Upperoceanresponsestocategory5typhoonMegiin...

Acta Oceanol. Sin., 2012, Vol. 31, No. 1, P. 51-58

DOI: 10.1007/s13131-012-0175-2

http://www.hyxb.org.cn

E-mail: [email protected]

Upper ocean responses to category 5 typhoon Megi in

the western north Pacific

CHEN Xiaoyan1,2, PAN Delu1,2∗, HE Xianqiang1,2, BAI Yan2, WANG Difeng2

1 Department of Earth Sciences, Zhejiang University, Hangzhou 310027, China

2 State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute ofOceanography, State Oceanic Administration, Hangzhou 310012, China

Received 27 May 2011; accepted 8 August 2011

© The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2012

AbstractCategory 5 typhoon Megi was the most intense typhoon in 2010 of the world. It lingered in theSouth China Sea (SCS) for 5 d and caused a significant phytoplankton bloom detected by thesatellite image. In this study, the authors investigated the ocean biological and physical responsesto typhoon Megi by using chlorophyll-a (chl-a) concentration, sea surface temperature (SST), seasurface height anomaly (SSHA), sea surface wind measurements derived from different satellitesand in situ data. The chl-a concentration (>3 mg/m3) increased thirty times in the SCS after thetyphoon passage in comparison with the mean level of October averaged from 2002 to 2009. Withthe relationship of wind stress curl and upwelling, the authors found that the speed of upwelling wasover ten times during typhoon than pre-typhoon period. Moreover, the mixed layer deepened about20 m. These reveal that the enhancement of chl-a concentration was triggered by strong verticalmixing and upwelling. Along the track of typhoon, the maximum sea surface cooling (6–8◦C) tookplace in the SCS where the moving speed of typhoon was only 1.4–2.8 m/s and the mixed layerdepth was about 20 m in pre-typhoon period. However, the SST drop at the east of the Philippinesis only 1–2◦C where the translation speed of typhoon was 5.5–6.9 m/s and the mixed layer depthwas about 40 m in pre-typhoon period. So the extent of the SST drop was probably due to themoving speed of typhoon and the depth of the mixed layer. In addition, the region with the largestdecline of the sea surface height anomaly can indicate the location where the maximum coolingoccurs.

Key words: sea surface temperature, chlorophyll-a concentration, sea surface height anomaly,

upwelling, typhoon Megi, ocean remote sensing

1 Introduction

Upper ocean biological and physical responses to

typhoons (also known as hurricanes or tropical cy-

clones) are always one of the hottest issues on air-

sea interaction research (Price, 1981; Lin et al., 2003;

Babin et al., 2004; Yin et al., 2007; Siswanto et al.,

2008). The development of satellite remote sensing,

especially microwave remote sensing, in combination

with numerical simulation, provide powerful tools to

further understand the upper ocean changes induced

by typhoon together with corresponding mechanisms.

The passage of typhoon can induce ocean mixing and

upwelling (Price, 1981), bringing the nutrient-rich wa-

ter of bottom layer to the euphotic zone where there

is abundant light for photosynthesis but often lack of

nutrients. Therefore it will contribute to the growth

of phytoplankton and take a significant impact on the

increase of ocean primary production (Lin et al., 2003;

Siswanto et al., 2008; Siswanto et al., 2009; Zhao et

al., 2009). Lin et al. (2003) suggested that the long-

neglected contribution of typhoons to the South China

Sea’s annual new production may be as much as 20%–

30%. Therefore the amounts of carbon fixation by

typhoons can not be ignored. At the same time, sea

surface cooling is another major feature after typhoon

passage (Price, 1981; Stramma et al., 1986; Sakaida et

al., 1998; Cione and Uhlhom, 2003; Tsai et al., 2008).

From the previous studies, the SST drop is due to

a unique combination of typhoon’s intensity and

Foundation item: The National Basic Research Program “973” Program of China under contract No. 2009CB421202; the NationalNatural Science Foundation of China under contract Nos 40976110 and 40706061; the Public Science and Technology ResearchFunds Projects of Ocean under contract No. 200905012; the Zhejiang Provincial Natural Science Foundation of China undercontract No. Y5090381.

