Sea surface temperature (SST) variability affects marine
ecosystems, fisheries, ocean primary productivity and human activities and
is the primary influence on typhoon intensity. SST drops of a few degrees in
the open ocean after typhoon passages have been widely documented; however,
few studies have focused on coastal SST variability. The purpose of this
study is to determine typhoon-induced SST drops in the near-coastal area
(within 1 km of the coast) and understand the possible mechanism. The
results of this study were based on extensive field data analysis.
Significant SST drop phenomena were observed at the Longdong Buoy in
northeastern Taiwan during 43 typhoons over the past 20 years (1998–2017).
The mean SST drop (ΔSST) after a typhoon passage was 6.1 ∘C,
and the maximum drop was 12.5 ∘C (Typhoon Fungwong in 2008). The
magnitude of the SST drop was larger than most of the observations in the open
ocean. The mean duration of the SST drop was 24 h, and on average, 26.1 h were
required for the SST to recover to the original temperature. The coastal SST
drops at Longdong were correlated with the moving tracks of typhoons. When a
typhoon passes south of Longdong, the strong and persistent longshore winds
induce coastal upwelling and pump cold water up to the surface, which is the
dominant cause of the SST drops along the coast. In this study, it was determined
that cold water mainly intruded from the Kuroshio subsurface into the Okinawa
Trough, which is approximately 50 km from the observation site. The
magnitude of coastal SST drops depends on the area of overlap between
typhoons generating strong winds and the Kuroshio. The dataset used in this
study can be accessed from 10.1594/PANGAEA.895002.
Introduction
Sea surface temperature (SST) drops after typhoon (hurricane) passages have
been widely known and reported in the world's oceans, including the
northwestern Pacific (Sakaida et al., 1998; Tsai et al., 2008a, b, 2013;
Chen et al., 2003; Wada, 2005; Wada et al., 2009; Chang et al., 2008; Wu et al.,
2008; Morimoto et al., 2009; Hung et al., 2010; Park and Kim, 2010;
Kuo et al., 2011; Sun et
al., 2015; Subrahmanyam, 2015), northeastern Pacific (Bingham, 2007), Indian
Ocean (Rao et al., 2004; Gopalakrishna et al., 1993) and South China Sea
(Shang et al., 2008; Jiang et al., 2009; Tseng et al., 2010; Chiang et al.,
2011). Upwelling, entrainment (vertical mixing) and the intrusion of cold
water are the possible mechanisms of the SST drop. Upwelling is the process that
the cold water in the subsurface rises toward the surface due to wind or
bathymetry effects. The entrainment is a vertical turbulent mixing (VTM)
process that causes ocean mixed layer (OML) deepening and results in a cooler
OML temperature. The intrusion of cold water may come from the surface in
another sea area or subsurface locally. SST drops are larger in scale
following a typhoon passage than under regular temperature variability and
may affect marine ecosystems and the primary productivity of the ocean (Lin
et al., 2003b; Siswanto et al., 2007). Cold water increases nutrients for
marine life. Several studies (Babin et al., 2004; Hanshaw et al., 2008; Liu
et al., 2009; Kawai and Wada, 2011; Cheung et al., 2013; Xu et al., 2017)
have reported that chlorophyll a increases when SST drops after the passages of
tropical cyclones. In contrast, fish species that cannot tolerate cold may
die if the water temperature drops dramatically over a short period of time.
In addition, the water temperature has a major impact on human comfort and
safety in leisure activities.
The SST drop caused by typhoons rarely exceeds 6 ∘C (Wentz et
al., 2000). Price (1981) presented SST drops of 3 and 1 ∘C in US waters
during Hurricane Eloise in 1975 and Hurricane Belle in 1976, respectively. He noted that the SST decrease beneath a moving
hurricane was mainly caused by entrainment. Stronger wind stress and the
associated curl surface wind trigger more substantial ocean mixing and
induce the mixing of sea surface water with colder and deeper waters. Wada
et al. (2009) studied the role of VTM in sea surface cooling during typhoon
Rex in 1998 in the northwestern Pacific Ocean near Japan, during which the
SST dropped by nearly 3 ∘C. They concluded that sea surface
cooling was caused by shear-induced VTM during the fast-moving phase of the
typhoon; in contrast, sea surface cooling was caused by Ekman pumping during
the slow phase of the typhoon. Notably, unless the waters are very shallow,
the wind-mixing mechanism usually occurs through the action of
wind-generated waves. Such wave-induced mixing has been studied in tropical
cyclone conditions (Ghantous and Babanin, 2014) and through measurements
obtained during tropical cyclones (Toffoli et al., 2012), and this mixing
was shown to cool the surface on a scale of a few hours of cyclone forcing.
Turbulence plays an important role in the heat, momentum and energy
balances of the ocean. Huang et al. (2012) measured the upper ocean
turbulence dissipation associated with wave–turbulence interactions in the
South China Sea. Their results contribute to understanding the SST drop
induced by wave mixing.
The South China Sea (SCS) is one of the largest semienclosed marginal seas
subject to frequent typhoons. Chiang et al. (2011) reported that the average
SST cooling in the northern SCS during the passage of a typhoon was approximately
4.3±2∘C in 1958–2008. Tseng et al. (2010) and
Lin et al. (2003b) observed an SST drop of more than 9 ∘C in the
northern SCS during Typhoon Kai-tak in 2000. They concluded that this drastic
SST drop could mainly be ascribed to continual wind-forced upwelling, a
preexisting, relatively shallow thermocline, local bathymetry and a slow
propagation speed of typhoons. Furthermore, Chiang et al. (2011) estimated
that the upwelling contribution to SST drop is twice that of entrainment for
the case of Typhoon Kai-tak in 2000. A larger SST drop in the central SCS was
observed by Shang et al. (2008) during Typhoon Lingling in 2001. Prior to
Typhoon Lingling, the SST was approximately 27–30 ∘C; however, the SST was reduced by 11 ∘C after the typhoon
passed. This extreme SST drop was mainly attributed to preexisting eddies
that were driven by the northeastern monsoon. Zheng et al. (2010) also
considered that the preexisting eddy is a favored condition for intensive
cooling after a typhoon passage.
