Historical Aspects of El Niño/Southern Oscillation - Information from Tree Rings

Abstract

Characteristics of the annual growth rings of trees can provide accurately dated proxy climatic information for periods prior to the start of instrumental climate records. To derive information about El Niño/Southern Oscillation (ENSO) events from tree rings depends upon the trees being located at sites where climate is significantly and consistently influenced by ENSO events and also the trees' ability to faithfully record this influence. Such a tree ring/ENSO linkage is illustrated with tree-ring data from sites in western North America that have been used to develop large-scale climate reconstructions back to 1602. The instrumental surface temperature and precipitation fields in parts of North America show significant teleconnections with ENSO events and these linkages are demonstrated to be preserved in dendroclimatic reconstructions of the same climate variables. A significantly calibrated and verified reconstruction of a Southern Oscillation (SO) index has been developed back to 1601 from the western North American tree-ring data base. The practical significance of this reconstruction is assessed by comparison with an independent record of historical El Niño events. Dendroclimatic techniques are limited in their application to tree species from temperate and sub-polar latitudes due to problems in dating species from tropical regions. Thus, no tree-ring data are presently available from the land areas in the tropics where climate is most strongly influenced by ENSO events. Prospects for deriving more information about past ENSO events from tree-ring data in temperate regions outside of North America are discussed. Ultimately, the most reliable history of climatic phenomena associated with ENSO events for periods prior to instrumental records will be obtained by combining a variety of independent sources of proxy climate information.