∗Corresponding author, E-mail: [email protected]

1

52 CHEN Xiaoyan et al. Acta Oceanol. Sin., 2012, Vol. 31, No. 1, P. 51-58

translation speed and oceanic conditions prior to the

typhoon’s arrival, such as the location of cold eddies

(Zheng et al., 2010), upper thermal structure and the

depth of 26◦ isotherm (Lin et al., 2008). In addition, it

may also be affected by the Kuroshio for the typhoons

which pass through the East China Sea or the Taiwan

Strait (Tsai et al., 2008).

Megi, a category 5 typhoon on the Saffir-Simpson

hurricane scale, was the most intense typhoon both in

2010 of the world and in the last 20 a of the west-

ern north Pacific. In this paper, we investigated the

changes of chl-a concentration, SST and SSHA in the

western north Pacific before and after the passage of

typhoon Megi, using MODIS ocean color data, ocean

microwave data and in situ profiles. Moreover, the

mechanisms to induce these changes were discussed,

which make us to further understand the interaction

between typhoon and ocean and provide the powerful

proofs for the previous conclusions.

2 Data and methods

2.1 Typhoon data

The typhoon track data used in this study

were downloaded from http://weather.unisys.com/

hurricane/w−pacific, which is based on the best hur-

ricane track data from the Joint Typhoon Warming

Center (JTWC). The data include typhoon center po-

sition in latitude and longitude, time (UTC), maxi-

mum sustained winds in knot and typhoon scale ev-

ery 6 h. The track of typhoon Megi is shown in Fig.

1, in which different color points stand for different

intensities according to the Saffir-Simpson hurricane

scale. Developed as a tropical depression in the west-

ern north Pacific on 13 October, Megi intensified in

the east of the Philippines and reached category 5 on

17 October. After dropping to a category 2 typhoon

after passing through the Philippines into the SCS on

18:00 UTC 18 October, Megi quickly regained strength

and intensified to category 4 on 18:00 UTC 19 Octo-

ber. It lingered in the SCS for 5 d with a relative slow

moving speed of 1.4–2.8 m/s and made landfall on the

southern coast of Fujian Province on 23 October.

2.2 Satellite data

Daily L3A chl-a concentration data derived from

MODIS were obtained from the Second Institute

of Oceanography, the State Oceanic Administration,

with a 0.01◦ spatial resolution. Monthly mean chl-a

products with a 4 km×4 km spatial resolution were

Fig.1. Location of study area and track of

Typhoon Megi. Megi’s 6 h positions are de-

picted with color points. Based on the Saffir-

Simpson hurricane scale, blue point stands for

tropical depression, green tropical storm, cyan

category 1, yellow category 2, pink category 3,

purple category 4, red category 5.

obtained from NASA Goddard Earth Sciences Data

and Information Services Center (GES DISC) in this

study.

Merged SST daily products with a 0.25◦ spatial

resolution retrieved from the Tropical Rainfall Mea-

suring Mission (TRMM), Microwave Imager (TMI)

and the Advanced Microwave Scanning Radiometer

for EOS (AMSR-E) are available from the Remote

Sensing Systems (http://www.remss.com/). Com-

pared with infrared sensors, the TMI and the AMSR-E

can provide continuous measurement of the SST even

when there is typhoon with heavy clouds.

Daily near-real time SSHA data with 0.25◦ spa-

tial resolution are provided by Archiving, Validation

and Interpretation of Satellite Oceanographic data

(AVISO). They were used to identify the positions

of the cold eddies (negative SSHA) and warm eddies

(positive SSHA) in this study.

As QuikSCAT stopped working on 23 November

2009, sea surface wind data are provided by the ad-

vanced scatterometer (ASCAT) in this study, which

was launched by the European Space Agency (ESA)

aboard the MetOp-A platform on 19 October 2006.

The measurement footprint is approximately 25 km.

The near-real time L2 products were interpolated to

get the daily gridded data with 0.25◦ spatial resolu-

tion.

2.3 In situ data

In situ data, which include Argo, XBT, CTD and

all other available profiles of temperature and salinity

during 13 October 2010 to 31 October 2010, were used

CHEN Xiaoyan et al. Acta Oceanol. Sin., 2012, Vol. 31, No. 1, P. 51-58 53

in this study from Coriolis Data Center. The mixed

layer depth can be defined from a threshold method,

for which the mixed layer depth is the depth at which

temperature changes more than 0.2◦C relative the sur-

face temperature.

