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Numerical study of typhoon-induced ocean thermal content variations on the northern shelf of the South China Sea
Continental Shelf Research 42 (2012) 64–77
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Research papers
Numerical study of typhoon-induced ocean thermal content variations on the northern shelf of the South China Sea Yaling Tsai, Ching-Sheng Chern n, Joe Wang Institute of Oceanography, National Taiwan University, P.O. Box 23-13, Taipei 106, Taiwan, Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2011 Received in revised form 27 April 2012 Accepted 3 May 2012 Available online 11 May 2012
This paper investigates the downwelling caused by tropical cyclones near the continental shelf. A numerical ocean model is used to simulate the ocean’s response to storms moving along shelf across the northern South China Sea. Five westward-moving storms from this region were selected for the study. Data on the sea surface temperature and the height deviations showed that there was an increase in the water levels and delayed cooling on the shelf to the right of the track during these storms. Such phenomena were observed in regions south of China’s coast and near Hainan Island, and the model’s results also revealed that subsurface warming occurred at both locations. The site south of China’s coast is associated with the strong post-storm convergent phase when warm water is brought both downward and seaward generating a temperature front on the shelf. The land barrier resulted in a cross-shelf circulation, which helped to block the movement of cold water towards the shore in the following divergent phases; therefore, the downwelling front continued for several days. The site east of Hainan resulted from an interesting combination of a westward storm path and coastal topography. The position of Hainan in relation to these storms greatly diminished the off-shelf path of stormgenerated flows, so the region became a significant convergence zone. Therefore, the downwelling in this region increased with the storm’s approach, and the warming continued until the storm passed. These examples indicate that the tropical cyclones’ influence on the ocean’s upper level heat content in the shelf region can be positive and may possibly have a significant effect on local or regional weather systems. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Typhoon Downwelling Numerical study Continental shelf Northern South China Sea
1. Introduction Ocean cooling is a common phenomenon induced by tropical cyclones (TCs). The cooling is caused by cold thermocline water being transported and mixed into the surface layer to replace the strong surface current forced out from the storm’s center by the wind. The upper ocean temperature is thus effectively reduced; the effect is easily observed by satellite measurements. Numerous studies on the oceanic response to TCs have focused on this upwelling and mixing process, whereas the warm surface water emanating from the storm’s center has received less investigative attention because the heat carried away by the divergent water may disperse into the surrounding environment, appearing to be negligible in the open ocean. However, as a storm moves closer to the shelf region, the shallow depth and landmasses become the boundaries that confine the redistribution of heat. In other words, the heat transported by the warm surface water may accumulate and be carried downwards at these boundaries
n
Corresponding author. Tel.: þ886 2 33661374; fax: þ886 2 23626092. E-mail address: [email protected] (C.-S. Chern).
0278-4343/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2012.05.004
where, in conjunction with the wind direction, downwelling may occur. It may become rather significant, as was the case with Hurricane Ivan, when during the storm’s progression off the Gulf Coast, Mitchell et al. (2005) observed a near-bottom temperature increase of 3 1C in the outer continental shelf. Although Ivan may have produced a significant increase in the bottom temperature, the downwelling was restricted by the storm’s path, which was perpendicular to the coast, as is often the case for Gulf Coast TCs. The winds that favor downwelling for TCs on this track can persist for only the first half of the forced period, and the temperature increase will quickly disappear once the storm center moves on. Presumably, in the northern hemisphere, notable downwelling could occur when the along-shelf wind is strong and persistent and has the coast to its right. A region characterized by this condition is the continental shelf area in the northern South China Sea (NSCS, Fig. 1a) where westward-moving TCs frequently occur (Wu et al., 2005). In this region, the southwest monsoons produce noticeable coastal upwelling during the summer months (Jing et al., 2009). The coastal areas east of Hainan Island (HNI), south of the Shantou Coast and east of the Leizhou Peninsula, are all areas of upwelling where observations showed that the sea surface temperature
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Fig. 1. (a) Bathymetry map of the SCS; (b) Transect locations used in SST and model output plots; (c–g) Tracks of five storm cases.
(SST) was 2 1C lower than the off-shore water (Jing et al., 2009). Therefore, it is interesting to determine how these features can be modified by storms passing over the NSCS. Su et al. (2011) compared a pre-storm and post-storm hydrographic survey conducted off eastern HNI during two upwelling seasons, and they found that TC Pabuk’s (2007) strong northerly winds reduced the thermocline’s strength in the coastal seas and that the upwelling strength was decreased by the development of a frontal zone between the mixed coastal water and the offshore water. However, because Pabuk’s track passed through the northern edge of the region (along 221N, see Fig. 14a), its influence on these upwelling features was presumably small, based on the study by Kuo et al. (2011), who found that major downwelling caused by a westward-moving TC along 201N into the SCS occurred near the shelf area along the coast of China and in the Taiwan Strait. Storms that cross the NSCS along the shelf, but further to the south, should produce response that is similar but of greater magnitude. Because the SCS is a relatively enclosed ocean basin and an important source of moisture for the surrounding regions’ summer rainfall (Simmonds et al., 1999), it is essential to elucidate the storm-induced heat distribution process. The purpose of this study is to determine how the shelf ocean is modified by TCs moving along-shelf in the NSCS. In the next section, satellite data acquired during some storm events are examined to identify the corresponding downwelling features. A process-oriented numerical study was adopted to explain the correlations.
2. Data and analysis From the 1985–2005 TC database of the Regional Specialized Meteorological Center (RSMC) Tokyo—Typhoon Center (Fig. 1c–g), five westward-moving storms in the NSCS were selected as case studies to determine how along-shelf storms may affect
shelf circulation. The storm tracks are mostly north of 181N, and all passed HNI from the south, so the storm-induced circulation can interact with the bathymetry and coastal topography. The satellite data during these five storms have been searched to ascertain whether the downwelling process on the shelf is evident. Only three cases, Luke, Wukong and Damrey, were found to have corresponding sea surface height anomaly (SSHA) data that showed how these storms affected the ocean’s volume distribution on the shelf. The pre- and post-storm SSHA fields were obtained from the AVISO program (http://www.aviso.ocea nobs.com/), and their differences were calculated (Fig. 2). The data indicate that the SSHA was generally lower along the storm track but higher on the shelf side. The SSHA increment was particularly evident in the case of Wukong, in which a significant negative center ( 18 cm) was generated to the right of the track where the storm lingered for 18 h. Correspondingly, a strong positive center (18 cm) occurred on its northern shelf near 1141E. This localized maximum south of Hong Kong was also observed in the case of Damrey and Luke. In the Wukong and Damrey cases, a large increase in the SSHA appeared around the northeast region off HNI, which, together with the Leizhou Peninsula, juts out into the NSCS shelf and is on the path of these westward-moving storms. It appears that only TCs of typhoon intensity can produce an appreciable accumulation of shelf water off northeastern HNI because such patterns were not associated with tropical storm Luke. Using daily SST data, it was possible to examine how the downwelling process varied over time in this region. The multiple-satellite blended SST data were obtained from the NOAA Coast Watch program (http://coastwatch.pfeg.noaa.gov/) with a spatial resolution of 0.11. However, data were available only for Damrey case. Fig. 3 shows the time series plots for three transects (Fig. 1b) along 19.91N, 111.21E and the 100 m isobath (transect A). The SST variations near the shelf, as a result of Damrey’s passage, are depicted as a cooling event starting on approximately September 24th when the storm entered the region.
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Fig. 2. SSHA deviation (contour interval: 3 cm) in 7 day spanned over Luke (a), Wukong (b) and Damrey (c) event with their tracks superimposed; dashed line is the 500 m depth contour.
The subsequent cooling became noticeable with the strengthening of the storm, and its maximum occurred between September 25th and 26th. However, there were several areas on the shelf where cooling was delayed for at least one day, until September 25th. Off of northeastern HNI, the delayed cooling that occurred was visible west of 1121E (Fig. 3a) and north of 211N (Fig. 3b). This is exceptional because the storm passed over this region between 112–1141E at maximum strength and with a significant cold patch ( 4 1C) next to it. A similar delay in cooling was also observed south of Hong Kong in the mid-section of transect A, where even post-storm cooling is weak. In summary, satellite observations show that, during these westward TCs, the ocean’s surface height appears to be elevated in some areas on the shelf and that surface cooling is delayed. Although the resolution of the satellite data may be poor for clarifying the cooling-impeding and water-accumulation features, they still indicate that the heat redistribution process on the shelf in the SCS, during westward-moving storms, is complicated. In the next section, a numerical model is used to simulate those five case studies to provide insight into the dynamics related to these processes.