SST drops also frequently occur in the waters off northeastern Taiwan.
Kuroshio flows through this region, which is the most important current that
transports warm water from the tropical ocean. The SST drop off northeastern
Taiwan mainly occurs during the winter monsoon rather than the summer season
(Tsai et al., 2008a; Jan et al., 2013). Bathymetry-induced upwelling, rather
than entrainment mixing, is considered to be the primary cause of the SST drops
in this region (Tsai et al., 2008a). The numerical modeling results of Tsai
et al. (2008b, 2013) suggest that the Taiwan Strait outflow is blocked by
northerly winds, facilitating Kuroshio intrusion and thus leading to SST
drops during the first half of a typhoon passage. This mechanism is similar
to that involved in the winter monsoon. In contrast, Morimoto et al. (2009)
demonstrated that the northward flow of the Kuroshio is mainly because of
the continuous, strong southerly winds, which accelerate the Kuroshio and
force its axis shoreward, resulting in the intrusion of the Kuroshio towards
the shelf and SST drops offshore. Furthermore, Wu et al. (2008) indicated
that internal waves were generated after Typhoon Nari's departure in 2001
and that this was a minor cause of the SST drops. SST drops that occur after
a typhoon passage are rapid and occur within a short period of time (Tsai et
al., 2013). According to previous studies, these temperature decreases in
the waters off northeastern Taiwan are approximately 4–8 ∘C after
a typhoon passage (Chang et al., 2008; Wu et al., 2008;
Tsai et al., 2008a).
Table 1 summarizes the records of the SST drops after typhoon passages reported
in the literature. Most studies on drops in SST have been conducted in the
open ocean. There have been comparatively few studies conducted on
near-coastal waters (i.e., less than 1 km from the coastline). In
addition, most previous studies on SST drops have been conducted based on
numerical modeling or satellite images because long-term field observations
of the SSTs are relatively rare in typhoon-prone areas. Thus, the purpose of
this research is to study SST drops following typhoon passages in coastal
areas. Unlike previous studies, this study was conducted based on an
analysis of field data. Coastal SST variability substantially affects both
coastal environmental ecosystems and human activities, and therefore,
typhoon-induced coastal SST variability requires a dedicated study.
Records of the SST drops due to typhoon passages in the literature.
Sea areaSST dropTyphoonMain analysis dataReferenceVarious regions1–8 ∘C16 typhoons from 1958 to 1988ModelingBender et al. (1993)Gulf of Mexico2 ∘CEloise in 1975Field dataPrice (1981)NW Pacific (off Taiwan coast)8 ∘CGerald in 1987Field dataTsai et al. (2008)NW Pacific (off Japan coast)9 ∘CT8914/T8915 in 1989Satellite imageSakaida et al. (1998)South China Sea1 ∘CErnie in 1996ModelingChu et al. (2000)NW Pacific (off Taiwan coast)9 ∘CHerb in 1996R/V dataChen et al. (2003)Indian Ocean6–7 ∘CChennai in 1997ModelingRao et al. (2004)NW Pacific3 ∘CRex in 1998Modeling and R/V dataWada et al. (2005, 2009)N South China Sea9 ∘CKai-tak in 2000ModelingTseng et al. (2010)N South China Sea10.8 ∘CKai-tak in 2000ModelingChiang et al. (2011)Middle South China Sea11 ∘CLingling in 2001Satellite imageShang et al. (2008)NW Pacific (off Taiwan coast)5 ∘CNari in 2001Satellite imageWu et al. (2008)N South China Sea5.3 ∘CKrovanh in 2003ModelingJiang et al. (2009)NW Pacific (Luzon Strait)1.8 ∘CDujuan in 2003ModelingKuo et al. (2011)NE Pacific (N Carolina)1–3 ∘CIsabel in 2003Field dataBingham (2007)NW Pacific (Kuroshio region)3 ∘CMegi in 2004Satellite imageWei et al. (2014)NW Pacific (Kuroshio region)4 ∘CMorakot in 2009Modeling and Argo dataZheng et al. (2014)NW Pacific (off Taiwan coast)4.5 ∘CHaitang in 2005Satellite imageChang et al. (2008)NW Pacific (off Taiwan coast)13 ∘CHaitang in 2005Field dataMorimoto et al. (2009)NW Pacific (Luzon Strait)3.5 ∘CPabuk in 2007ModelingKuo et al. (2011)NW Pacific (off Taiwan coast)2–4 ∘CFungwong in 2008R/V dataHung et al. (2010)South China Sea5–6 ∘CNuri in 2008ModelingSun et al. (2015)NW Pacific2 ∘CKaemi in 2006Satellite imageSubrahmanyam (2015)NW Pacific (Kuroshio region)0.61–4.93 ∘C22 typhoons from 2001 to 2010SST maps and Argo dataLiu and Wei (2015)NW Pacific (off Taiwan coast)7 ∘CMorakot in 2009ModelingTsai et al. (2013)South China Sea8 ∘CMegi in 2010Modeling and Satellite imageKo et al. (2014)South China Sea4.2 ∘CMegi in 2010Modeling and MooringGuan et al. (2014)Study area and dataStudy area
This research was conducted on the Longdong coast in northeastern Taiwan, as
shown in Fig. 1. The Longdong coast is characterized by its irregular
coastline and rapidly changing bathymetry. The Longdong coastline is
oriented northwest–southeast at approximately 160 degrees from the north. The
average sea bottom slope at Longdong is ∼1/50. An important
North Pacific warm western boundary current, known as Kuroshio, flows along
the eastern waters of Taiwan. The observed maximum flow velocity of Kuroshio
varies between 0.7 and 1.4 m s-1 and is located at depths, ranging from 20 to
100 m (Jan et al., 2011). The distance between Taiwan's coast and the main
stream of Kuroshio is varied. Morimoto et al. (2009) demonstrated that the
western edge of the Kuroshio streamflows approaches Taiwan during the typhoon
period. In this study, the shift in Kuroshio during typhoon Haitang in 2005
is estimated and plotted in Fig. 1, according to Morimoto et al. (2009).