2.4 Methods

The spatial variation of wind stress produces a

corresponding variation in the resulting Ekman trans-

port in the ocean surface layer, causing convergence in

some places and divergence in other places. This leads

to “Ekman pumping” (Enriquez and Friehe, 1995). In

this paper, Ekman pumping velocity, that is, the veloc-

ity of upwelling, is computed by the following equation

given by Enriquez and Friehe (1995):

WE =1

ρf(∇× τ), (1)

where WE is the Ekman pumping velocity; ρ, f and τ

are the seawater density, the Coriolis parameter and

wind stress, respectively.

The curl of the wind stress is given by

∇× τ =1

Rcosϕ

[∂τy

∂λ−

∂

∂ϕ(τxcosϕ)

]

, (2)

where R is the radius of the earth; ϕ and λ are ge-

ographic latitude and longitude, respectively; and τx

and τy are zonal and meridional wind stress.

In order to get the curl of wind stress for grid

(i, j), a finite-difference scheme is then applied as fol-

lows:

∇× τi,j =1

Rcosϕi,j

[

(τy)i+1,j − (τy)i−1,j

2∆λ−

(τxcosϕ)i,j+1 − (τxcosϕ)i,j−1

2∆ϕ

]

. (3)

3 Results

3.1 Chl-a concentration

Because of cloud coverage during the typhoon pe-

riod, only one image was useful to show the high chl-a

concentration which was induced by typhoon Megi in

the study area (Fig. 2a). This image was received

by MODIS on 25 October 2010, just 2 d after land-

fall of typhoon Megi. Figure 2a shows that high chl-a

concentration with value larger than 1 mg/m3 can be

observed in the large regions in the SCS west of the

Luzon Strait. Moreover, the maximum concentration

is about 3 mg/m3. Owing to be lack of chl-a con-

centration data before the passage of typhoon Megi,

we chose the monthly mean chl-a concentration data

of October averaged from 2002 to 2009 for compari-

son. The monthly mean chl-a concentration is approx-

imately 0.1–0.3 mg/m3 in the study area (Fig. 2b).

Therefore, typhoon Megi had an important impact on

the increase of chl-a concentration in the SCS and the

maximum increase reached to 30 times, which greatly

promoted the growth of the primary production.

3.2 Sea surface temperature

Figures 3a-f show the sea surface temperature be-

fore, during and after the passage of typhoon Megi in

the study area. Before the passage of typhoon, the

temperature along the track of typhoon was very uni-

form and around 30◦C on 12 October (Fig. 3a). On 19

October, a little decrease of 1–2◦C took place at the

center of typhoon and its ambient water (Fig. 3b).

The SST fell rapidly with a remarkable drop of 6–8◦C

in the SCS on 21 October and appeared to the right

of the track of Megi, which matched the Price’s con-

clusion well (Price, 1981) (Fig. 3c). After the passage

of typhoon, the cold SST patch with a core of ap-

proximately 24◦C moved westward (Figs 3d-f). The

phenomenon almost lasted for half a month. Figure

3f also shows a 2◦C drop of the SST observed in the

whole western north Pacific compared with the SST

in Fig. 3a.

Seven points, the locations of which are shown

in Fig. 3a, were selected along the typhoon track to

depict the time series of the SST change: A (18.7◦N,

127.5◦E), B (17.6◦N, 124.2◦E), C (16.3◦N, 119.0◦E),

D (17.2◦N, 117.3◦E), E (19.4◦N, 117.4◦E), F (21.0◦N,

118.2◦E), G (22.8◦N, 118.2◦E). Figure 4 shows the

daily SST change of these points relative to that of 12

October from 12 October to 10 November 2010. The

maximum drop at Points A and B was about 1.5◦C

and lasted 2–3 d. After a short time of a little increase,

the SST began to decrease and sustain the same level

for both points since 28 October. For Points C and D,

the maximum drop was 3 and 4◦C, respectively, which

lasted about 2 weeks. The SST drop at Points E and

F was larger than other points, with 6.5 and 5.5◦C,

respectively. The maximum drop at Point E occurred

on 22 October, four days earlier than that of Point

F. The SST drop at Point G induced by typhoon was

about 2◦C. Moreover, the SST continued to decrease

since 26 October, which was due to not only typhoon

Megi, but also the weaker solar radiation induced by


Fig.2. Chl-a concentration data. Daily data of 25 October 2010 (a) and monthly mean data of Octoberaveraged from 2002 to 2009 (b). Black represents cloud coverage, gray for land and white for the area withvalues more than the maximum.