3. Numerical model and storm cases The model used in this study is a three-dimensional primitive equation model modified from the one used by Semtner and Mintz’s (1977). It assumes rigid-lid, hydrostatic and Boussinesq
Fig. 3. Time series plot of SST anomaly relative to the 21st of September along. 19.91N (a), 111.21E (b) and the along-shelf transect A (c) during the Damrey event; gap represents missing data.
approximations and uses a level-2 turbulence closure scheme to estimate the vertical eddy viscosity. The model covers most of the northern Pacific (1001E–1201W, 0–401N) and the SCS, with a grid resolution of 0.21. The model has 25 uneven vertical layers with a higher resolution in the mixed layer and thermocline. The maximum depth in the model is limited to 4 km, but the topography in the shelf region is represented realistically. A model-produced quasi-steady state for September is adopted as the initial field for all cases (Fig. 4). Detailed model configurations are reported by Kuo et al. (2011). A Rankine Vortex storm wind field (Holland, 1980) based on the RSMC’s best-track data (Fig. 5) was then superimposed on the initial field. It is a symmetric wind field parameterized by the storm’s radius of maximum winds and central pressure. During the 6-h best-track data interval, the intensity and size of the storm are assumed constant, but the
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Fig. 4. The upper 100 m mean of the model used initial field of temperature (shaded contour) and horizontal current (vector) in the SCS; dashed lines are the depth contours of 50, 100 and 250 m; the white circle marks one of the monsoon-induced upwelling regions.
center location is interpolated linearly within time steps between two issued positions. The five selected case studies have similar tracks but vary in size, lifespan and intensity (Figs. 1 and 5). TC Andy (1985), Brian (1989) and Wukong (2005) were all generated in the SCS. Wukong and Andy had similar tracks, with the former’s being stronger and larger, while Brian’s track closely followed the shelf break before it reached HNI. TC Luke (1994) and Damrey (2005) both formed east of Luzon and then moved westward across the NSCS. Luke was weak and passed through quickly, but Damrey was a large and slow-moving storm. Damrey traveled along the shelf break for more than a day before moving onto the shelf, while Luke went onto the shelf further east. The intensity peaks for the five cases are spread over 3 categories: Wukong, Damrey and, for a short time, Brian, were typhoons; Andy was a severe tropical storm, and Luke was a tropical storm. In each case, the model was integrated over 10 day, using the same initial conditions, so the differences in the ocean response were due to each storm’s individual characteristics.
4. Results 4.1. Forced period response Fig. 6 shows four snapshots of upper ocean temperatures anomaly adjacent to the storm’s eye for all five cases. The induced vertical varying temperature anomalies were averaged over a depth of 0–100 m (or the bottom, if shallower than 100 m, and hereinafter referred to as the ‘‘upper 100 m mean’’) to illustrate the net effect of the storm’s passage in redistributing heat. This forced period’s composite picture shows that the intensity of the ocean’s response is affected mainly by wind strength. The disturbance caused by tropical storm Luke was the weakest, that caused by the severe tropical storms Andy and Brian were much stronger, and that caused by typhoons Wukong and Damrey were the strongest of all. The model reproduced cooling along the storm tracks, but in every case, the warming patches were of particular interest, appearing in the front half and especially the right front quadrant of the storm. The warming is particularly
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Fig. 5. (a) Time series of the maximum wind speed for five cases from the RSMC (solid line) and Hong Kong Observatory (dashed line); the closed circle marks the time of landfall; (b) the same as (a) but for the radius of the maximum wind (black) and the estimated moving speed (gray) based on the RSMC data.
evident on the NSCS shelf when the storm was near HNI. The temperature in this initially cooled upwelling region (white circle in Fig. 4) changed with each storm’s passage to the extent that even weak tropical storm Luke produced a slight increase. The wind forcing in this region must reach a certain level so that the wind-generated flow may interact with the coast and ocean-bottom topography and generate downwelling. For example, by comparing the model results we found that, for a storm with a radius of maximum wind of 15 nm and travelling at a speed of 10 km/hr, it was necessary for the wind stress to be of a severe tropical storm strength (e.g., Brian) rather than of a tropical storm strength (e.g., Andy) to produce warming on the shelf at approximately 1121E (Fig. 6a and b). For a TC, however, the forcing strength depends not only on the storm intensity. Other factors such as storm’s size, path and moving speed must also be considered. Therefore, for storms with similar intensity, a larger wind field and a path relatively closer to shore may become necessary conditions in generating a warm pattern, such as the one that occurred on the northern shelf at 1161E with Damrey, but not with Wukong. 4.2. Relaxation period response In comparison with the forced period responses, Fig. 7 shows the upper 100 m mean temperatures anomaly for the five cases averaged over one inertial period after the storm moved out of each divided region, as shown in Fig. 6. It appears that most of the forced-period warming that occurred in front of the storm was transient. The temperature deviation along the storm track was negative, except near the east coast of HNI. On the other hand, the warming on the northern SCS shelf, although slightly weaker, remained and enlarged in terms of the coverage area. This pattern is evident with both Wukong and Damrey, whose wind intensities were sufficiently large to generate noticeable
changes on the shelf. Because Damrey appears to have interacted with the ocean shelf the most, the following discussion is based primarily on the results for its model run. To determine how long the shelf warming lasts, the time series for the upper 100 m mean vertical velocity and temperature anomaly of transect A were plotted in Fig. 8. The stations in transect A align, approximately, along the 100-m isobaths with station A1 closer to HNI and station A11 on the eastern end (Fig. 1b). Damrey’s path from September 23rd–25th is approximately aligned with the shelf break and transect A, and the storm traveled from east to west and intersected transect A at station A1. The wind forcing of this transect was thus greater towards A1 due to its shorter distance to the storm center and the increasing storm intensity. Fig. 8 clearly shows that significant warming occurred near stations A1 and A2, A3–A5, and A7–A9 in either the forced period or the relaxation period. The locations of these stations coincide approximately with the two positive SSHA regions discussed in Section 2. From this figure, we know that the storm-induced ocean cooling at most stations occurred as soon as the front half of the storm passed, which is consistent with the findings of the study by Price et al. (1994). At A1 and A2, however, the cooling was delayed until the end of the forced period. Besides the residual flow, the responses in the wake period comprised pronounced near-inertial period oscillation in every station. Near station A1, the much stronger upwelling and mixing process quickly deplete the positive temperature anomaly produced in forced period. On the other hand, warming appeared again after the first post-storm downwelling near stations A3–A5 and A7–A9 and has been the predominant phase since then. In summary, contrary to the response in the open ocean, storminduced warming in the upper ocean is significant in the shelf region of NSCS. In addition, the responses in the along-shelf directions are not unvaried because the coastal geometry in this region is
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Fig. 6. The model-calculated upper 100 m mean temperature anomaly around the storm center at the time specified on the map for five cases, (a) Andy (b) Brian (c) Luke (d) Wukong and (e) Damrey.
complicated with the appearance of the Leizhou Peninsula and the HNI. In the next three sections, the model results at stations A8, A4 and A1 are discussed separately so that the effect of coastal boundary to the storm-induced warming on the shelf can be clearly illustrated. 4.3. Warming near station A8 The noticeable temperature response at A8 is that, instead of cooling, warming was significant in the relaxation period. This station is located south of Hong Kong on the shelf and Damrey passed further south as a severe tropical storm. The wind stress at A8 had a dominant along-shore component that lasted for more than one day. Winds with the coastline to the right can generate shoreward tranport and it is thus necessary to examine the flow
and temperature structure in the cross-shelf direction. Therefore, the model results are further analyzed at transect B (Fig. 1b) which Damrey crossed over from the 19.71N shelf-break on September 24th. The temperature and current fields of transect B are shown in Fig. 9 for every 4 h during the wake period beginning on 06:00 September 25th for a total of 1 inertial period (IP, 36 h). The mean field (Fig. 9k and l) and the oscillating components (Fig. 9a–j) within this IP are discussed separately.
