10:45 AM - 12:15 PM
[ACG33-P01] Sea surface salinity variability and trade wind behavior over the past 70 years recorded in Tuvalu coral annual band
★Invited Papers
Keywords:Tuvalu, coral annual band, oxygen isotope ratio, sea surface salinity, trade wind, multidecadal-scale variation
The importance of the role of sea surface salinity (SSS) in addition to sea surface temperature (SST) has been recently pointed out in studies of the water cycle and ocean-atmosphere coupled phenomena in the tropics, but long-term salinity observations are very limited. The trace chemical composition of coral carbonate skeleton provides alternative information for the tropical ocean environment. In particular, the coral oxygen isotope ratio (d18Ocoral), which is determined by the SST and the oxygen isotope ratio of seawater (d18Osw), is useful as a paleo-thermometer. The oxygen isotope ratio of seawater is considered as a proxy of SSS, which is determined by run-offs from rivers, precipitation-evaporation processes, and advection. However, the contribution of SST and SSS to d18Ocoral is not constant, but varies depending on the oceanic region, geography, and climatic conditions. Reconstructions of SST and SSS from coral annual bands have been attempted in various oceanographic regions. We present the results of coral d18O analyses collected in March 2009 from Tuvalu, South Pacific (8°S, 179°E), which is located at the northern edge of the South Pacific Convergence Zone (SPCZ).
Tuvalu is an atoll island with an elevation of 3-4 m and no rivers. The climatology of Funafuti Atoll, the capital of Tuvalu, has an average SST of 29.5°C and a small annual cycle with the maximum of 29.9°C in March and the minimum of 29.1°C in September. The annual precipitation is 3-4,000 mm, and the rainy season is from December to March with the maximum in January, while the dry season is from May to October with the minimum in September. The easterly trade winds prevail from March to September, whereas westerly winds are observed from October to February. The SPCZ migration and the occurrence of El Niño/Southern Oscillation (ENSO) affect the abundance of water resources on the island.
A coral core (98 cm) collected from Funafuti Atoll showed 70-yr continuous annual bands (1940-2009) and was analyzed for monthly d18O. The d18O variability not only shows seasonal variations in SST, but also decadal and multi-decadal-scale variations. The d18O variability range over the past 70-yr is 0.98‰ (0.6‰ for 1978-2009).
In the last 30 years since 1977-78, when more instrumental records were available, no high correlation was found between d18Ocoral and observed SST (r=-0.45). Given that the annual SST range is within 1°C (equivalent to 0.2‰ of d18O), the d18Ocoral variation (0.6‰) in Tuvalu during the last 30 years (1978-2009) can be regarded as a mixture of SST variability of 0.2‰ and d18Osw variability of 0.4‰, which is equivalent to 2 psu in salinity.
From a comparison of d18Ocoral anomalies during the rainy season (December-March) since the 1980s with rainfall records and reanalyzed SSS, we found a low correlation with rainfall (r=-0.30), while the very high correlation with SSS (r=0.76). Furthermore, a correlation analysis between d18Ocoral anomalies during the rainy season (December-March) and the reanalyzed sea surface height around Tuvalu suggests that northwestward currents are enhanced when d18Ocoral values are high (advection of high salinity water masses). Therefore, the d18Ocoral variability in Tuvalu is expected to reflect SSS anomalies caused by the intensity of the trade winds modulated by the SPCZ migration, Walker circulation, and ENSO.
The reconstructed SSS for the 70-yr shows multidecadal-scale variations in d18Ocoral with higher values between the 1950s to 1977/78. A wavelet spectrum analysis of the 70-yr d18Ocoral record reveals significant spectra at both long-period and short-period, but both spectra became shorter in period throughout the 70-yr. The present results suggest that salinity variability in the tropical South Pacific may be modulated by ocean-atmosphere coupled phenomena such as the Interdecadal Pacific Oscillation (IPO) and ENSO.
Tuvalu is an atoll island with an elevation of 3-4 m and no rivers. The climatology of Funafuti Atoll, the capital of Tuvalu, has an average SST of 29.5°C and a small annual cycle with the maximum of 29.9°C in March and the minimum of 29.1°C in September. The annual precipitation is 3-4,000 mm, and the rainy season is from December to March with the maximum in January, while the dry season is from May to October with the minimum in September. The easterly trade winds prevail from March to September, whereas westerly winds are observed from October to February. The SPCZ migration and the occurrence of El Niño/Southern Oscillation (ENSO) affect the abundance of water resources on the island.
A coral core (98 cm) collected from Funafuti Atoll showed 70-yr continuous annual bands (1940-2009) and was analyzed for monthly d18O. The d18O variability not only shows seasonal variations in SST, but also decadal and multi-decadal-scale variations. The d18O variability range over the past 70-yr is 0.98‰ (0.6‰ for 1978-2009).
In the last 30 years since 1977-78, when more instrumental records were available, no high correlation was found between d18Ocoral and observed SST (r=-0.45). Given that the annual SST range is within 1°C (equivalent to 0.2‰ of d18O), the d18Ocoral variation (0.6‰) in Tuvalu during the last 30 years (1978-2009) can be regarded as a mixture of SST variability of 0.2‰ and d18Osw variability of 0.4‰, which is equivalent to 2 psu in salinity.
From a comparison of d18Ocoral anomalies during the rainy season (December-March) since the 1980s with rainfall records and reanalyzed SSS, we found a low correlation with rainfall (r=-0.30), while the very high correlation with SSS (r=0.76). Furthermore, a correlation analysis between d18Ocoral anomalies during the rainy season (December-March) and the reanalyzed sea surface height around Tuvalu suggests that northwestward currents are enhanced when d18Ocoral values are high (advection of high salinity water masses). Therefore, the d18Ocoral variability in Tuvalu is expected to reflect SSS anomalies caused by the intensity of the trade winds modulated by the SPCZ migration, Walker circulation, and ENSO.
The reconstructed SSS for the 70-yr shows multidecadal-scale variations in d18Ocoral with higher values between the 1950s to 1977/78. A wavelet spectrum analysis of the 70-yr d18Ocoral record reveals significant spectra at both long-period and short-period, but both spectra became shorter in period throughout the 70-yr. The present results suggest that salinity variability in the tropical South Pacific may be modulated by ocean-atmosphere coupled phenomena such as the Interdecadal Pacific Oscillation (IPO) and ENSO.