Study Sites Tropical Northwestern Pacific

The main area sampled for corals in the present study was the tropical northwestern Pacific, where published coral records are still rare compared with other areas in the tropics and subtropics (Fig. 1; Table 1). The study area covers a relatively wide range of latitude, from Pohnpei Island (7°N) in the equatorial Pacific to Ishigaki Island (24°N) in the Kuroshio Current region of the northwestern Pacific.

The western equatorial Pacific is one of the hottest and wettest tropical ocean regions on Earth. The area in which SST is higher than 28 °C is referred to as the western Pacific warm pool (WPWP) (Yan et al., 1992) and more recently as the Indo-Pacific Warm Pool (IPWP), which includes warm areas in the eastern Indian Ocean (Gagan et al., 2004). Climatic variability relating to the ENSO cycle is typically large along the equator. Corals from Micronesian islands located near the northern border of the WPWP, Palau, Chuuk, Pohnpei, and Majuro, lie along an east-west transect at about 7°N latitude, providing a unique opportunity to reconstruct the migration (E-W zonal displacement) and expansion (in total area and N-S breadth) of the WPWP, both seasonally and in response to the interannual ENSO cycle. Seasonal variations in SST and SSS show an east-west trend along this island transect (Fig. 1; Levitus et al., 1994). Annual mean SST increases, and SSS decreases, westward from Majuro to Palau, because Palau is close to the center of the WPWP.

Pandora Reef Ishigaki

Bicol

Palau

Chuuk

Pohnpei

Majuro

e c urf22

Pandora Reef Ishigaki

e c urf22

33 a

JSNJMM JMMJSN AODFAJ FAJAOD

Month

1 28

er p

33 a

JSNJMM JMMJSN AODFAJ FAJAOD

Month

Bicol

Palau

Chuuk

Pohnpei

Majuro

34 ce

JMMJSN JMMJSN JMMJSN FAJAOD FAJAOD FAJAOD

34 ce

JMMJSN JMMJSN FAJAOD FAJAOD

JMMJSN JMMJSN JMMJSN FAJAOD FAJAOD FAJAOD

Month r a

Figure 1: Oceanic settings of study sites in the western Pacific. (A) The location of the western Pacific warm pool (WPWP, mean annual SST >28 °C) is shown, together with the average position of the Intertropical Convergence Zone (ITCZ) in July and January. (B) Climatological monthly sea-surface temperature (SST), and sea-surface salinity (SSS) at each coral site. The DASILVA SMD94 climatology data (da Silva et al., 1994; http:// ingrid.ldeo.columbia.edu/SOURCES/.DASILVA/.SMD94/.climatology/) and data from Levitus et al. (1994) were used for SST and SSS, respectively.

Table 1: Porites coral samples examined in this study.

Site

Core id.

Locality

Depth (m MSL)

Date of collection

DASILVA SMD94

Microsampling interval (mm)

Reference

Lat.

Long

Mean

Max.

Min.

Micronesia

Chuuk Atoll

CHU99-01

8°N

152°E

3.9

March 11, 1999

28.8

29.4

28.1

400

This study

Pohnpei Reef

POH99-O1

7°N

158°E

1.0

March 15, 1999

28.8

29.3

28.3

400

Inoue et al. (2004)

Philippines

Bicol

SWGM01-01

13°N

124°E

6

March 2001

28.5

29.8

26.9

400

This study

Ishigaki I., Japan

24°N

124°E

25.7

29.0

22.4

Urasoko Bay

IU96-07

3.6

November 22, 1996

400

Suzuki et al. (1999)

Shiraho Reef

IS91-06

2

September, 1991

This study

Yasura Channel

IY99-01

0.7

February 2, 1999

200

Suzuki et al. (2003)

Yasura Channel

IY99-02

0.8

February 2, 1999

200

Suzuki et al. (2003)

Great Barrier Reef

19°S

146°E

26.3

28.9

24.2

Pandora Reef

PAN-1-B

5

January 14, 1999

200

Suzuki et al. (2003)

Pandora Reef

PAN-3

5

January 14, 1999

200

Suzuki et al. (2003)

MSL, mean sea level; SST, sea surface temperature.

Seasonal variation of rainfall in the tropic zone is generally controlled by the position of the Intertropical Convergence Zone (ITCZ; Fig. 1). The ITCZ is an area of low pressure where the northeast and southeast trades meet, and it is characterized by a band of heavy precipitation. On the other hand, interannual variations of SST and rainfall in the western equatorial Pacific are strongly influenced by the ENSO phenomenon (Fig. 2). From the Philippines to Micronesia cooler SSTs and relative drought characterize El Nino events, when the WPWP and the Indonesian low atmospheric pressure system migrate eastward toward the central Pacific. Cooling of SSTs is probably caused by the shallowing of the thermocline as a result of the relaxation of the trade winds across the Pacific basin. Lower than average rainfall in the WPWP area is a result of zonal displacement of convective centers of the Walker circulation during El Nino events.

Two of the four coral sites examined in this study, Bicol (13°N, 124°E), near the southeastern tip of Luzon Island in the Philippines, and Ishigaki Island, one of the Ryukyu Islands in the East China Sea, are located within the path of the warm Kuroshio Current (Fig. 1). Bicol is close to the point where the North Equatorial Current bifurcates, forming the northward flowing Kuroshio and the southward flowing Mindanao Current. Bicol appears to be located near the center of the area of reduced rainfall during El Nino events, but the local SST there shows little correlation with the Southern Oscillation Index (SOI; Trenberth and Hoar, 1996), which is a widely recognized index of ENSO activity (Fig. 2). The seasonal variation in SST increases from the Bicol region northward toward Ishigaki Island (Fig. 1). In terms of interannual variation of SST at Ishigaki Island the majority of the highest SST measurements have been obtained during summers following the onset of El Nino events (warm phase of ENSO), indicating a telecon-nection linking Ishigaki SST to ENSO (Suzuki et al., 2000, 2003). Park and Oh (2000) reported a teleconnection pattern between the East China Sea and the equatorial Pacific: SSTs in the East China Sea show significant coherence with SSTs in the Nino 3.4 region (Trenberth, 1997), with a phase lag of 5-9 months. Therefore, the anomalously warm SSTs around the Ryukyus in 1998, which resulted in severe coral bleaching in the area, may have been related to the 1997-1998 El Nino.

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