4.3.1. Response associated with the residual flow The net influences of the storm on transect B within this IP (Fig. 9k and l) are obtained by applying the time averages to the model results. It is clearly observed that the isotherms were
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Fig. 7. The same as Fig. 6 but averaged over one inertial period (36 h) after the storm moved out of each sub-region.
uplifted near the shelf break but tilted downwards on the shelf, which produced a temperature front at mid-shelf that was cold in the south but warm in the north (Fig. 9l). This structure results from the disturbance exerted by a storm with its center passing from the south and a downwelling generated near the coast. A westward current developed through geostrophic adjustment near the shelf-break (Fig. 9k), which subsequently affected the oceanic conditions downstream (see Section 4.5). Shoreward from this current, the flow was moving mainly to the east because of the interaction between the coastline and the recirculation of the storm-induced flow. The cross-shelf component during this IP was stronger near the surface where it went off-shelf due to a strong convergent flow towards the storm track. 4.3.2. Response associated with the oscillatory flow The oscillating motions in this post-storm IP (Fig. 9a–j) are obtained by subtracting from the model results the IP-averaged
current field (Fig. 9k). The time frame for these plots is specified in IP relative to the beginning of the storm’s wake period (Fig. 9a). To highlight the warming feature on the shelf, the initial but not the IP-averaged temperature field was removed from the model results. Unlike the response in the open ocean where the inertial motions in the response center and its vicinity are relatively in unity, apparent phase difference was found in the motions between the response center and the coast. This is because the outreach of the storm-induced flows was confined by the land boundary and the horizontal unity was thus disappeared on the shelf. For example, horizontal flow divergence occurred near the coast when the response center was in convergent phase (Fig. 9a–d), and vice versa (Fig. 9e–h). Phase lag can also be observed in the vertical velocities associated with the horizontal divergence or convergence as seen in Fig. 9e in which upwelling and downwelling occurred at the shelf break and the coast, respectively. In addition, the horizontal flow directions in the
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study by Austin and Lentz (2002), the almost unstratified ocean caused by the downwelling resulted in such fast weakening of the cross-shelf circulation. The mid-shelf stratification (near A8), however, remained strong enough to support the internal motion. In this region, the warm front varied with a smaller vertical velocity, and the horizontal current maximum was also found here (Fig. 10b and c). Near the shelf break, Damrey disturbs the stratification so significantly that large vertical velocity and temperature deviations oscillated along this latitude with time (Fig. 10a). 4.4. Warming near station A4
Fig. 8. Time series of the upper 100 m mean vertical velocity (line contour, m/hr) and temperature anomaly (shaded contour) of transect A in the Damrey case; green arrows are the wind stress with magnitudes 40.3 Pa.
vertical also exhibit a two-layer structure due to the stratification on the shelf (e.g., Fig.9a and b, e and f). This feature persisted for a few days and is clearly illustrated in Fig. 10b and c which show that the model’s currents in layers 2 ( 18.75 m) and 6 ( 68.75 m) were almost out of phase after the first post-storm IP. In this way, a cross-shelf circulation was formed in response to the storms passage, which largely contributed to the long-lasting of the warm anomaly at A8 ( 20.61N). The variation of the temperature field in Fig. 9 shows that the slight warming on the shelf formed in the forced period (Fig. 9a) was much enhanced during the first post-storm convergent phase (Fig. 9b–d). In the following divergent phase, however, the warm anomaly on the shelf reduced a little but did not disappear as that in the region south of the storm track (Fig. 9e–h). This feature was due to the cross-shelf circulation which blocked the massive intrusion of the cold water onto the shelf and thus retained the heat trapped earlier, while inertial motion alone did not generate much temperature changes in the mean profile, after averaged over an IP, as in the region south of the track (Fig. 9l).
4.3.3. Seaward propagation of the internal waves The ocean response at A8 is similar to the downwelling front generated by the wind along the southern shores of Lake Ontario, where, according to Simon (1978), near-inertial internal waves propagated off-shelf from the front, while coastal jets were generated through geostrophic adjustment, helping to maintain the front for a few IPs. In our study, the seaward propagation speed, along the transect B, of the near-inertial wave estimated from Fig. 10b and c is about 4.8 m/s. It is also noted that the current signal propagated southwards with larger decaying rate found in layer 2 than in layer 6, which is consistent with the observations of Mitchell et al. (2005). The internal motions on the shelf were affected by the stratification modified by the storminduced downwelling. In the inner shelf region (north of A8), the model result that both vertical velocity and temperature changes were weak after the first post-storm IP (Fig. 10a) indicates a quick diminishing of Damrey’s influence over the motions. Based on the
Station A4 is located southwest of station A8, and Damrey passed it from its south. As in A8, the storm wind forcing also lasted for more than one day in A4, but the strength was stronger in A4 because Damrey’s intensity had strengthened to typhoon (Fig. 8). Another difference between A4 and A8 is that A4 is much closer to the HNI, so the influence of NSCS’s western boundaries (i.e., the Leizhou Peninsula and HNI) to the ocean became more substantial than they were in A8. Therefore, at A4, in addition to the blocking effect of northern boundary, the western boundaries will further confine the storm-induced flow and produced stronger downwelling (the mechanism discussed in Section 4.5). As a result, ocean cooling at A4, as that which occurred in the middle of the forced period at A8, though more significant due to being closer to a more intense storm, was delayed until the late forced period (Fig. 8). In the wake period, besides the cross-shelf circulation that similarly slowed the massive intrusion of cold water onto the shelf, the horizontal circulation in this region contributes further to the temperature field’s recovery. Normally, TC-induced circulation during the storm’s wake period appears to be cyclonic near the track but much weaker and anti-cyclonic on the sides. In this region, the anti-cyclonic circulation on the track’s right-hand side was enhanced considerably by the coastal boundaries. To observe this, the upper 100 m mean post-storm residual flows of this shelf area were calculated and the results show that small-scale circulations developed on the shelf north of the storm track (Fig. 11). Station A4 is located inside an anti-cyclonic circulation where the downwelling velocity is relatively high. Therefore, although Damrey produced substantial cooling on the shelf with the maximum strength in its lifetime (Fig. 7e), periodic warming still emerged in the wake period at A4. The coastal shape here appears to favor the formation and development of anti-cyclonic circulation with respect to the flows induced by westwardmoving storms. This is an important process that helped to accelerate the retreat of the cold water from the shelf region and restore it to its pre-storm condition. 4.5. Warming near the HNI Price et al. (1994) noted that large currents exist below the mixed layer, even during the forced stage, because of the intense perturbation produced by hurricanes. It can also be seen in Fig. 10b that a subsurface jet is formed between 20–211N and that it flowed westward along shelf with a magnitude of approximately 30 cm/s during the second half of the forced period (September 24th–25th). Because Damrey was a relatively large and slow moving storm, this westward-moving jet could propagate faster than the storm. Therefore, in the regions off eastern HNI, this jet formed in advance of the storm center. Its velocity grew stronger downstream closer to HNI as the storm strengthened. With the presence of HNI, the shelf not only narrows here but also changes its alignment from west to south. Therefore, with the land boundaries located to the north and west and a storm approaching from the east, the region
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Fig. 9. (a–j) Vertical profiles of temperature and velocity anomaly fields along transect B every 4 h beginning 06:00 September 25th (0 IP) in the Damrey case. Shaded contours are the temperature anomaly field; vectors are the current anomaly composed of vertical velocity ( 500) and cross-shelf velocity; line contours are the alongshelf current anomaly at interval of 15 cm/s (yellow: northeastward, green: southwestward). Note the current anomalies are relative to the IP-averaged field (shown in (k)), while temperature anomalies are relative to the initial field (black contour in (l)), not to the IP-averaged field (color contour in (l)). (For interpretation of the references to color in this figure legend, the reader is reffered to the web version of this article.)
northeast of HNI becomes a flow convergent zone. This is similar to the Louisiana ‘‘boot’’ effect on the circulation pattern produced by Hurricane Ivan, as discussed by Mitchell et al. (2005). Ivan crossed the continental shelf with the boot on its left, which forced the flow to accelerate through the narrow region between this particular
landmass and the storm’s eye. Thus, the estimated transport was greater on the storm track’s left side. The Damrey model results for the region show that the downwelling and the rising of temperatures began before the storm center arrived (Fig. 12a). As the storm proceeded westward, the exit path for the off-shelf flow
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Fig. 10. (a) The model-calculated time series of the upper 100 m mean vertical velocity (shaded contour) and temperature anomaly (black contour, 1C) of transect B in the Damrey case; green arrows are the wind stress with magnitudes 40.3 Pa; (b) The model-calculated time series of the along-shelf current anomaly at layer 6 (shaded contour) and layer 2 (black contour, cm/s) of transect B in the Damrey case; (c) The same as (b) but for the cross-shelf current anomaly. (For interpretation of the references to color in this figure legend, the reader is reffered to the web version of this article.)
between the storm and HNI gradually diminished. Meanwhile, the storm also grew stronger, with its center moving shoreward to the northwest (Fig. 12b, c). As a result, the downwelling and heat flux convergence intensified, and temperatures on the shelf rose even higher (Fig. 12d). On the other hand, when the westward-moving storm began to cross the shelf, the strong upwelling and mixing associated with its center occurred in this region (Fig. 12e). The warming in this region quickly decreased after the storm’s passage because of the direct impact of the storm-induced upwelling and mixing.
5. Discussion The model results have shown that the heat in the ocean may accumulate on the northern shelf of the SCS during the passage of a storm moving along-shelf. Consequently, the satellite data for these events show an increased SSHA and a reduced/delayed SST cooling in this region. Generally, TC winds produce vertical mixing and upwelling in the ocean, which will reduce the SST and the heat and moisture fluxes into the atmosphere, thus hindering the cyclone’s development and intensification. However, the
Fig. 11. The model-calculated upper 100 m mean anomaly field of temperature (shaded contour), vertical velocity (green contour, m/hr) and horizontal velocity vectors averaged over two inertial periods (72 h from 12:00 September 26th) in the Damrey case; Station A4 is also plotted on the map; dashed line represents the track of Damrey. (For interpretation of the references to color in this figure legend, the reader is reffered to the web version of this article.)