Locations of the study area and field stations. The gray belt is
the main stream of Kuroshio; however, the dashed gray belt is the shift of
the Kuroshio during Typhoon Haitang in 2005 according to measurements by
Morimoto et al. (2009).
DataSST measured by moored buoys
SST can be measured by satellite technology, ships and floating or moored
buoys (Matthews, 2013). Satellite observations provide the spatial
distribution of the SST; however, moored buoys record the time series of the SST. In
this study, the main data are the SST recorded by a 2.5 m discus-shaped
buoy deployed in the water along the Longdong coast. The Longdong Buoy was
deployed by the Coastal Ocean Monitoring Center of National Cheng Kung
University, as assigned by the Taiwan Central Weather Bureau (CWB) in 1998.
This buoy is approximately 0.6 km off the Longdong coast and is situated in
the water at 23 m depth. The buoy is anchored to the sea bottom. The buoy
was equipped with sensors of water and air temperatures, wind, pressure and
wave, as well as power unit, data transmission unit and control unit. Every
hour, the buoy automatically switches on to collect the oceanographic and
atmospheric data. The sampling rates for all sensors are 2 Hz. The sampling
duration for wind and wave data is 10 min to the hour and it is 1 min
to the hour for pressure and temperature data. The water temperature sensor
is installed at 0.6 m below the sea surface. The procedures of sensor
calibration, system integration, operation and maintenance have been
qualified by ISO 9001:1994 since 2000.
The SST is measured by a platinum resistance temperature detector (RTD),
which is capable of covering the range from -10 to 70 ∘C. The
sensor provides ±0.1 % F.S. accuracy for critical temperature
monitoring applications. Before integrating the temperature sensor with the
buoy, the sensor is submitted to the National Meteorological Instruments
Center in CWB for calibration to confirm the sensor accuracy. All new or
retrieved sensors from the field were submitted for calibration. After
integrating the water temperature sensor into the buoy, the temperature
measurements are compared with those of another sensor to confirm the
system's accuracy before sea deployment. The buoy SST data used in this
study can be accessed from 10.1594/PANGAEA.895002.
Water temperature measured by tide station
In addition to the Longdong Buoy, SST data were collected from buoys at
the Gueishandao, Suao and Hualien and tide stations at Linshanbi, Keelung
and Fulong, respectively. The locations of these stations are shown in
Fig. 1. The buoys at Gueishandao, Suao and Hualien are 10.0, 1.0 and 0.6 km from the coast and are situated in the water at depths of 38,
20 and 21 m, respectively. All tide stations are located inside the
harbors and are equipped with water temperature sensors installed at the
bottom (depth varies from 2 to 5 m) of the stations. Water temperature
measured by bottom-mounted ADCP near Linshanbi tide station was also
collected in this study for data quality check use. The SST data from tide
stations and used in this study can be accessed from 10.1594/PANGAEA.895002.
Current data
Current data observed by acoustic Doppler current profilers (ADCPs) deployed
at Longdong and Linshanbi were also collected and used for validation. The
ADCPs were bottom-mounted and upward-looking and measured the current profile of
the sea column. Current profile data from the Longdong ADCP were collected
from June 2008 to June 2009 and the data from four typhoons (Kalmaegi,
Fungwong, Sinlaku and Jangmi) were recorded. The Linshanbi ADCP only
obtained recordings in September 2013, which included data from the passage
of Typhoon Usagi. The ADCPs measured the current profile in the range -4
to -23 m water depth. The current data used in this study can be
accessed from 10.1594/PANGAEA.895002.
Satellite images
Except for the field data, multiscale ultra-high-resolution (MUR) SST
analyzed satellite images (downloaded from the NOAA website:
http://coastwatch.pfeg.noaa.gov/erddap/griddap/jplMURSST.graph?analyzed_sst,
last access: 14 April 2017) were also collected for cross analysis. In an optimal way, this dataset
combines data from the advanced very high-resolution radiometer, moderate
imaging spectroradiometer's Terra and Aqua, and advanced microwave
spectroradiometer-EOS instruments to produce 1 km global SST maps. Data have
been released since 2003, and one image is produced per day. The SST images
during Typhoon Jangmi in 2008 were collected in this study.
Spatial wind field
To discuss the possible mechanism of the SST drop, the cross-calibrated
multiplatform (CCMP) gridded surface vector winds for the East Asia area
(115–130∘ E, 18–30∘ N) were collected. CCMP is one of
the productions provided by the scientific research company, remote-sensing
systems (RSS), located in California, USA. The CCMP version 2.0 dataset
integrates observations from satellites, moored buoys and model results and
provides a long-term and high-resolution record of global ocean surface
(10 m) winds (Wentz et al., 2015). The spatial and temporal resolutions of CCMP
wind are 0.25 degree and 6 h, respectively. CCMP has a wide-ranging
appeal to users in educational, operational and research environments. In
this study, data obtained during Typhoon Bilis in 2000, Fungwong in 2008,
Morakot in 2009 and Fanapi in 2010 were downloaded from
http://www.remss.com/measurements/ccmp (last access: 19 September 2018).
