected and served as a direct mechanism of the observed shortterm active biological responses, such as phytoplankton blooms. In late September and early October 2011, two strong typhoons (Neast and Nalgae) affected the biological environment of the inner northern South China Sea. In October 2011, the local averaged CHL increased 1.5-fold compared with the historical climatology data for previous Octobers when there were no typhoons. The variability in the CHL concentration following typhoons effectively demonstrated a stronger response to the right of the typhoon tracks. More studies, particularly those carried out in quasireal-time and full-depth observations during the passage of a typhoon and along its track, are needed to further our underst and ing of the ocean’s physical and biological response mechanisms to typhoons.

The authors are grateful for the comments of two anonymous reviewers. And we also would like to thank Chief Scientists LIU Zhifei (Tongji University) and TIAN Jiwei (Ocean University of China), Captain JIANG Liujia and the crew of R/V Dongfanghong NO 2 for enthusiastic support of the project. This study is as a partial for fulfillment for LIU’s Ph. D. requirement at IOCAS.