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Fig. 12. The model-calculated upper 100 m mean temperature anomaly (1C) and the 50–100 m mean horizontal current anomaly (vector) every 6 h from 18:00 September 24th to 00:00 September 26th in the Damrey case; note that the areas of positive temperature anomaly are shaded and the thin solid line represents the track of Damrey.
downwelling feature discussed in this study tends to weaken this ocean’s negative feedback on the storm. This is particularly noteworthy for the more intense feature near HNI because of the time that it occurred. Fig. 13 shows how the OHC in the layer above
the 26 1C isotherm (OHC26), an important factor in storm prediction (DeMaria et al., 2006), was modified in this region with the approach of the storms. In this figure, the OHC26 anomaly and the wind stress distributions for the five case studies were plotted
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Fig. 13. (a) Tracks of five storm cases near the HNI; the dashed line is the 500 m depth contours; (b–f) the model-calculated time variation of the OHC26 (shaded contour) and the wind stress (line contour, Pa) of the meridional transects that passed those points shown in (a).
along some meridional transects that pass through the storm’s position 6 to 9 h before landfall (Fig. 13a). In four cases, the OHC26 anomalies overlapped the storm passage in both time and space, indicating that interactions with the current storm system were likely to occur. The anomalies generally become clear after the eye has passed, and the positive anomaly often surpassed the negative one in strength. The maximum increase for stronger examples, such as Wukong and Damrey, can reach 30 kJ/cm2. The spatial average for this region’s anomaly can still be significant. For example, Damrey’s OHC26 anomaly peak, averaged over the shelf area north of transect subsection A1–A4, was as high as 11 kJ/cm2. This heat increase is comparable to the amount of surface heat loss at Hurricane Isidore’s eye wall (Shay and Uhlhorn, 2008). Although the positive OHC26 anomalies may have less influence on small, fast-moving storms such as Luke, they could all be energy sources for the other four cases when the storm circulation radius concept of De Maria et al. (2006) ( 2 times the radius of maximum wind) is applied to the model’s results. To some extent, this energy source could offset the influence that landmasses and cold wakes have on the reduction of storm intensity (Shen and Ginis, 2001), as was the case with Wukong, whose intensity exhibited a break in a decaying trend for several hours when it moved over this area (Fig. 5a). Furthermore, this energy source could support a TC’s rapid intensification when suitable atmospheric conditions exist, as in the case of Damrey (Shi et al., 2006). Damrey’s growth rate was even more extreme in the
best track data from the Hong Kong Observatory. This database comprised observations from local stations and may better reflect the presence of this energy source (Fig. 5a). Greater intensities are also found associated with Andy and Brian in this database, while for Wukong, the break of the decaying trend extends all the way until the time of landfall. On the other hand, the warming influence south of China’s coastline may be less on the current storm than on the weather system after the storm has passed. In Damrey’s case, the OHC26 anomaly averaged over the shelf area north of transect A reached its peak of 1.3 kJ/cm2 and remained positive for several days. Therefore, when there are consecutive storms passing through this region, the ocean’s heat accumulation may increase the amount of moisture and heat fluxes into the atmosphere. Such increases may increase Taiwan’s summer rainfall, which is affected by the southwesterly monsoons originating from the SCS (Chen et al., 2010). A case of unusually high summer rainfall involving TC circulation occurred during category-2 typhoon Morakot (2009). In this event, record-breaking rainfall ( 42400 mm in three days) had caused severe damage and casualties in Taiwan. According to Hong et al. (2010), the precipitation was caused by the convergence of the northwesterly flows associated with Morakot’s circulation and the southwesterly flows associated with the monsoon gyre. The southwesterly flows are found to be greatly enhanced by the presence of TC Goni (2009), which contributed approximately one-third of the rainfall in Taiwan (Huang et al.,
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Fig. 14. (a) Tracks of Pabuk (2007), Goni (2009) and Morakot (2009) based on the RSMC database. (b) Zonal (21–221N) averaged daily SST (1C) during Goni and Morakot events. Note the SST data are obtained from www.ssmi.com with resolution of 0.251.
2011). Goni is a tropical storm which lingered around the NSCS a few days ahead of Morakot’s approach to Taiwan (Fig. 14a). Regarding the process discussed in our study, we may expect the warm water to accumulate to the right of Goni’s track, which is the region southwest of Taiwan. This inference is supported by the daily SST data in this region during the Goni–Morakot event. The zonal (21–221N) averaged SST on the shelf in the region between 113–1191E during this event (Fig. 14b) shows an increase of SST (with maximum of 29.6 1C) in the region between 116.5–1181E from 5 August to 10 August as the warm water accumulated during the passage of Goni and Morakot. Since SST is highly related to the convective available potential energy (CAPE) as demonstrated by Bhat et al. (1996), the finding by Huang et al. (2011) appears to support our argument that the heat accumulation on the shelf in NSCS by TC passage is able to facilitate the occurrence of convective instability near Taiwan. Their study have concluded that the high CAPE ( 43000 J/kg) in the NSCS during the Morakot event was a necessary preconditioning for the synoptic environment near Taiwan to account for extreme rainfall in Taiwan. It is interesting to note that TC Fanapi (2010) had a
similar track to that of Morakot, but it produced much less rainfall in Taiwan because there was no TC in the NSCS during Fanapi’s passage over Taiwan. The process discussed above may still be effective when the TC system is not involved in the circulation. The moisture transported by southwesterly monsoon alone can produce extremely heavy rain in Taiwan. One example of this is the floods of July 26–27, 2010, when accumulations of over 200 mm of rain were common across southern Taiwan. No TC remained near Taiwan, but typhoons Conson (2010) and Chanthu (2010) had passed over the SCS in quick succession only a few days previously, which may have resulted in the increase in moisture there.
6. Conclusion This study shows that westward-moving TCs across the NSCS may induce significant flow convergence against the northern boundary and generate downwelling on the shelf region. The ocean’s subsurface warming produced by this process is evident,
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and the accumulated heat may persist on the shelf for several days. In this way, the storm-induced SST cooling is reduced during either the forced period or in the wake period, depending on the shape of the coastal boundaries and the storm’s characteristics. This storminduced warm feature can influence the weather system in two ways. Locally, the heat trapped on the shelf may be great enough to perturb the thermodynamic budget of the storm system, and the storm intensity may become stronger. If weather forecasting is performed without consideration of this process, the storm will be underestimated, and even worse, there will be little time for correction because most of the TCs will make landfall soon after they move to the shelf region. Regionally, the environmental conditions may be modified in the post-storm period that moistening of the atmosphere may be increased in the part of the SCS where the heat is accumulated. If this region is covered by the southwesterly monsoon-conveying belt, failure to consider this moisture source may lead to a large deviation in the rainfall prediction for the downstream region.
Acknowledgements This study is sponsored by the grant from the National Science Council of the Republic of China, NSC98-2611-M-002-015-MY3. References Austin, J.A., Lentz, S.J., 2002. The inner shelf response to wind-driven upwelling and downwelling. Journal of Physical Oceanography 32, 2171–2193. Bhat, G.S., Srinivasan, J., Gadgil, S., 1996. Tropical deep convection, convective available potential energy and sea surface temperature. Journal of the Meteorological Society of Japan 74, 155–166. Chen, J.-M., Li, T., Shih, C.-F., 2010. Tropical cyclone- and monsoon-induced rainfall variability in Taiwan. Journal of Climate 23, 4107–4120.