Data quality check
Checking data quality is necessary and crucial to field data analysis.
Incorrect data may yield misleading results, and inaccurate observations may
have a greater negative impact than a lack of observations. In addition to
the satellite image and wind field data that were downloaded from qualified
websites, all field data were strictly verified. The list of field data used
in this study are shown in Table 2. The field measurements are equipped with
a solid data-quality-checking (QC) system (Doong et al., 2007), including
both automatic and manual verifications of raw data and statistical data,
respectively. The automatic machine verification is used to cull the
suspicious data according to the rationality, continuity and correlation of
data. The core work for automatic data quality check is to filter the
outliers. The data outlier can be divided into a system outlier and general
outlier. The system outlier is its measurement value clearly exceeding the
limitations of the measurements systems or environmental conditions such as
breaking wave height. These system outliers with such obviously unreasonable
extreme values can be detected easily. The general outliers are measurements
within the limitations but still look suspicious due to their rather larger
deviations from the rest of measurements. The outliers can be detected when
its deviations exceed the pre-determined range in the ranked deviation
series. Doong et al. (2007) proposed that the multiple of standard deviation has
a correlation with sample sizes and the confidence level of this statistical
test. When the measured data are located outside of the upper and lower limits, they
have to be filtered.
∗ All buoys have sampling interval 2 h from 1998 to 2003 and 1 h from
2004 to 2017.
The manual verification is used to double-check the suspicious data
according to spectrum, nearby observations and the QC engineers' knowledge
and experience. Except for QC procedures, data are correlated with nearby
measurements every month, season and year to develop quality accuracy (QA)
and increase confidence in the data use. Figure 2 shows one SST drop event
in 2013 during Typhoon Usagi as an example. The SST drops were measured by
the Longdong Buoy, Linshanbi ADCP and Linshanbi tide station. The
simultaneous observations of the SST drops using different instruments proves
that the phenomenon cannot be ascribed to instrumental error.
SST drop observed by various types of instruments during Typhoon
Usagi in 2013.
Typhoons
There were 108 typhoon datasets observed by the Longdong Buoy from 1998 to
2017. Typhoons are complex atmospheric phenomena and have high variability
in intensity, moving track and speed; therefore, not all typhoons induced
SST drops. For 43 typhoons, SST drops significantly along the coast
of Longdong. Table 3 shows the list of the cases. The intensity of the
typhoons is categorized according to the Saffir–Simpson classification
method. The maximum significant wave height of each typhoon is shown in the
table. Typhoon parameters are highly time dependent. The values of typhoon
intensity and maximum sustained wind shown in Table 3 are the numbers
obtained when the typhoons were closest to Taiwan.
Significant SST drops observed at the Longdong Buoy (NE Taiwan
coast) during 43 typhoon passages from 1998 to 2017.
MaximumDurationDurationMovingsustainedMaxof theof theTyphoonTyphoonTrackIntensityspeed∗wind∗HsSSTSSTCooling rateNo.namedatescategory∗category∗(m s-1)(m s-1)(m)ΔSSTdrop (h)recovery (h)(∘ C h-1)1Zeb10–17 Oct 1998DV6.1386.03.714200.262Babs14–30 Oct 1998EIV4.2153.62.42840.093Maggie1–9 Jun 1999CIII6.7384.35.614260.404Kai-tak3–12 Jul 2000DI10.6302.43.546500.085Bilis18–27 Aug 2000BV6.1535.010.012240.836Xangsane25 Sep –2 Oct 2000DIII9.2334.92.828920.107Chebi19–24 Jun 2001EIII8.1332.53.524200.158Utor1–7 Jul 2001CI9.2385.19.024120.389Toraji25 Jul–1 Aug 2001BIII4.7383.23.618120.2010Nari5–21 Sep 2001AIII1.7402.32.611270.2411Lekima22–30 Sep 2001BII1.4354.47.626380.2912Morakot31 Jul–4 Aug 2003BI5.3231.74.028320.1413Dujuan27 Aug–3 Sep 2003CIV8.3435.46.012280.5014Mindulle21 Jun–4 Jul 2004DIV4.2283.98.015260.5315Nockten14–26 Oct 2004BIII5.6408.23.51150.3216Matsa30 Jul–8 Aug 2005AII3.9405.23.423140.1517Sanvu9–14 Aug 2005CI6.4203.27.32360.3218Longwang25 Sep–3 Oct 2005BIV6.4517.56.722140.3019Chanchu8–18 May 2006EIV11.9252.73.52460.1520Bilis8–16 Jul 2006BTS5.0254.85.312300.4421Kaemi17–27 Jul 2006BI4.7383.37.630470.2522Sepat12–20 Aug 2007BV5.6484.89.53080.3223Kalmaegi13–20 Jul 2008BII5.6333.15.115180.3424Fungwong23–30 Jul 2008BII4.7437.912.517350.7425Sinlaku8–21 Sep 2008BIV2.2387.36.818200.3826Jangmi23 Sep–1 Oct 2008BV5.05111.28.019440.4227Morakot2–11 Aug 2009BI3.3358.212.320160.6228Merant6–10 Sep 2010CI3.3151.54.626420.1829Fanapi14–21 Sep 2010BIII5.6457.210.521260.5030Nanmadol21–31 Aug 2011CV2.5352.98.927300.3331Saola26 Jul–5 Aug 2012BII4.2308.35.410140.5432Tembin17–30 Aug 2012DIV3.1302.53.810420.3833Trami16–24 Aug 2013AI12.8303.12.421100.1134Usagi16–24 Sep 2013CV5.3534.36.420410.3235Hagibis13–18 Jun 2014ETS3.6151.04.576600.0636Matmo16–25 Jul 2014BII5.6384.310.422290.4737Fungwong17–24 Sep 2014DTS6.1253.43.543100.0838Nepartak2–10 Jul 2016BV4.7553.67.536290.2139Meranti8–16 Sep 2016CV5.6583.98.321190.4040Megi22–29 Sep 2016BIV6.44512.510.029180.3441Aere4–14 Oct 2016CTS6.4183.92.642460.0642Nesat25–30 Jul 2017BII4.2402.46.311220.5743Hato19–24 Aug 2017CIII7.8202.05.05190.10
The asterisk indicates that the values were obtained when typhoons were close to
Taiwan.