Fig.3. Sea surface temperature before, during and after the passage of typhoon Megi. 12 October(a), 19October(b), 21 October (c), 23 October (d), 31 October (e) and 7 November (f).


Fig.4. Daily SST change of seven points rel-

ative to that of 12 October from 12 October

to 10 November.

the seasonal variability.

3.3 Sea surface height anomaly

Figures 5a-f show the sea surface height anomaly

(SSHA) before, during and after the passage of ty-

phoon Megi in the study area. Before the passage of

typhoon, the positive SSHA (warm-core eddies) was

the main feature along the typhoon track while the

negative SSHA (cold-core eddies) only appeared in the

vicinity of Point A (Fig. 5a). As shown in Fig. 5b, the

SSHA in the east of the Philippines on 19 October was

almost the same as that of 12 October. However, the

SSHA in the SCS west of the Luzon Strait decreased

approximately 20–30 cm along the typhoon track. At

the same time, there was little change with the SST in

the same place (Fig. 3b). Therefore, the SST change

lagged behind the SSHA. The region with the largest

decline of the SSHA can indicate the location where

the maximum cooling occurs. Reversely, the SSHA

near the land in the northern SCS rose. Since 21

October, the cold eddies continued to strengthen and

moved westward consistent with the SST (Figs 5c–f).

They were also present in the SCS until 15 November

(Fig. 5f).

4 Discussion

4.1 Typhoon-induced phytoplankton bloom

Zhao et al. (2005) analyzed the seasonal and spa-

tial distributions of chl-a concentration in the SCS and

showed that the maximum chl-a concentration west of

the Luzon Strait was low in all seasons except win-

ter. It was mainly because this area was controlled by

downwelling all year round except in winter and little

affected by other factors. In addition, chl-a concentra-

tion is high in subsurface layer in the SCS (Takahashi

and Hori, 1984; Gao et al., 2010). However, a signifi-

cant phytoplankton bloom was detected by MODIS 2

d after the landfall of typhoon Megi in this area where

nutrients are deficient in the upper ocean layer. Fig-

ures 6a-c show the Ekman pumping velocity on 13 Oc-

tober, 20 October and 24 October, respectively. The

upwelling was weak (≈0.2×10−6m/s) and its distri-

bution was relatively small on 13 October when ty-

phoon had not reached the SCS. On 20 October, the

upwelling was apparently stronger with wide distribu-

tion. The Ekman pumping velocity increased about

ten times. Just 1 d after typhoon’s landfall on 24 Oc-

tober, the Ekman pumping velocity returned to the

level as 13 October. Therefore, the strong upwelling

triggered by strong wind stress vectors contributed a

lot to this event. On the one hand, the passage of ty-

phoon Megi resulted in vertical mixing, bringing the

phytoplankton in subsurface to the upper layer and

causing the increase of chl-a concentration. On the

other hand, the nutrients on the bottom layer were

transported to the euphotic zone by strong upwelling

induced by typhoon and promote the growth of phy-

toplankton, leading to the enhancement of chl-a con-

centration.

4.2 Typhoon-induced sea surface cooling

A remarkable sea surface cooling (6–8◦C) was ob-

served after the passage of typhoon Megi in the SCS

(Figs 3c-e), corresponding to the track D-F (Fig.3a).