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DeMaria, M., Knaff, J.A., Kaplan, J., 2006. On the decay of tropical cyclone winds crossing narrow landmasses. Journal of Applied Meteorology and Climatology 45, 409–491. Holland, G.J., 1980. An analytic model of the wind and pressure profiles in hurricanes. Monthly Weather Review 108, 1212–1218. Hong, C.-C., Lee, M.-Y., Hsu, H.-H., Kuo, J.-L., 2010. Role of submonthly disturbance and 40–50 day ISO on the extreme rainfall event associated with Typhoon Morakot (2009) in southern Taiwan. Geophysical Research Letters 37, L08805, http://dx.doi.org/10.1029/2010GL042761. Huang, C.-Y., Wong, C.-S., Yeh, T.-C., 2011. Extreme rainfall mechanisms exhibited by Typhoon Morakot (2009). Terrestrial Atmospheric and Oceanic Sciences 22. Jing, Z.-Y., Qi, Y.-Q., Hua, Q.-L., Zhang, H., 2009. Numerical study on the summer upwelling system in the northern continental shelf of the South China Sea. Continental Shelf Research 29 (2), 467–478. Kuo, Y.-C., Chern, C.-S., Wang, J., Tsai, Y.-L., 2011. Numerical study of upper ocean response to a typhoon moving zonally across the Luzon Strait. Ocean Dynamics, http://dx.doi.org/10.1007/s10236-011-0459-7. Mitchell, D.A., Teague, W.J., Jarosz, E., Wang, D.W., 2005. Observed currents over the outer continental shelf during Hurricane Ivan. Geophysical Research Letters 32, L11610, http://dx.doi.org/10.1029/2005GL023014. Price, F.J., Sandford, T.B., Forristall, G.Z., 1994. Forced stage response to a moving hurricane. Journal of Physical Oceanography 24, 233–260. Semtner, A.J., Mintz, Y., 1977. Numerical simulation of the Gulf Stream and midocean eddies. Journal of Physical Oceanography 7, 208–230. Shay, L.K., Uhlhorn, E.R., 2008. Loop current response to Hurricanes Isidore and Lili. Monthly Weather Review 136, 3248–3273. Shen, W., Ginis, I., 2001. The impact of ocean coupling on hurricanes during landfall. Geophysical Research Letters 28 (14), 2839–2842. Shi, H.-Y., Li, W.-H., Huang, H.-H., 2006. Characteristic analysis of the Typhoon No. 0518 ‘‘Damrey’’. Mar. Forecasts 23 (4), 59–64. Simmonds, I., Bi, D., Hope, P., 1999. Atmospheric water vapor flux and its association with rainfall over China in summer. Journal of Climate 12, 1353–1367. Simons, T.J., 1978. Generation and propagation of downwelling fronts. Journal of Physical Oceanography 8, 571–581. Su, J.-L., Wang, J., Pohlman, T., Xu, D.-F., 2011. The influence of meteorological variation on the upwelling system off eastern Hainan during summer 20072008. Ocean Dynamics 61, 717–730. Wu., L.-G., Wang, B., Geng, S.-Q., 2005. Growing typhoon influence on East Asia. Geophysical Research Letters 32, L18703, http://dx.doi.org/10.1029/2005GL022937.
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Research papers
Numerical study of typhoon-induced ocean thermal content variations on the northern shelf of the South China Sea Yaling Tsai, Ching-Sheng Chern n, Joe Wang Institute of Oceanography, National Taiwan University, P.O. Box 23-13, Taipei 106, Taiwan, Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2011 Received in revised form 27 April 2012 Accepted 3 May 2012 Available online 11 May 2012
This paper investigates the downwelling caused by tropical cyclones near the continental shelf. A numerical ocean model is used to simulate the ocean’s response to storms moving along shelf across the northern South China Sea. Five westward-moving storms from this region were selected for the study. Data on the sea surface temperature and the height deviations showed that there was an increase in the water levels and delayed cooling on the shelf to the right of the track during these storms. Such phenomena were observed in regions south of China’s coast and near Hainan Island, and the model’s results also revealed that subsurface warming occurred at both locations. The site south of China’s coast is associated with the strong post-storm convergent phase when warm water is brought both downward and seaward generating a temperature front on the shelf. The land barrier resulted in a cross-shelf circulation, which helped to block the movement of cold water towards the shore in the following divergent phases; therefore, the downwelling front continued for several days. The site east of Hainan resulted from an interesting combination of a westward storm path and coastal topography. The position of Hainan in relation to these storms greatly diminished the off-shelf path of stormgenerated flows, so the region became a significant convergence zone. Therefore, the downwelling in this region increased with the storm’s approach, and the warming continued until the storm passed. These examples indicate that the tropical cyclones’ influence on the ocean’s upper level heat content in the shelf region can be positive and may possibly have a significant effect on local or regional weather systems. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Typhoon Downwelling Numerical study Continental shelf Northern South China Sea
1. Introduction Ocean cooling is a common phenomenon induced by tropical cyclones (TCs). The cooling is caused by cold thermocline water being transported and mixed into the surface layer to replace the strong surface current forced out from the storm’s center by the wind. The upper ocean temperature is thus effectively reduced; the effect is easily observed by satellite measurements. Numerous studies on the oceanic response to TCs have focused on this upwelling and mixing process, whereas the warm surface water emanating from the storm’s center has received less investigative attention because the heat carried away by the divergent water may disperse into the surrounding environment, appearing to be negligible in the open ocean. However, as a storm moves closer to the shelf region, the shallow depth and landmasses become the boundaries that confine the redistribution of heat. In other words, the heat transported by the warm surface water may accumulate and be carried downwards at these boundaries
n
Corresponding author. Tel.: þ886 2 33661374; fax: þ886 2 23626092. E-mail address: [email protected] (C.-S. Chern).
0278-4343/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2012.05.004
where, in conjunction with the wind direction, downwelling may occur. It may become rather significant, as was the case with Hurricane Ivan, when during the storm’s progression off the Gulf Coast, Mitchell et al. (2005) observed a near-bottom temperature increase of 3 1C in the outer continental shelf. Although Ivan may have produced a significant increase in the bottom temperature, the downwelling was restricted by the storm’s path, which was perpendicular to the coast, as is often the case for Gulf Coast TCs. The winds that favor downwelling for TCs on this track can persist for only the first half of the forced period, and the temperature increase will quickly disappear once the storm center moves on. Presumably, in the northern hemisphere, notable downwelling could occur when the along-shelf wind is strong and persistent and has the coast to its right. A region characterized by this condition is the continental shelf area in the northern South China Sea (NSCS, Fig. 1a) where westward-moving TCs frequently occur (Wu et al., 2005). In this region, the southwest monsoons produce noticeable coastal upwelling during the summer months (Jing et al., 2009). The coastal areas east of Hainan Island (HNI), south of the Shantou Coast and east of the Leizhou Peninsula, are all areas of upwelling where observations showed that the sea surface temperature
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Fig. 1. (a) Bathymetry map of the SCS; (b) Transect locations used in SST and model output plots; (c–g) Tracks of five storm cases.
(SST) was 2 1C lower than the off-shore water (Jing et al., 2009). Therefore, it is interesting to determine how these features can be modified by storms passing over the NSCS. Su et al. (2011) compared a pre-storm and post-storm hydrographic survey conducted off eastern HNI during two upwelling seasons, and they found that TC Pabuk’s (2007) strong northerly winds reduced the thermocline’s strength in the coastal seas and that the upwelling strength was decreased by the development of a frontal zone between the mixed coastal water and the offshore water. However, because Pabuk’s track passed through the northern edge of the region (along 221N, see Fig. 14a), its influence on these upwelling features was presumably small, based on the study by Kuo et al. (2011), who found that major downwelling caused by a westward-moving TC along 201N into the SCS occurred near the shelf area along the coast of China and in the Taiwan Strait. Storms that cross the NSCS along the shelf, but further to the south, should produce response that is similar but of greater magnitude. Because the SCS is a relatively enclosed ocean basin and an important source of moisture for the surrounding regions’ summer rainfall (Simmonds et al., 1999), it is essential to elucidate the storm-induced heat distribution process. The purpose of this study is to determine how the shelf ocean is modified by TCs moving along-shelf in the NSCS. In the next section, satellite data acquired during some storm events are examined to identify the corresponding downwelling features. A process-oriented numerical study was adopted to explain the correlations.
2. Data and analysis From the 1985–2005 TC database of the Regional Specialized Meteorological Center (RSMC) Tokyo—Typhoon Center (Fig. 1c–g), five westward-moving storms in the NSCS were selected as case studies to determine how along-shelf storms may affect
shelf circulation. The storm tracks are mostly north of 181N, and all passed HNI from the south, so the storm-induced circulation can interact with the bathymetry and coastal topography. The satellite data during these five storms have been searched to ascertain whether the downwelling process on the shelf is evident. Only three cases, Luke, Wukong and Damrey, were found to have corresponding sea surface height anomaly (SSHA) data that showed how these storms affected the ocean’s volume distribution on the shelf. The pre- and post-storm SSHA fields were obtained from the AVISO program (http://www.aviso.ocea nobs.com/), and their differences were calculated (Fig. 2). The data indicate that the SSHA was generally lower along the storm track but higher on the shelf side. The SSHA increment was particularly evident in the case of Wukong, in which a significant negative center ( 18 cm) was generated to the right of the track where the storm lingered for 18 h. Correspondingly, a strong positive center (18 cm) occurred on its northern shelf near 1141E. This localized maximum south of Hong Kong was also observed in the case of Damrey and Luke. In the Wukong and Damrey cases, a large increase in the SSHA appeared around the northeast region off HNI, which, together with the Leizhou Peninsula, juts out into the NSCS shelf and is on the path of these westward-moving storms. It appears that only TCs of typhoon intensity can produce an appreciable accumulation of shelf water off northeastern HNI because such patterns were not associated with tropical storm Luke. Using daily SST data, it was possible to examine how the downwelling process varied over time in this region. The multiple-satellite blended SST data were obtained from the NOAA Coast Watch program (http://coastwatch.pfeg.noaa.gov/) with a spatial resolution of 0.11. However, data were available only for Damrey case. Fig. 3 shows the time series plots for three transects (Fig. 1b) along 19.91N, 111.21E and the 100 m isobath (transect A). The SST variations near the shelf, as a result of Damrey’s passage, are depicted as a cooling event starting on approximately September 24th when the storm entered the region.