Statistics on coastal SST dropSST drop determination
To estimate the scale and rate of each SST drop event, the starting and
ending times and temperatures of an SST drop process were determined. The
background SST, which is defined as the mean SST over the 7 days before
the SST drop occurrence, is first obtained to determine the starting point
of the event. The starting time of each SST drop event was defined based on
the point at which the water temperature rapidly dropped to a value lower
than the background SST. The lowest SST was the minimum water temperature
value during the typhoon. The sea surface temperature drop (ΔSST)
was the difference between the background SST and the lowest SST. The
duration and further cooling rate of an SST drop event are then estimated.
The cooling rate represents how rapidly a typhoon exerted its effects on the ocean.
The significant coastal SST drop event
Typhoon Fungwong occurred in 2008 and was a Category 2 typhoon when it was
close to Taiwan. The typhoon exhibited a maximum wind speed of 43 m s-1 and a
minimum central air pressure of 948 hpa. Fungwong occupied an area at
22∘ N and 136∘ E and traveled approximately along the latitude of
22∘ N at an average speed of 4.7 m s-1. The intensity of the
typhoon increased to that of a medium typhoon during the second half of
26 July and subsequently changed direction to the northwest. Figure 3 shows the
track of the typhoon and the time series of the SST, wind speed, wind
direction and significant wave height observed at the Longdong Buoy during
Fungwong. Before the typhoon approached, the background SST was 29.1 ∘C.
The mean wind speed was lower than 10 m s-1, and the wind
directions were irregular. On 28 July, Fungwong landed on the eastern coast
of Taiwan, and the mean wind speed at Longdong rapidly increased and reached
a maximum value of 21.4 m s-1. The wind direction shifted northward and
continued for approximately 1 day. The significant wave height increased
to 7.9 m on 28 July from less than 0.5 m on 26 July. Approximately 7 h
later, the SST began to drop. Cold water at a temperature of 16.6 ∘C
was observed on 29 July. The total SST drop was 12.5 ∘C within 17 h. Then, the SST took 35 h to recover to its
background temperature level. Typhoon Fungwong in 2008 induced the maximum
SST drop in Longdong.
The significant SST drop event after the passage of Typhoon
Fungwong in 2008. (a) The typhoon track, (b) SST, (c) wind speed and
direction, and (d) significant wave height. The data were observed by a data
buoy in the Longdong coastal waters of northeastern Taiwan.
Statistical results
To reduce the measurement uncertainty, only SST drops larger than 2 ∘C
were considered in this study. Forty percent (43 of 108) of
typhoons triggered a significant SST drop in Longdong in the past 20 years
(1998–2017). Among these 43 typhoons, the mean SST drop was 6.1 ∘C, and the maximum drop was 12.5 ∘C (Typhoon Fungwong in 2008).
The mean drop duration was 24 h, and the mean recovery duration was 26.1 h.
The mean cooling rate was 0.32 ∘C h-1; however, the maximum
cooling rate reached 0.83 ∘C h-1, which occurred during Typhoon
Bilis in 2000. The entire statistic of coastal SST drops is shown in Table 3.
Figure 4 shows the distribution of the SST drop magnitude. Typhoon
passages that caused SSTs to drop by 3–4 ∘C
occurred most frequently. Six typhoons caused coastal SSTs to drop by more
than 10 ∘C. These include Typhoon Bilis in 2000, Fungwong in
2008, Morakot in 2009, Fanapi in 2010, Matmo in 2014 and Megi in 2016. The
typhoon tracks and time series of the SSTs are shown in the Appendix. The
intensities of Typhoon Fungwong (Category 2) and Morakot (Category 1) were
relatively weak, but these typhoons induced the largest and second-largest
SST drops on the Longdong coast.
Distribution of the SST drop magnitude for 43 typhoons.
Mechanisms of coastal SST dropTyphoon dependenceTyphoon intensity
The scale of the typhoon-induced SST drop depends on the typhoon's
characteristics, such as the intensity measured by the maximum surface wind
speed, moving speed and size. Zhu and Zhang (2006) quantified the influence of
SST variability on typhoon intensity using a numerical model. However, this
is not the case for the coastal ocean at Longdong. Of the 43 typhoons that
triggered significant coastal SST drops, eight were categorized as
Category 1 typhoons, seven as Category 2 typhoons, eight as Category 3 typhoons,
eight Category 4 typhoons and eight Category 5 typhoons. Another four typhoons were
categorized as tropical storms (TS). The uniform intensity distribution of
all typhoons causing SST drops demonstrates that intensity may not be a
significant factor triggering the coastal SST drop. This can also be
validated according to weak typhoons (for example, Typhoon Hagibis in 2014)
that triggered larger coastal SST drops than stronger typhoons (for example,
Category IV Typhoon Tembin in 2012). We used both the minimum central air
pressure and central maximum wind speed as typhoon intensity indicators to
understand their influences on SST drops. The regression results show that
the determination coefficients of the typhoon intensity indicators (min
central pressure or max wind speed) with the SST drop scale (ΔSST)
were smaller than 0.15 as shown in Fig. 5a, b. Again, it was suggested
that typhoon intensity is not the dominant factor that influences coastal
SST drops.
