A CORAL-BASED RECONSTRUCTION OF INTERTROPICAL CONVERGENCE ZONE VARIABILITY OVER CENTRAL-AMERICA SINCE 1707

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 deltaO-18 analyses on a massive coral from Secas Island (7-degrees 59'N, 82-degrees 3'W) in the Gulf of Chiriqui, Panama'. The land area that drains into the Gulf of Chiriqui has served to amplify the rainfall effect on nearshore surface waters and coral deltaO-18 composition. During the protracted wet season in Panama, the deltaO-18 of precipitation (deltaO-18ppt) is reduced on average by 10 parts per thousand and sea surface salinity (SSS) along the western coast is reduced up to 11 parts per thousand. Calibration of the coral deltaO-18 from Secas Island against instrumental sea surface temperature (SST), SSS, precipitation and deltaO-18(ppt) data indicate that seasonal rainfall induced variations in seawater deltaO-18 are responsible for approximately 80% of the annual deltaO-18 variance. Past El Nino-Southern Oscillation (ENSO) events are recorded as minor 0.2 to 0.4 parts per thousand deltaO-18 changes superimposed on the dominant annual deltaO-18seawater and salinity variations. The annual cycle in coral deltaO-18 (average 0.9 parts per thousand) 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 Panama in every northern hemisphere summer. Significant interannual and interdecadal deltaO-18 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 deltaO-18 from nearshore in the Gulf of Panama. SST data spanning the last 40 years show no decadal changes. This indicates that decadal oscillations in the Gulf of Chiriqui deltaO-18 record are regional features not related to SST changes, but are caused by ITCZ precipitation effects on the deltaO-18 of seawater. A 9-year period in Panama precipitation supports this conclusion and provides a potential link between interannual coral deltaO-18 variations and ITCZ precipitation. It is also shown that the period of the average 9-year interannual period in coral deltaO-18 varies from approximately 7.5 years to approximately 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 deltaO-18 time series also contains a long-term trend of -0.40 parts per thousand suggesting an increase in precipitation and/or SST since the early 1800s. As the Gulf of Chiriqui coral deltaO-18 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.