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Fig. 2. SSHA deviation (contour interval: 3 cm) in 7 day spanned over Luke (a), Wukong (b) and Damrey (c) event with their tracks superimposed; dashed line is the 500 m depth contour.
The subsequent cooling became noticeable with the strengthening of the storm, and its maximum occurred between September 25th and 26th. However, there were several areas on the shelf where cooling was delayed for at least one day, until September 25th. Off of northeastern HNI, the delayed cooling that occurred was visible west of 1121E (Fig. 3a) and north of 211N (Fig. 3b). This is exceptional because the storm passed over this region between 112–1141E at maximum strength and with a significant cold patch ( 4 1C) next to it. A similar delay in cooling was also observed south of Hong Kong in the mid-section of transect A, where even post-storm cooling is weak. In summary, satellite observations show that, during these westward TCs, the ocean’s surface height appears to be elevated in some areas on the shelf and that surface cooling is delayed. Although the resolution of the satellite data may be poor for clarifying the cooling-impeding and water-accumulation features, they still indicate that the heat redistribution process on the shelf in the SCS, during westward-moving storms, is complicated. In the next section, a numerical model is used to simulate those five case studies to provide insight into the dynamics related to these processes.
3. Numerical model and storm cases The model used in this study is a three-dimensional primitive equation model modified from the one used by Semtner and Mintz’s (1977). It assumes rigid-lid, hydrostatic and Boussinesq
Fig. 3. Time series plot of SST anomaly relative to the 21st of September along. 19.91N (a), 111.21E (b) and the along-shelf transect A (c) during the Damrey event; gap represents missing data.
approximations and uses a level-2 turbulence closure scheme to estimate the vertical eddy viscosity. The model covers most of the northern Pacific (1001E–1201W, 0–401N) and the SCS, with a grid resolution of 0.21. The model has 25 uneven vertical layers with a higher resolution in the mixed layer and thermocline. The maximum depth in the model is limited to 4 km, but the topography in the shelf region is represented realistically. A model-produced quasi-steady state for September is adopted as the initial field for all cases (Fig. 4). Detailed model configurations are reported by Kuo et al. (2011). A Rankine Vortex storm wind field (Holland, 1980) based on the RSMC’s best-track data (Fig. 5) was then superimposed on the initial field. It is a symmetric wind field parameterized by the storm’s radius of maximum winds and central pressure. During the 6-h best-track data interval, the intensity and size of the storm are assumed constant, but the
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Fig. 4. The upper 100 m mean of the model used initial field of temperature (shaded contour) and horizontal current (vector) in the SCS; dashed lines are the depth contours of 50, 100 and 250 m; the white circle marks one of the monsoon-induced upwelling regions.
center location is interpolated linearly within time steps between two issued positions. The five selected case studies have similar tracks but vary in size, lifespan and intensity (Figs. 1 and 5). TC Andy (1985), Brian (1989) and Wukong (2005) were all generated in the SCS. Wukong and Andy had similar tracks, with the former’s being stronger and larger, while Brian’s track closely followed the shelf break before it reached HNI. TC Luke (1994) and Damrey (2005) both formed east of Luzon and then moved westward across the NSCS. Luke was weak and passed through quickly, but Damrey was a large and slow-moving storm. Damrey traveled along the shelf break for more than a day before moving onto the shelf, while Luke went onto the shelf further east. The intensity peaks for the five cases are spread over 3 categories: Wukong, Damrey and, for a short time, Brian, were typhoons; Andy was a severe tropical storm, and Luke was a tropical storm. In each case, the model was integrated over 10 day, using the same initial conditions, so the differences in the ocean response were due to each storm’s individual characteristics.
4. Results 4.1. Forced period response Fig. 6 shows four snapshots of upper ocean temperatures anomaly adjacent to the storm’s eye for all five cases. The induced vertical varying temperature anomalies were averaged over a depth of 0–100 m (or the bottom, if shallower than 100 m, and hereinafter referred to as the ‘‘upper 100 m mean’’) to illustrate the net effect of the storm’s passage in redistributing heat. This forced period’s composite picture shows that the intensity of the ocean’s response is affected mainly by wind strength. The disturbance caused by tropical storm Luke was the weakest, that caused by the severe tropical storms Andy and Brian were much stronger, and that caused by typhoons Wukong and Damrey were the strongest of all. The model reproduced cooling along the storm tracks, but in every case, the warming patches were of particular interest, appearing in the front half and especially the right front quadrant of the storm. The warming is particularly
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Fig. 5. (a) Time series of the maximum wind speed for five cases from the RSMC (solid line) and Hong Kong Observatory (dashed line); the closed circle marks the time of landfall; (b) the same as (a) but for the radius of the maximum wind (black) and the estimated moving speed (gray) based on the RSMC data.
evident on the NSCS shelf when the storm was near HNI. The temperature in this initially cooled upwelling region (white circle in Fig. 4) changed with each storm’s passage to the extent that even weak tropical storm Luke produced a slight increase. The wind forcing in this region must reach a certain level so that the wind-generated flow may interact with the coast and ocean-bottom topography and generate downwelling. For example, by comparing the model results we found that, for a storm with a radius of maximum wind of 15 nm and travelling at a speed of 10 km/hr, it was necessary for the wind stress to be of a severe tropical storm strength (e.g., Brian) rather than of a tropical storm strength (e.g., Andy) to produce warming on the shelf at approximately 1121E (Fig. 6a and b). For a TC, however, the forcing strength depends not only on the storm intensity. Other factors such as storm’s size, path and moving speed must also be considered. Therefore, for storms with similar intensity, a larger wind field and a path relatively closer to shore may become necessary conditions in generating a warm pattern, such as the one that occurred on the northern shelf at 1161E with Damrey, but not with Wukong. 4.2. Relaxation period response In comparison with the forced period responses, Fig. 7 shows the upper 100 m mean temperatures anomaly for the five cases averaged over one inertial period after the storm moved out of each divided region, as shown in Fig. 6. It appears that most of the forced-period warming that occurred in front of the storm was transient. The temperature deviation along the storm track was negative, except near the east coast of HNI. On the other hand, the warming on the northern SCS shelf, although slightly weaker, remained and enlarged in terms of the coverage area. This pattern is evident with both Wukong and Damrey, whose wind intensities were sufficiently large to generate noticeable
changes on the shelf. Because Damrey appears to have interacted with the ocean shelf the most, the following discussion is based primarily on the results for its model run. To determine how long the shelf warming lasts, the time series for the upper 100 m mean vertical velocity and temperature anomaly of transect A were plotted in Fig. 8. The stations in transect A align, approximately, along the 100-m isobaths with station A1 closer to HNI and station A11 on the eastern end (Fig. 1b). Damrey’s path from September 23rd–25th is approximately aligned with the shelf break and transect A, and the storm traveled from east to west and intersected transect A at station A1. The wind forcing of this transect was thus greater towards A1 due to its shorter distance to the storm center and the increasing storm intensity. Fig. 8 clearly shows that significant warming occurred near stations A1 and A2, A3–A5, and A7–A9 in either the forced period or the relaxation period. The locations of these stations coincide approximately with the two positive SSHA regions discussed in Section 2. From this figure, we know that the storm-induced ocean cooling at most stations occurred as soon as the front half of the storm passed, which is consistent with the findings of the study by Price et al. (1994). At A1 and A2, however, the cooling was delayed until the end of the forced period. Besides the residual flow, the responses in the wake period comprised pronounced near-inertial period oscillation in every station. Near station A1, the much stronger upwelling and mixing process quickly deplete the positive temperature anomaly produced in forced period. On the other hand, warming appeared again after the first post-storm downwelling near stations A3–A5 and A7–A9 and has been the predominant phase since then. In summary, contrary to the response in the open ocean, storminduced warming in the upper ocean is significant in the shelf region of NSCS. In addition, the responses in the along-shelf directions are not unvaried because the coastal geometry in this region is
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Fig. 6. The model-calculated upper 100 m mean temperature anomaly around the storm center at the time specified on the map for five cases, (a) Andy (b) Brian (c) Luke (d) Wukong and (e) Damrey.
complicated with the appearance of the Leizhou Peninsula and the HNI. In the next three sections, the model results at stations A8, A4 and A1 are discussed separately so that the effect of coastal boundary to the storm-induced warming on the shelf can be clearly illustrated. 4.3. Warming near station A8 The noticeable temperature response at A8 is that, instead of cooling, warming was significant in the relaxation period. This station is located south of Hong Kong on the shelf and Damrey passed further south as a severe tropical storm. The wind stress at A8 had a dominant along-shore component that lasted for more than one day. Winds with the coastline to the right can generate shoreward tranport and it is thus necessary to examine the flow
and temperature structure in the cross-shelf direction. Therefore, the model results are further analyzed at transect B (Fig. 1b) which Damrey crossed over from the 19.71N shelf-break on September 24th. The temperature and current fields of transect B are shown in Fig. 9 for every 4 h during the wake period beginning on 06:00 September 25th for a total of 1 inertial period (IP, 36 h). The mean field (Fig. 9k and l) and the oscillating components (Fig. 9a–j) within this IP are discussed separately.