Typhoon track and moving speed
We classify typhoon moving tracks into five paths, as shown in Fig. 6.
Tracks A, B and C represented typhoons that traveled from southeast to
northwest. Track A was north of waters off Longdong, whereas tracks B and C
were south of Longdong. Typhoons on track B made landfall, whereas track C
typhoons traveled along southern Taiwan. The typhoon numbers (of a total of
43 cases) and their corresponding mean temperature decreases for each track
are listed in Fig. 6. Typhoons that traveled along tracks B and C occupied
70 % of those typhoons that triggered SST drops, and the mean decrease in
temperature for the sea surface at Longdong is greater than 6 ∘C
(7.6 ∘C for track B; 6.4 ∘C for track C). This
indicates that the mean distance between track C typhoons and Longdong is
more than 500 km. Typhoons that traveled along track A were closer to the
waters off Longdong, but of the typhoons that induced an SST decrease along
this track, the scale of SST decrease was relatively small. Typhoons that
passed along the southern side of Longdong had greater induced SST drops than
other typhoons. These results were consistent with those of previous studies
conducted in the open ocean (Price, 1981; Wada, 2005; Wada et al., 2009), which
have proposed that the SST response is larger on the right side of a typhoon.
The correlation of ΔSST with two typhoon intensity
indicators,
(a) min central pressure and (b) max wind speed.
Slow-moving typhoons induced larger SST drops in the open sea because they
facilitate more substantial air–sea interactions (Tsai et al., 2008b; Wada et
al., 2009; Tseng et al., 2010; Kuo et al., 2011). This study correlated the
typhoon moving speeds with the magnitude of coastal SST drops and found no
correlation. The regression result is shown in Fig. 7. The coefficient of
determination is 0.018.
Typhoon wind distribution
The above results show that the coastal SST drop at Longdong is correlated
with the typhoon track. Therefore, it is interesting to look directly at the
wind distribution during typhoons. Figure 8 shows the CCMP wind patterns for
the four significant cases (Typhoon Bilis in 2000; Fungwong in 2008; Morakot
in 2009; and Fanapi in 2010). Because of the output time limitation for the
operational model, the CCMP wind fields are not exactly at the starting time
of the SST drop, but the maximum values are different within 2 h. All 4
cases show strong winds off the northeastern Taiwan waters, and the wind
directions are parallel with the Kuroshio direction. The coverage of the
Kuroshio region with large wind speeds is a significant factor. We found
that when the area of strong wind overlapping with Kuroshio is large (for
example, Typhoons Fungwong and Morakot in Fig. 8b and c), there was a
very large SST drop along the Longdong coast. We suggest that the
interaction between typhoon wind and Kuroshio plays an important role in
triggering coastal SST drops in the northeastern corner of Taiwan.
The SST drops for various typhoon tracks. The two numbers in
parentheses show the typhoon number and the mean SST drop magnitude in the
corresponding typhoon track.
The correlation of typhoon moving speed with ΔSST.
Wind patterns at the time close to the start of the SST drop. (a) Typhoon
Bilis in 2000. The SST started to decrease on 23 August 2000 at 10:00 UTC+8.
The wind pattern was observed on 23 August 2000 at 08:00 UTC+8.
(b) Typhoon Fungwong in
2008. The SST started to decrease on 28 July 2008 at 18:00 UTC+8. The wind pattern
was observed on 28 July 2008 at 20:00 UTC+8. (c) Typhoon Morakot in 2009. The SST
started to decrease on 8 August 2009 at 13:00 UTC+8. The wind pattern was observed on
8 August 2009 at 14:00 UTC+8. (d) Typhoon Fanapi in 2010. The SST
started to decrease
on 19 September 2010 at 22:00 UTC+8. The wind pattern was observed on
19 September 2010 at 20:00 UTC+8.
Vertical Kuroshio intrusion
Seeking the source of the cold water is the most interesting issue in this
study. Because the Longdong Buoy observation site is located in near-coastal
water (0.6 km from the coastline at 23 m water depth), the cold water may
originate from three sources: river discharge from the land, adjacent
surface water, or subsurface water.
The Shuangsi River is the only stream near Longdong. However, the discharge
of the river is small, and the river water temperature ranges between 26 and
30 ∘C during the summer typhoon season, although the mean low SST
in the waters off Longdong was 21.5 ∘C. This fact allows for
rejection of the hypothesis that cold water was supported by land.
We assume that the cold water was pumped from the subsurface of Longdong.
According to the simultaneous measurement of wind, we observed southerly
winds during the SST drop periods (Figs. 3 and 8, as examples). The
prevailing wind directions during these typhoons were between 164
and 189∘. The Longdong coastline lies at an angle of 160∘ from
the north. Thus, typhoons created winds parallel to the Longdong coastline and
induced coastal upwelling. The subsurface water is usually cooler than the
surface water it replaces. To prove this assumption, the current profile
data were analyzed.
