A coral-based reconstruction of Intertropical Convergence Zone variability over Central America since 1707

Authors

  • Braddock K. Linsley,

  • Robert B. Dunbar,

  • Gerard M. Wellington,

  • David A. Mucciarone


Abstract

Seasonal movements of the Intertropical Convergence Zone (ITCZ) control precipitation patterns and cloud cover throughout the tropics. In this study we have reconstructed seasonal and interannual variability of the eastern Pacific ITCZ from 1984 to 1707 using subseasonal δ18O analyses on a massive coral from Secas Island (7°59′N, 82°3′W) in the Gulf of Chiriquí, Panamá. The land area that drains into the Gulf of Chiriquí has served to amplify the rainfall effect on nearshore surface waters and coral δ18O composition. During the protracted wet season in Panamá, the δ18O of precipitation (δ18Oppt) is reduced on average by 10‰ and sea surface salinity (SSS) along the western coast is reduced up to 11‰. Calibration of the coral δ18O from Secas Island against instrumental sea surface temperature (SST), SSS, precipitation and δ18Oppt data indicate that seasonal rainfall induced variations in seawater δ18O are responsible for ∼80% of the annual δ18O variance. Past El Niño-Southern Oscillation (ENSO) events are recorded as minor 0.2 to 0.4‰ δ18O changes superimposed on the dominant annual δ18Oseawater and salinity variations. The annual cycle in coral δ18O (average 0.9‰) accounts for the largest component of variance at 51% and is the direct result of the annual northward expansion of the eastern Pacific ITCZ. The regularity of the reconstructed seasonal ITCZ cycle indicates that over the length of the record the zone of maximum rainfall in the eastern Pacific has always expanded north to at least Panamá in every northern hemisphere summer. Significant interannual and interdecadal δ18O oscillations occur at average periods near 9, 3–7 (ENSO band), 17 and 33 years (listed in order of decreasing variance). Over the past 20 years similar decadal shifts are apparent in coral δ18O from nearshore in the Gulf of Panamá. SST data spanning the last 40 years show no decadal changes. This indicates that decadal oscillations in the Gulf of Chiriquí δ18O record are regional features not related to SST changes, but are caused by ITCZ precipitation effects on the δ18O of seawater. A 9-year period in Panamá precipitation supports this conclusion and provides a potential link between interannual coral δ18O variations and ITCZ precipitation. It is also shown that the period of the average 9-year interannual period in coral δ18O varies from ∼7.5 years to ∼11.8 years. Variance near 11 years is strongest throughout the 1800s, however, a poor direct correlation with sunspot number and solar irradiance leaves the origin of this interannual oscillation in question. The δ18O time series also contains a long-term trend of −0.40‰ suggesting an increase in precipitation and/or SST since the early 1800s. As the Gulf of Chiriquí coral δ18O time series is the first paleoclimatic record of past variations in the ITCZ, other seasonal-resolution reconstructions of the past behavior of the ITCZ are required to test whether the interannual and long-term variability observed in the eastern Pacific ITCZ is more than regional in scale.

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