4.3.1. Response associated with the residual flow The net influences of the storm on transect B within this IP (Fig. 9k and l) are obtained by applying the time averages to the model results. It is clearly observed that the isotherms were
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Fig. 7. The same as Fig. 6 but averaged over one inertial period (36 h) after the storm moved out of each sub-region.
uplifted near the shelf break but tilted downwards on the shelf, which produced a temperature front at mid-shelf that was cold in the south but warm in the north (Fig. 9l). This structure results from the disturbance exerted by a storm with its center passing from the south and a downwelling generated near the coast. A westward current developed through geostrophic adjustment near the shelf-break (Fig. 9k), which subsequently affected the oceanic conditions downstream (see Section 4.5). Shoreward from this current, the flow was moving mainly to the east because of the interaction between the coastline and the recirculation of the storm-induced flow. The cross-shelf component during this IP was stronger near the surface where it went off-shelf due to a strong convergent flow towards the storm track. 4.3.2. Response associated with the oscillatory flow The oscillating motions in this post-storm IP (Fig. 9a–j) are obtained by subtracting from the model results the IP-averaged
current field (Fig. 9k). The time frame for these plots is specified in IP relative to the beginning of the storm’s wake period (Fig. 9a). To highlight the warming feature on the shelf, the initial but not the IP-averaged temperature field was removed from the model results. Unlike the response in the open ocean where the inertial motions in the response center and its vicinity are relatively in unity, apparent phase difference was found in the motions between the response center and the coast. This is because the outreach of the storm-induced flows was confined by the land boundary and the horizontal unity was thus disappeared on the shelf. For example, horizontal flow divergence occurred near the coast when the response center was in convergent phase (Fig. 9a–d), and vice versa (Fig. 9e–h). Phase lag can also be observed in the vertical velocities associated with the horizontal divergence or convergence as seen in Fig. 9e in which upwelling and downwelling occurred at the shelf break and the coast, respectively. In addition, the horizontal flow directions in the
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study by Austin and Lentz (2002), the almost unstratified ocean caused by the downwelling resulted in such fast weakening of the cross-shelf circulation. The mid-shelf stratification (near A8), however, remained strong enough to support the internal motion. In this region, the warm front varied with a smaller vertical velocity, and the horizontal current maximum was also found here (Fig. 10b and c). Near the shelf break, Damrey disturbs the stratification so significantly that large vertical velocity and temperature deviations oscillated along this latitude with time (Fig. 10a). 4.4. Warming near station A4
Fig. 8. Time series of the upper 100 m mean vertical velocity (line contour, m/hr) and temperature anomaly (shaded contour) of transect A in the Damrey case; green arrows are the wind stress with magnitudes 40.3 Pa.
vertical also exhibit a two-layer structure due to the stratification on the shelf (e.g., Fig.9a and b, e and f). This feature persisted for a few days and is clearly illustrated in Fig. 10b and c which show that the model’s currents in layers 2 ( 18.75 m) and 6 ( 68.75 m) were almost out of phase after the first post-storm IP. In this way, a cross-shelf circulation was formed in response to the storms passage, which largely contributed to the long-lasting of the warm anomaly at A8 ( 20.61N). The variation of the temperature field in Fig. 9 shows that the slight warming on the shelf formed in the forced period (Fig. 9a) was much enhanced during the first post-storm convergent phase (Fig. 9b–d). In the following divergent phase, however, the warm anomaly on the shelf reduced a little but did not disappear as that in the region south of the storm track (Fig. 9e–h). This feature was due to the cross-shelf circulation which blocked the massive intrusion of the cold water onto the shelf and thus retained the heat trapped earlier, while inertial motion alone did not generate much temperature changes in the mean profile, after averaged over an IP, as in the region south of the track (Fig. 9l).
4.3.3. Seaward propagation of the internal waves The ocean response at A8 is similar to the downwelling front generated by the wind along the southern shores of Lake Ontario, where, according to Simon (1978), near-inertial internal waves propagated off-shelf from the front, while coastal jets were generated through geostrophic adjustment, helping to maintain the front for a few IPs. In our study, the seaward propagation speed, along the transect B, of the near-inertial wave estimated from Fig. 10b and c is about 4.8 m/s. It is also noted that the current signal propagated southwards with larger decaying rate found in layer 2 than in layer 6, which is consistent with the observations of Mitchell et al. (2005). The internal motions on the shelf were affected by the stratification modified by the storminduced downwelling. In the inner shelf region (north of A8), the model result that both vertical velocity and temperature changes were weak after the first post-storm IP (Fig. 10a) indicates a quick diminishing of Damrey’s influence over the motions. Based on the
Station A4 is located southwest of station A8, and Damrey passed it from its south. As in A8, the storm wind forcing also lasted for more than one day in A4, but the strength was stronger in A4 because Damrey’s intensity had strengthened to typhoon (Fig. 8). Another difference between A4 and A8 is that A4 is much closer to the HNI, so the influence of NSCS’s western boundaries (i.e., the Leizhou Peninsula and HNI) to the ocean became more substantial than they were in A8. Therefore, at A4, in addition to the blocking effect of northern boundary, the western boundaries will further confine the storm-induced flow and produced stronger downwelling (the mechanism discussed in Section 4.5). As a result, ocean cooling at A4, as that which occurred in the middle of the forced period at A8, though more significant due to being closer to a more intense storm, was delayed until the late forced period (Fig. 8). In the wake period, besides the cross-shelf circulation that similarly slowed the massive intrusion of cold water onto the shelf, the horizontal circulation in this region contributes further to the temperature field’s recovery. Normally, TC-induced circulation during the storm’s wake period appears to be cyclonic near the track but much weaker and anti-cyclonic on the sides. In this region, the anti-cyclonic circulation on the track’s right-hand side was enhanced considerably by the coastal boundaries. To observe this, the upper 100 m mean post-storm residual flows of this shelf area were calculated and the results show that small-scale circulations developed on the shelf north of the storm track (Fig. 11). Station A4 is located inside an anti-cyclonic circulation where the downwelling velocity is relatively high. Therefore, although Damrey produced substantial cooling on the shelf with the maximum strength in its lifetime (Fig. 7e), periodic warming still emerged in the wake period at A4. The coastal shape here appears to favor the formation and development of anti-cyclonic circulation with respect to the flows induced by westwardmoving storms. This is an important process that helped to accelerate the retreat of the cold water from the shelf region and restore it to its pre-storm condition. 4.5. Warming near the HNI Price et al. (1994) noted that large currents exist below the mixed layer, even during the forced stage, because of the intense perturbation produced by hurricanes. It can also be seen in Fig. 10b that a subsurface jet is formed between 20–211N and that it flowed westward along shelf with a magnitude of approximately 30 cm/s during the second half of the forced period (September 24th–25th). Because Damrey was a relatively large and slow moving storm, this westward-moving jet could propagate faster than the storm. Therefore, in the regions off eastern HNI, this jet formed in advance of the storm center. Its velocity grew stronger downstream closer to HNI as the storm strengthened. With the presence of HNI, the shelf not only narrows here but also changes its alignment from west to south. Therefore, with the land boundaries located to the north and west and a storm approaching from the east, the region
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Fig. 9. (a–j) Vertical profiles of temperature and velocity anomaly fields along transect B every 4 h beginning 06:00 September 25th (0 IP) in the Damrey case. Shaded contours are the temperature anomaly field; vectors are the current anomaly composed of vertical velocity ( 500) and cross-shelf velocity; line contours are the alongshelf current anomaly at interval of 15 cm/s (yellow: northeastward, green: southwestward). Note the current anomalies are relative to the IP-averaged field (shown in (k)), while temperature anomalies are relative to the initial field (black contour in (l)), not to the IP-averaged field (color contour in (l)). (For interpretation of the references to color in this figure legend, the reader is reffered to the web version of this article.)
northeast of HNI becomes a flow convergent zone. This is similar to the Louisiana ‘‘boot’’ effect on the circulation pattern produced by Hurricane Ivan, as discussed by Mitchell et al. (2005). Ivan crossed the continental shelf with the boot on its left, which forced the flow to accelerate through the narrow region between this particular
landmass and the storm’s eye. Thus, the estimated transport was greater on the storm track’s left side. The Damrey model results for the region show that the downwelling and the rising of temperatures began before the storm center arrived (Fig. 12a). As the storm proceeded westward, the exit path for the off-shelf flow
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Fig. 10. (a) The model-calculated time series of the upper 100 m mean vertical velocity (shaded contour) and temperature anomaly (black contour, 1C) of transect B in the Damrey case; green arrows are the wind stress with magnitudes 40.3 Pa; (b) The model-calculated time series of the along-shelf current anomaly at layer 6 (shaded contour) and layer 2 (black contour, cm/s) of transect B in the Damrey case; (c) The same as (b) but for the cross-shelf current anomaly. (For interpretation of the references to color in this figure legend, the reader is reffered to the web version of this article.)
between the storm and HNI gradually diminished. Meanwhile, the storm also grew stronger, with its center moving shoreward to the northwest (Fig. 12b, c). As a result, the downwelling and heat flux convergence intensified, and temperatures on the shelf rose even higher (Fig. 12d). On the other hand, when the westward-moving storm began to cross the shelf, the strong upwelling and mixing associated with its center occurred in this region (Fig. 12e). The warming in this region quickly decreased after the storm’s passage because of the direct impact of the storm-induced upwelling and mixing.