The current profile data were measured very close to the Longdong Buoy by an
ADCP from 2008 to 2009. There were four typhoon-induced surface cooling
cases observed during the ADCP measurement period: Typhoon Kalmaegi
(ΔSST = 5.1 ∘C), Typhoon Fungwong (ΔSST = 12.5 ∘C),
Typhoon Sinlaku (ΔSST = 6.8 ∘C) and
Typhoon Jangmi (ΔSST = 8.0 ∘C). The current profiles
obtained during Typhoon Fungwong are shown in Fig. 9. In the waters off
Longdong, currents flowed offshore, while the alongshore winds blew during
typhoons. The sea current in the area generally flows shoreward, but
instead, the current flowed seaward. The data demonstrated that typhoons
generate an alongshore wind and pump cold water from the subsurface of
Longdong to cool the surface.
The mean SST drop in the waters off Longdong was estimated to be 6.1 ∘C;
however, the Longdong Buoy is situated in water that is 23 m
deep. The difference in water temperature between the sea surface and sea
bottom is only approximately 2–3 ∘C. It was
assumed that the observed cold water was not from the subsurface water at
the Longdong Buoy location but may be transferred from offshore deep seawaters. In this study, we referred to the data of the mean water temperature
profile from the Ocean Data Bank (ODB) of the Ministry of Science and
Technology of Taiwan. The data have been collected by research vessels since
1985. At a deep-sea location (122.5∘ E, 25.25∘ N) in
waters off Longdong, the temperature is 22.9 ∘C at a depth of 50 m,
18.8 ∘C at 100 m and 14.5 ∘C at 200 m. The mean
lowest SST for those 43 events was 21.5 ∘C and was 16.1 ∘C
for the extreme case. Therefore, we determined that the cold
water was being pumped from a maximum depth of 155 m and then intruded into the
coastal area. This finding reaches the maximum value that Narayan et al. (2010)
proposed in which cooler waters from 100 to 150 m depths are able to be
pumped via coastal upwelling.
To identify the movement path of cold water being pumped from the deep
ocean, the starting time of the SST drop was assessed at several stations in the
research area, as shown in Fig. 1. The analysis results of Typhoon Morakot
(ΔSST = 12.3 ∘C) are shown in Table 4 as an example. The
lag time shown in the table is the start time differences in the SST drops
between the stations for the Longdong Buoy; in the table, a positive number
indicates that the SST drop observed at the station occurred later than that
observed at the Longdong Buoy. As Table 4 shows, we found that coastal SST
drops occurred earliest in Longdong waters. We suggest, according to the
bathymetry off northeastern Taiwan, that the cold water was pumped from the
Kuroshio subsurface (∼155 m depth) in the Okinawa Trough and
reached the Longdong area first, and then the cold water was transported
north to Keelung and south to Suao. Figure 10 shows a sketch
of the cold-water movement path. This assumption can partially prove that no
significant SST drop occurred at the Hualien Buoy.
Quantities of the SST drop, the lowest SST and their lag time
corresponding to the Longdong Buoy during Typhoon Morakot in 2009. A
positive lag time value indicates that the SST drop observed at the station
occurred later than that observed at the Longdong Buoy. A dash means no
significant SST drop was observed.
The exchange of water masses off northeastern Taiwan is complex. Chen et al. (1995)
showed that at least six water masses take part in the mixing
processes in this region, including the Kuroshio Surface Water (SW),
Kuroshio Tropical Water (TW), Kuroshio Intermediate Water (IW), East China
Sea Water (ECSW), Coastal Water (CW) and the Taiwan Strait Water (TSW).
According to extensive investigations, the intrusion of the Kuroshio into
the East China Sea (ECS) occurs northeast of Taiwan (Hsueh et al., 1992;
Tang et al., 1999; Guo et al., 2006; Yang et al., 2011, 2018; Wu et al., 2017). The mechanism leading to the Kuroshio intrusion into the
ECS is still being researched. Recently, Zhou et al. (2018) indicated that
the Kuroshio subsurface water could intrude into the ECS shelf from
northeastern Taiwan and reach north of 29∘ N. Yang et al. (2018)
explained that a topographic beta spiral occurs when the Kuroshio encounters
the shelf break and induces strong upwelling. These researchers suggested
that the topographic beta spiral provides a dynamic channel through which to bring the
cold deepwater from Kuroshio to the continental shelf. Our findings in this
study provide direct evidence from long-term buoy measurements.
Current profile and corresponding tide level observed in Longdong
during Typhoon Fungwong in 2008.
The suggested movement path of cold water. The cold water was
pumped from the Kuroshio subsurface in the Okinawa Trough and reached the
Longdong coastal waters first. Then, the cold water was transported north to
Keelung and south to Suao.
Spatial cold-water intrusion
In addition to coastal upwelling, the cold water in the coastal area of
Longdong may also come from offshore surfaces, as many studies have
confirmed that a cold dome exists in the waters off northeastern Taiwan.
Numerous observational and modeling studies have reported occurrences of
cold water and isotherm doming in northeastern Taiwan, which is known as the
cold dome (Tang et al., 1999; Yang et al., 2011; Shen et al., 2011; Jan et
al., 2011; Gopalakrishnan et al., 2013; Cheng et al., 2018). When the
Kuroshio flows near the northeastern Taiwan shelf, a weaker northwestward
branch intrudes into the ECS shelf (Tang et al., 1999; Lee and Matsuno, 2007).
Recently, Cheng et al. (2018) demonstrated a 4–6 year interannual
variability in the cold dome. Then, the cold dome forms because of the
on-shelf intrusion of the Kuroshio subsurface water. Gopalakrishnan et al. (2013)
established a numerical model and found that the cold dome
occurrences appeared to be connected with seasonal variability in the
Kuroshio. Jan et al. (2011) used field observation data and satellite images
to better understand that the center of the cold dome is located at
approximately 25.625∘ N, 122.125∘ E. The diameter of the
cold dome is approximately 100 km, and it has a weak counterclockwise
circulation. The SST of the cold dome is ∼3∘C
below the temperature of the ambient shelf waters.