For the track A-B in the east of the Philippines, the

SST drop was only 1–2◦C. Figure 1 shows that the

wind speed along the track A-B was stronger than

that of D-F. So why did not the stronger wind speed

induce greater SST drop? This is mainly attributed

to the typhoon translation speed and the mixed layer

depth. Price (1981) documented that the response of

the sea surface cooling increases with decreasing ty-

phoon translation speed and with increasing hurricane

intensity. Typhoon Megi with rapid translation speed

(≈5.5–6.9 m/s) along the track A-B, moved slowly

(≈1.4–2.8 m/s) along the track D-F which agreed with

Price (1981) well.

In situ profiles were used to compute the mixed

layer depth. The profiles revealed the vertical tem-

perature distribution in both Points A and D within

1◦ (Figs 7a and b). The time typhoon passed Point A

was 00:00 UTC on 17 October and the typhoon passed

Point D was 00:00 UTC on 20 October. After the ty-


phoon, the depth of the mixed layer was deepened

from the original 40 to 70 m, which resulted in tem-

perature dropping from 30 to 28.5◦C. Unfortunately,

the in situ profiles at Point D in pre-typhoon period

were not available so that only the mixed layer depth

after typhoon shown in Fig. 7b was 40 m. But the

typhoon will inevitably lead to the deepening of the

mixed layer. So the mixed layer depth at Point D was

bound to be less than 40 m in pre-typhoon period.

From the climatological data (de Boyer Montegut et

al., 2004), the mean mixed layer depth at Point D in

October was approxomately 20–30 m.Therefore, the

mixed layer depth at Point A was deeper than that

of Point D. Zheng et al. (2010) pointed out that the

deeper the mixed layer depth is, the greater the sea

surface cooling is. Therefore, the temperature drop at

Point D was greater than that at Point A.

Zheng et al. (2010) showed that the existence of

cold eddies (negative sea surface height anomaly) is

able to act as good predictors for forecasting where

extreme cooling will take place. However, the location

of the maximum SST drop was not consistent with

the location of preexisting cold eddies at Point A for

typhoon Megi (Figs 3 and 5). Lin et al. (2008) consid-

ered that in the gyre central region (10◦–21◦N, 121◦–

170◦E) where the background climatological warm

layer (typically the depth of the 26◦C isotherm (D26))

is deep, the background itself is already sufficient to

restrain the self induced cooling. Figure 7 shows that

the D26 was about 110 m at Point A and only 60 m

at Point D. This provided further evidence to explain

why the extreme cooling take place in the certain

Fig.5. Sea surface height anomaly before, during and after the passage of typhoon Megi. 12 October (a),19 October (b), 21 October (c), 23 October (d), 31 October (e) and 15 November (f).


Fig.6. The Ekman pumping velocity. 13 October (a), 20 October (b) and 24 October (c).

Fig.7. Vertical temperature profiles from in situ data at Point A (a) and at Point D (b).

region rather than others.

5 Conclusions

In this study, we investigated the ocean biolog-

ical and physical responses to typhoon Megi using

the chlorophyll-a (chl-a) concentration, the sea surface

temperature(SST), the sea surface height anomaly

(SSHA), the sea surface wind measurements derived

from different satellites and the in situ data. The re-

sults show that (1) strong upwelling was induced by

typhoon Megi and its velocity increased an order of


magnitude, which provided perfect conditions for the

growth of phytoplankton and caused a significant phy-

toplankton bloom in the SCS. Relative to historical

levels over the same period, the maximum chloro-

phyll a concentration increased by about 30 times;

(2) the maximum temperature drop existed in the

South China Sea. This was mainly attributed to slow

translation speed, shallow mixed layer depth and D26

which enabled the cold water to be easily transported

from subsurface layer; (3) the surface temperature

changes lagged behind the sea surface height. So the

region with the largest decline of the sea surface height

anomaly can indicate the location where the maximum

cooling occurs; (4) after the passage of typhoon, the

cold patches moved westward and lasted for 2 weeks.

Same as the cold patches, the cold eddies induced by

typhoon moved westward and lasted for more than 3

weeks.

Acknowledgements

The authors thank for the satellite data receiv-

ing station of the Second Institute of Oceanography

of the State Oceanic Administration of China and the

NASA Goddard Earth Sciences Data and Information

Services Center (GES DISC) for providing the MODIS

data. Remote Sensing Systems, AVISO, NASA and

Coriolis Data Center are also thanked for providing

SST, SSHA, wind and in situ data respectively.

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