5. Discussion The model results have shown that the heat in the ocean may accumulate on the northern shelf of the SCS during the passage of a storm moving along-shelf. Consequently, the satellite data for these events show an increased SSHA and a reduced/delayed SST cooling in this region. Generally, TC winds produce vertical mixing and upwelling in the ocean, which will reduce the SST and the heat and moisture fluxes into the atmosphere, thus hindering the cyclone’s development and intensification. However, the
Fig. 11. The model-calculated upper 100 m mean anomaly field of temperature (shaded contour), vertical velocity (green contour, m/hr) and horizontal velocity vectors averaged over two inertial periods (72 h from 12:00 September 26th) in the Damrey case; Station A4 is also plotted on the map; dashed line represents the track of Damrey. (For interpretation of the references to color in this figure legend, the reader is reffered to the web version of this article.)
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Fig. 12. The model-calculated upper 100 m mean temperature anomaly (1C) and the 50–100 m mean horizontal current anomaly (vector) every 6 h from 18:00 September 24th to 00:00 September 26th in the Damrey case; note that the areas of positive temperature anomaly are shaded and the thin solid line represents the track of Damrey.
downwelling feature discussed in this study tends to weaken this ocean’s negative feedback on the storm. This is particularly noteworthy for the more intense feature near HNI because of the time that it occurred. Fig. 13 shows how the OHC in the layer above
the 26 1C isotherm (OHC26), an important factor in storm prediction (DeMaria et al., 2006), was modified in this region with the approach of the storms. In this figure, the OHC26 anomaly and the wind stress distributions for the five case studies were plotted
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Fig. 13. (a) Tracks of five storm cases near the HNI; the dashed line is the 500 m depth contours; (b–f) the model-calculated time variation of the OHC26 (shaded contour) and the wind stress (line contour, Pa) of the meridional transects that passed those points shown in (a).
along some meridional transects that pass through the storm’s position 6 to 9 h before landfall (Fig. 13a). In four cases, the OHC26 anomalies overlapped the storm passage in both time and space, indicating that interactions with the current storm system were likely to occur. The anomalies generally become clear after the eye has passed, and the positive anomaly often surpassed the negative one in strength. The maximum increase for stronger examples, such as Wukong and Damrey, can reach 30 kJ/cm2. The spatial average for this region’s anomaly can still be significant. For example, Damrey’s OHC26 anomaly peak, averaged over the shelf area north of transect subsection A1–A4, was as high as 11 kJ/cm2. This heat increase is comparable to the amount of surface heat loss at Hurricane Isidore’s eye wall (Shay and Uhlhorn, 2008). Although the positive OHC26 anomalies may have less influence on small, fast-moving storms such as Luke, they could all be energy sources for the other four cases when the storm circulation radius concept of De Maria et al. (2006) ( 2 times the radius of maximum wind) is applied to the model’s results. To some extent, this energy source could offset the influence that landmasses and cold wakes have on the reduction of storm intensity (Shen and Ginis, 2001), as was the case with Wukong, whose intensity exhibited a break in a decaying trend for several hours when it moved over this area (Fig. 5a). Furthermore, this energy source could support a TC’s rapid intensification when suitable atmospheric conditions exist, as in the case of Damrey (Shi et al., 2006). Damrey’s growth rate was even more extreme in the
best track data from the Hong Kong Observatory. This database comprised observations from local stations and may better reflect the presence of this energy source (Fig. 5a). Greater intensities are also found associated with Andy and Brian in this database, while for Wukong, the break of the decaying trend extends all the way until the time of landfall. On the other hand, the warming influence south of China’s coastline may be less on the current storm than on the weather system after the storm has passed. In Damrey’s case, the OHC26 anomaly averaged over the shelf area north of transect A reached its peak of 1.3 kJ/cm2 and remained positive for several days. Therefore, when there are consecutive storms passing through this region, the ocean’s heat accumulation may increase the amount of moisture and heat fluxes into the atmosphere. Such increases may increase Taiwan’s summer rainfall, which is affected by the southwesterly monsoons originating from the SCS (Chen et al., 2010). A case of unusually high summer rainfall involving TC circulation occurred during category-2 typhoon Morakot (2009). In this event, record-breaking rainfall ( 42400 mm in three days) had caused severe damage and casualties in Taiwan. According to Hong et al. (2010), the precipitation was caused by the convergence of the northwesterly flows associated with Morakot’s circulation and the southwesterly flows associated with the monsoon gyre. The southwesterly flows are found to be greatly enhanced by the presence of TC Goni (2009), which contributed approximately one-third of the rainfall in Taiwan (Huang et al.,
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Fig. 14. (a) Tracks of Pabuk (2007), Goni (2009) and Morakot (2009) based on the RSMC database. (b) Zonal (21–221N) averaged daily SST (1C) during Goni and Morakot events. Note the SST data are obtained from www.ssmi.com with resolution of 0.251.
2011). Goni is a tropical storm which lingered around the NSCS a few days ahead of Morakot’s approach to Taiwan (Fig. 14a). Regarding the process discussed in our study, we may expect the warm water to accumulate to the right of Goni’s track, which is the region southwest of Taiwan. This inference is supported by the daily SST data in this region during the Goni–Morakot event. The zonal (21–221N) averaged SST on the shelf in the region between 113–1191E during this event (Fig. 14b) shows an increase of SST (with maximum of 29.6 1C) in the region between 116.5–1181E from 5 August to 10 August as the warm water accumulated during the passage of Goni and Morakot. Since SST is highly related to the convective available potential energy (CAPE) as demonstrated by Bhat et al. (1996), the finding by Huang et al. (2011) appears to support our argument that the heat accumulation on the shelf in NSCS by TC passage is able to facilitate the occurrence of convective instability near Taiwan. Their study have concluded that the high CAPE ( 43000 J/kg) in the NSCS during the Morakot event was a necessary preconditioning for the synoptic environment near Taiwan to account for extreme rainfall in Taiwan. It is interesting to note that TC Fanapi (2010) had a
similar track to that of Morakot, but it produced much less rainfall in Taiwan because there was no TC in the NSCS during Fanapi’s passage over Taiwan. The process discussed above may still be effective when the TC system is not involved in the circulation. The moisture transported by southwesterly monsoon alone can produce extremely heavy rain in Taiwan. One example of this is the floods of July 26–27, 2010, when accumulations of over 200 mm of rain were common across southern Taiwan. No TC remained near Taiwan, but typhoons Conson (2010) and Chanthu (2010) had passed over the SCS in quick succession only a few days previously, which may have resulted in the increase in moisture there.
6. Conclusion This study shows that westward-moving TCs across the NSCS may induce significant flow convergence against the northern boundary and generate downwelling on the shelf region. The ocean’s subsurface warming produced by this process is evident,
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and the accumulated heat may persist on the shelf for several days. In this way, the storm-induced SST cooling is reduced during either the forced period or in the wake period, depending on the shape of the coastal boundaries and the storm’s characteristics. This storminduced warm feature can influence the weather system in two ways. Locally, the heat trapped on the shelf may be great enough to perturb the thermodynamic budget of the storm system, and the storm intensity may become stronger. If weather forecasting is performed without consideration of this process, the storm will be underestimated, and even worse, there will be little time for correction because most of the TCs will make landfall soon after they move to the shelf region. Regionally, the environmental conditions may be modified in the post-storm period that moistening of the atmosphere may be increased in the part of the SCS where the heat is accumulated. If this region is covered by the southwesterly monsoon-conveying belt, failure to consider this moisture source may lead to a large deviation in the rainfall prediction for the downstream region.
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