Interannual sea surface salinity changes in the western Pacific from ...

7 downloads 64 Views 316KB Size Report
western Pacific from 1954 to 2000 based on coral isotope analysis, Geophys. Res. Lett. .... taken using a coring drill driven by compressed air from. SCUBA tanks ... determine sub-sampling lines along the maximum growth axis. After ultrasonic ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L04608, doi:10.1029/2004GL022026, 2005

Interannual sea surface salinity changes in the western Pacific from 1954 to 2000 based on coral isotope analysis Hiroko Iijima,1 Hajime Kayanne,1 Maki Morimoto,2 and Osamu Abe2 Received 17 November 2004; revised 4 January 2005; accepted 31 January 2005; published 24 February 2005.

[1] Changes in sea surface salinity (SSS) in the western Pacific warm pool (WPWP) provide information on the variability of the El Nin˜o Southern Oscillation (ENSO) because SSS reflects the balance between evaporation and precipitation. We reconstructed the long-term variability of SSS at the northwestern edge of the WPWP from oxygen isotope analysis of annual coral bands. The estimated SSS corresponded well with in situ observations, demonstrating that isotopic records from coral can be used as a paleosalinometer. Estimated SSS between 1954 and 2000 peaked during the El Nin˜os of 1972– 73, 1982 – 83, and 1997 – 98. These SSS peaks were caused by anomalously low precipitation accompanying the eastward shift in the WPWP. Citation: Iijima, H., H. Kayanne, M. Morimoto, and O. Abe (2005), Interannual sea surface salinity changes in the western Pacific from 1954 to 2000 based on coral isotope analysis, Geophys. Res. Lett., 32, L04608, doi:10.1029/ 2004GL022026.

1. Introduction [2] The surface ocean in the western tropical Pacific, the western Pacific warm pool (WPWP), is characterized by high annual average sea surface temperatures (SST > 28C) and relatively low sea surface salinity (SSS < 35.0 PSU). The high SST makes the WPWP a large source of atmospheric water vapor. Temporal and spatial variability in the WPWP are linked with El Nin˜o Southern Oscillation (ENSO) events. During an El Nin˜o, the WPWP shifts eastward because of weakened trade winds, which makes the atmosphere over the western Pacific dryer than usual during the northern winter [Ropelewski, 1992]. Several studies have reported expansion or contraction of the WPWP, related to ENSO, based on changes in SSS [Delcroix et al., 1998; Maes, 2000; Ioualalen and Henin, 2001; Delcroix and McPhaden, 2002; Wang and Chao, 2004]. These studies used the limited observations of SSS or SSS calculated from ocean circulation models, because instrumental observations of SSS are limited temporally and spatially. A longer record of SSS is necessary to understand decadal variability of the WPWP and its relationship to ENSO. [3] Massive corals from tropical and subtropical regions provide records of climate variation for the last several hundred years, including variability of ENSO events [Cole et al., 1993; Urban et al., 2000]. The oxygen isotope 1 Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan. 2 Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan.

Copyright 2005 by the American Geophysical Union. 0094-8276/05/2004GL022026

composition of coral skeleton (d18Ocoral) is controlled by both SST and d18O of seawater (d18Oseawater), which is closely related to SSS via changes in the hydrological balance between precipitation, evaporation, and water advection [e.g., Gagan et al., 2000]. In coral living where seasonal variation in SSS is negligible, d18Ocoral is a proxy for SST [Dunbar et al., 1994; Wellington et al., 1996]. Conversely, where the seasonal variation in SST is low, d18Ocoral can be a proxy for SSS [Le Bec et al., 2000]. However, seasonal variation in both SST and SSS yields measurable effects on d18Ocoral in most locations, so reconstruction of past ENSO variability based on these assumptions is restricted to qualitative estimates. Recent studies have tried to remove the contribution of SST from d18Ocoral using skeletal Sr/Ca, which represents temperature when the skeleton was formed, to estimate SSS, and have shown good agreement with observed SSS [Quinn and Sampson, 2002; Ren et al., 2002]. These studies regarded the residual component of d18Ocoral after removing the SST signal (i.e., d18Oseawater) as a proxy for SSS because of the lack of a local quantitative relationship between d18Oseawater and SSS. [4] Morimoto et al. [2002] reported a quantitative relationship between d18Ocoral, SST, d18Oseawater, and SSS for the Palau Islands in the WPWP for 2.5 years from 1998 to 2000, and showed a significant relationship between d18Ocoral and SSS. In this study, we extend this analysis to the last 50 years, using d18Ocoral records from the Palau Islands, and calculate SSS using the local SSS calibration of Morimoto et al. [2002]. We show the SSS change for the last 50 years and discuss the relationship between SSS and ENSO.

2. Climatology at Palau [5] The Palau Archipelago, in the Western Pacific (6530 to 8120N and 134080 to 134440E) (Figure 1), consists of hundreds of islands and a lagoon surrounded by a barrier reef [Kayanne et al., 2002]. The Palau Islands are in the Intertropical Convergence Zone (ITCZ) on the northwestern edge of the WPWP, where precipitation exceeds evaporation [Chaen et al., 1987], and are located oceanographically in the North Equatorial Current (NEC), which flows eastward between 7N to 25N. [6] Weather observations have been recorded at a NOAA station on Koror Island, Palau, since 1924 (NOAA, National Climatic Data Center; 17200N, 134290W). Over the last 50 years, annual precipitation averaged approximately 3,700 mm (Figures 2b and 2c) with maximum precipitation from July to August, and minimum precipitation from February to April. Between January and April in 1973, 1983, and 1998, monthly precipitation was below the mean value because of El Nin˜os. While El Nin˜o events resulted in distinctly dryer conditions in Palau, La Nin˜a events were not associated with distinct precipitation anomalies.

L04608

1 of 4

L04608

IIJIMA ET AL.: SSS RECORDS FROM WEST PACIFIC CORALS

Figure 1. Map of the equatorial Pacific. The WPWP is the area with SST exceeding 28C. The Palau Islands are located on the northwestern edge of the WPWP. [7] Monthly SST data for a 1  1 box centered on Palau (8N, 134E) since 1854 were obtained from the NOAA National Climate Data Center (NOAA NCDC ERSST ver.2; Extended reconstruction of global sea surface temperature data based on COADS data). Over the last 50 years, average annual SST was 28.8C, with two annual maxima from May to June and October to November, and an annual range of