Daily satellite images (Fig. 11) show the spatial distribution of SSTs
during Typhoon Jangmi in 2008. The cold dome moved shoreward along the
movement of the typhoons. The temperature difference between the coastal
area of Longdong and the center of the cold water is generally less than 3 ∘C.
However, the scale of the SST drop in the Longdong area was much
higher. Although the contributions from the north (cold dome) and deep sea
were not decomposed, it was suggested that cold water coming from the deep
sea dominates the coastal SST drops in the Longdong area.
Movement of the cold dome off northeastern Taiwan during Typhoon
Jangmi in 2008. The typhoon track is shown in the upper panel. The lower
panel shows the satellite images of the SST.
Data availability
The dataset used in this study was deposited in the World Data Center
PANGAEA (10.1594/PANGAEA.895002, Doong, 2018). The contents and
format of the data are included in the “readme” file provided with the
data.
Conclusions
Seawater temperature affects marine environmental ecosystems and human
activities. The variability in seawater temperature also influences typhoon
intensity. It is widely known that the SST may drop a few degrees after the
passage of a typhoon. However, in this study, we found that following summer
typhoon passages in the coastal waters off Longdong in Taiwan, the SST may
decrease to values lower than the annual minimum temperature (which always
occurs in winter).
Long-term SST field data from the Longdong Buoy (which is located 0.6 km
offshore at a water depth of 23 m) over the past 20 years (1998 to 2017)
were analyzed to study coastal SST drops. These decreases were observed
after the passage of 43 typhoons. The mean SST drop during the 43 events was
6.1 ∘C. The lowest SST was 16.1 ∘C, which was observed
during Typhoon Morakot in 2009; however, the maximum SST drop was 12.5 ∘C,
observed during Typhoon Fungwong in 2008. This scale of
decrease is much larger than that in the open ocean. The mean duration of
the SST drop was 24 h, and on average, 26.1 h were required for the
SST to recover to the background temperature.
Previous studies on the open ocean have proposed that the scale of the SST drop
is related to typhoon intensity and speed. However, we found that the scale
of typhoon-induced coastal SST drops in the northeastern Taiwan area were
mainly correlated with the typhoon track. Typhoon intensity and moving speed
do not appear to be significant factors driving coastal SST drops in this
location. Typhoons that moved south of Longdong (i.e., Longdong is to the
right of the typhoon) accounted for more than 70 % of coastal SST
drops and exhibited reduction on extremely large scales, irrespective of whether
these typhoons traveled near or far from Longdong.
Wind-driven coastal upwelling was confirmed as the main mechanism involved
in substantial coastal SST drops after a typhoon passage at Longdong. The
measurements indicated that many typhoons were accompanied by alongshore
winds blowing in a constant direction. Such winds induce coastal upwelling
and pump bottom seawater up to the surface. This was verified through
measurements of the current profile collected at Longdong. This discovery
explains the conclusion that SST drops are mainly influenced by typhoon
tracks. However, the cold water was not directly supplied from the
subsurface of Longdong. We suggest that the coldest water may originate from
depths of 155 m in the Okinawa Trough, which is ∼50 km from
Longdong. These waters are the subsurface waters of Kuroshio. We found that
the coverage of a large wind speed region by the Kuroshio is a significant
factor that triggers the coastal SST drop. When the strong wind area largely
overlapped with Kuroshio, there was a very large SST drop on the Longdong
coast. By analyzing SST drop processes and the lag times between field
stations, we suggest that the cold water intrudes first at Longdong and is
then transported along the coast. Except for the vertical source of cold
water, the cold water from the known cold dome off northeastern Taiwan may
also penetrate and cool the coastal area. An analysis of satellite images
indicated that the cold dome moves towards the northern coast of Taiwan after
a typhoon passage and contributes to coastal SST drops. In this study, the
contributions of the offshore surface cold water and Kuroshio subsurface
cold water were not decomposed, but we suggest that the Kuroshio subsurface
cold water is the main source of the Longdong coastal SST drop. The
presentation of the coastal SST dataset with a significant drop may help to
understand the interaction between Kuroshio and typhoons, and can be used
to calibrate and validate the numerical models of such interactions.
Six cases of coastal SST drops larger greater than 10 ∘C observed
by the Longdong Buoy after a typhoon passage. (Left figure shows
the typhoon tracks, and the time series of SSTs are shown on the right.)
(1) Typhoon Bilis in 2000, max ΔSST = 10.0 ∘C.
(2) Typhoon Fungwong in 2008, max ΔSST = 12.5 ∘C.
(3) Typhoon Morakot in 2009, max ΔSST = 12.3 ∘C.
(4) Typhoon Fanapi in 2010, max ΔSST = 10.5 ∘C.
(5) Typhoon Matmo in 2014, max ΔSST = 10.4 ∘C.
(6) Typhoon Megi in 2016, max ΔSST = 10.0 ∘C.
Author contributions
DJD was the main contributor to this paper.
He initiated the idea, collected the data, designed the experiment and wrote
the manuscript. JPP checked the data quality, worked on the analysis and
plotted the figures. AVB joined the discussions and provided
constructive suggestions on writing the manuscript.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This research was performed with support from the Ministry of Science and
Technology (MOST) of Taiwan under grant no. MOST 106-2628-E-006-008-MY3. The
buoys that measure SST data are operated by the Coastal Ocean Monitoring
Center of National Cheng Kung University in Tainan, Taiwan. The authors
would like to thank all their colleagues at the center. In addition, the
authors acknowledge the Industrial Technology Research Institute (ITRI) for
providing the ADCP current data.
Edited by: Giuseppe M. R. Manzella
Reviewed by: two anonymous referees
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