AN ANALYSIS OF THE TIME SCALES OF VARIABILITY IN CENTURIES-LONG ENSO-SENSITIVE RECORDS IN THE LAST 1000 YEARS

We document the characteristic time scales of variability for seven cl imate indices whose time-dependent behavior is sensitive to some aspec t of the El Nino/Southern Oscillation (ENSO). The ENSO sensitivity ari ses from the location of these long-term records on the periphery of t he Indian and Pacific Oceans. Three of the indices are derived princip ally from historical sources, three others consist of tree-ring recons tructions (one of summer temperature, and the other two of winter rain fall), and one is an annual record of oxygen isotopic composition for a high-elevation glacier in Peru. Five of the seven indices sample at least portions of the Medieval Warm Period (approximately A.D. 950 to 1250). Time series spectral analysis was used to identify the major ti me scales of variability among the different indices. We focus on two principal time scales: a high frequency band (approximately 2-10 yr), which comprises most of the variability found in the modem record of E NSO activity, and a low frequency band to highlight variations on deca dal to century time scales (11 < P < 150 yr). This last spectral band contains variability on time scales that are of general interest with respect to possible changes in large-scale air-sea exchanges. A techni que called evolutive spectral analysis (ESA) is used to ascertain how stable each spectral peak is in time. Coherence and phase spectra are also calculated among the different indices over each full common peri od, and following a 91-yr window through time to examine whether the r elationships change. In general, spectral power on time scales of appr oximately 2-6 yr is statistically significant and persists throughout most of the time intervals sampled by the different indices. Assuming that the ENSO phenomenon is the source of much of the variability at t hese time scales, this indicates that ENSO has been an important part of interannual climatic variations over broad areas of the circum-Paci fic region throughout the last millennium. Significant coherence value s were found for El Nino and reconstructed Sierra Nevada winter precip itation at approximately 2-4 yr throughout much of their common record (late 1500s to present) and between 6 and 7 yr from the mid-18th to t he early 20th century. At decadal time scales each record generally te nds to exhibit significant spectral power over different periods at di fferent times. Both the Quelccaya Ice Cap deltaO-18 series and the Qui nn El Nino event record exhibit significant spectral power over freque ncies approximately 35 to 45 yr; however, there is low coherence betwe en these two series at those frequencies over their common record. The Sierra Nevada winter rainfall reconstruction exhibits consistently st rong variability at periods of approximately 30-60 yr.