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

Seasonal movements of the Intertropical Convergence Zone (ITCZ) contro l precipitation patterns and cloud cover throughout the tropics. In th is study we have reconstructed seasonal and interannual variability of the eastern Pacific ITCZ from 1984 to 1707 using subseasonal deltaO-1 8 analyses on a massive coral from Secas Island (7-degrees 59'N, 82-de grees 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 o n nearshore surface waters and coral deltaO-18 composition. During the protracted wet season in Panama, the deltaO-18 of precipitation (delt aO-18ppt) is reduced on average by 10 parts per thousand and sea surfa ce salinity (SSS) along the western coast is reduced up to 11 parts pe r thousand. Calibration of the coral deltaO-18 from Secas Island again st 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 an nual deltaO-18 variance. Past El Nino-Southern Oscillation (ENSO) even ts are recorded as minor 0.2 to 0.4 parts per thousand deltaO-18 chang es 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 i s the direct result of the annual northward expansion of the eastern P acific ITCZ. The regularity of the reconstructed seasonal ITCZ cycle i ndicates that over the length of the record the zone of maximum rainfa ll in the eastern Pacific has always expanded north to at least Panama in every northern hemisphere summer. Significant interannual and inte rdecadal 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 de ltaO-18 from nearshore in the Gulf of Panama. SST data spanning the la st 40 years show no decadal changes. This indicates that decadal oscil lations in the Gulf of Chiriqui deltaO-18 record are regional features not related to SST changes, but are caused by ITCZ precipitation effe cts on the deltaO-18 of seawater. A 9-year period in Panama precipitat ion supports this conclusion and provides a potential link between int erannual coral deltaO-18 variations and ITCZ precipitation. It is also shown that the period of the average 9-year interannual period in cor al deltaO-18 varies from approximately 7.5 years to approximately 11.8 years. Variance near 11 years is strongest throughout the 1800s, howe ver, a poor direct correlation with sunspot number and solar irradianc e leaves the origin of this interannual oscillation in question. The d eltaO-18 time series also contains a long-term trend of -0.40 parts pe r thousand suggesting an increase in precipitation and/or SST since th e early 1800s. As the Gulf of Chiriqui coral deltaO-18 time series is the first paleoclimatic record of past variations in the ITCZ, other s easonal-resolution reconstructions of the past behavior of the ITCZ ar e required to test whether the interannual and long-term variability o bserved in the eastern Pacific ITCZ is more than regional in scale.